power ic in the saddle

POWER ICs IN THE SADDLE

B. JAYANT BALlC,A North Carolina State University

Higher efficiency and lower lossgs are sought for devices for a range of applications, from microwave ovens to hig h-voltage dc transmission

placed between the power source and any load to which power flow must be controlled. The load may be inductive (like the motors encountered at

HE FOUNDATION for the Information Age that we live in today was laid half a century ago, with the invention of the T transistor in 1947 and the IC in 1958.

home in mixers, vacuum cleaners, and power tools) or capacitive (like a transducer or display) or resistive (like an electric space-heating unit). The contents of the black box fall into three cate- gories: the digital interface circuitry used to en- code and decode signals for communication with the system's microprocessors; the analog circuits for self-protection, and high-voltage or high-current devices for regulating the power flow from source to load [see diagram opposite]. This mix- ture of disparate technologies presents tough design and manufacturing problems.

For example, the interface circuits in smart power chips are implemented in CMOS technology not unlike that used in very large-scale integrated (VLSI) memory and microprocessor circuitry. But the harsh environment they can expect imposes special design challenges. For example, the ambi- ent temperature is higher (typically 60-1 25 "C) than for VLSI circuits. Moreover, the monolithic integration of power switches with the interface circuitry raises chip temperature because of the power the switches dissipate during circuit operation. The CMOS circuits must therefore be designed not to risk latch-up and its paralyzing effects amid these adverse circumstances. In addition, some of the CMOS circuits that are used to provide the gate drive voltage to the power switches must be able to operate at relatively high voltages, typically * 15 V.

IEEE SPECTRUM JULY 1995

Smart power technology

Power

r

4 Power MOSFETs / i

/ /

/

/

/

/

1 1

\ \

\ \ \ \

1 Insulated-gate bipolar transistors

/ /

/ /

Load

\ \ \ \ \ \ \

control I Power t

Sensing and protection

1 1

b MOS-controlled thvristors I I - I

30-V CMOS Drive circuits

-

High-voltage level sh i f t

High-speed bipolar transistors

Operational amplifiers

Analog circuits 1

Detection circuits

Logic circuits __I High-density CMOS i

As for the analog circuits in a smart power chip, they are used for creating the sensors necessary for self-protection and for providing a rapid feedback loop, which can terminate chip operation harmlessly when the system conditions exceed expectations. For example, smart power chips must be designed to shut down without damage when a short circuit occurs across a load such as a motor winding. Under these conditions, the current soars at a rate typically in excess of 100 N p s . With smart power technology, the load current is monitored, and whenever this current exceeds a preset limit, the drive voltage to the power switches is shut off.

In addition to this over-current protection feature, over-voltage and over-temperature protection are commonlv included to prevent destructive failure. These analog circuits require the integration on chip of high-speed bipolar transistors with cut- off frequencies above 1 CHz for the feedback loop and for the band-gap reference used to mon- itor chip temperature.

Of course, the most unusual feature of smart power chips is the ability to operate at high voltages and sometimes at high current levels. The power electronics applications impose demands ranging from 50 V to 10 000 V with current-han-

dling capability ranging from 10 mA to more than 1000 A [see diagram, p. 361. Two prominent applications at low operating voltages (50-100 v) are power supplies for computers and multiplexed-bus systems for automotive electronics.

The relatively high currents of up to t 00 A that must be controlled by the chips compels the design of power switches with a vertical architecture like that used in discrete devices. Isolation between multiple power switches as well as from the rest of the digital and analog IC components, however, is achieved by imbedding the power switch in a high-resistivity substrate. The substrate has deep isolation diffusions that must extend 10-15 pm through device drift regions typically in the range of 10 pm.

Power MOSFETs are used in these applications because they can be designed with extremely low on-state voltage drop and fast switching that min- imizes power losses even above 100 kHz. Further- more, these devices require that gate drive currents be less than 1 pA in the steady state because of the MOS gate structure. This structure makes for a far simpler control circuit than would power bipolar transistors, one that is amenable to monolithic integration with the digital and analog circuits.

A Smart power technology can be viewed as a black box that interfaces a power source to any load. The box’s interface function is realized with high- density CMOS logic circuits, its sensing and protection function with bipolar analog and detection circuits, and its power control function with power devices and their associated drive circuits.

HAL.IC.4 - I’OW i R IC, IN THF SAUDLF 35

Defining terms Band-gap reference: a circuit in which the semiconductor band gap is used to create a reference voltage source. Cell: the basic cross-sectional unit of a device structure that typically carries a small fraction of the device's total current, Typical power devices have thou- sands of cells. Channel density: the width of an MOS gate integrated within 1 cm2 of the area of a power device. DMOS (Double-diffused MOS): a process (and the resulting device structure) that determines the MOSFET channel length through a double diffusion of p- and n-type impurities from the edge of a refractory gate material. Figure of merit for power devices: an expression relating the fundamental properties of semiconductor materials to the specific on-resistance of the drift

region that supports the voltage during the blocking state. Fusible links: thin metal bars that can be fused by the application of a current pulse, so as to disconnect defective regions of a device from the fully func- tional portions. Hard switching: switching through the simultaneous application of high voltage and high current for a short period of time. Isolation diffusion: a deep semiconductor region (typically 10-1 5 microme- ters) produced by a diffusion process that provides electrical isolation between the components within an integrated circuit. Level shifting: the delivery of control signals to devices connected directly to the high-voltage power bus in power electronics systems. Resurf: a process and device struc-

1000

100

a $ 10 .- .w z 2

V l

.w

3

0.1

High-voltage dc

Power supplies Traction

-

A

I

Factory automation

I utomotive - electronics

Motor control

.amp iallasts

inications

0.01 10 100 1000 10 000

Blocking voltage rating, V

A The current and voltage ratings that must be satisfied by power devices and smart power ICs range from several hundred volts at a fraction of an ampere for display devices to about 10 000 V at 1000 A for high-voltage dc transmission. Two key applications, power supplies for computers and those for telecommunications systems, require technology operating at relatively low voltages but at high current levels. The other applications demand increasing current-handling capability with rising operating voltages.

ture for producing a reduced surface electric field in lateral high-voltage power devices. SLIC: the subscriber-line interface card used in telecommunication systems. Snubber: a resistance-inductance-cap- acitance circuit used in conjunction with power switches to reduce electrical stress during the switching of an inductive load. SOA (safe operating area): the voltage versus the current domain within which a power switch can operate without destructive failure. Soft switching: switching through use of resonance, rather than simultaneous high voltage and current UMOS: a process (and the resulting device structure) that forms the MOSFET channel by etching trenches with vertical walls and refilling them with polysilicon for the gate structure.

The first power MOSFETs were developed in the early 1970s. They used a V- groove technology but were not commercially viable because the high electric field at the bottom of their sharply pointed groove affected reliability and breakdown [see diagram at top middle of opposite page]. To make matters worse, instabilities were created by potassium contamination from the potassium hydroxide solution used to etch the grooves.

These problems were solved by the development of the double-diffused MOS (DMOS) process that i s used today to manufacture most power MOSFETs and insulated-gate bipolar transistors. In this technology, the MOSFET channel is formed by diffusing the p-base region and the n+ source region with the polysilicon gate as the common masking boundary [see left of diagram on next page]. This procedure allows the channel length to be controlled to within submicrometer dimen- sions by the use of thermal cycles during fabrication. And the control is maintained without using the expensive high-resolution printing tools needed for VLSI chips. This innovation has been the key that enables millions of DMOS cells to be integrated in parallel within a single chip while retaining an acceptable manufacturing yield, typically more than 70 percent.

The use of smaller design rules, however, is still important to the design of the power DMOSFET because the smaller feature size in essence shrinks the area that is occupied by the diffusion windows. The shrinkage greatly improves thc L:!: rent distribution within the drift region, allowing reduction in the specific on-resistance (resistance for t cm2 of cell area). Using more aggressive design rules with technology

36 I t E E SPECTRUM l U l Y IVJ?

Source

JFET Region

n- - DMOS structure

l-----4 40 um

VMOS structure

L UMOS structure

0.7

E 0.6

2 0.5

c 0.4 5

0.3 t: 0 0.2

yl 0.1

N

0

$

.-

.- t

n Ln

0

50-V power MOSFETs

Theoretical limit for silicon

1980 '82 '84 '86 '88 '90 '94 Year

borrowed from VLSl process tools, manufacturers have steadily reduced the specific on-resistance from about 0.7 pQ m2 in 1980 to about 0.06 pL2m2 in the 1990s [see graph above]. Power MOSFETs with on-resistances below 10 mL2 based on the DMOS technology are obtainable from many companies, including International Rectifier, Motorola, and Siemens.

A further innovation in power MOS- FET technology has occurred with the introduction of the UMOS structure. To fabricate this structure, vertically walled trenches are etched after the p-base and n- source diffusions [right in top diagram]. The trenches are then refilled with polysilicon, which is planarized to expose the contacts to the source and base regions. UMOS allows fabrication of a far smaller cell structure than in DMOS technology while simultaneously eliminating the resistance contributed by the junc-

tion field-effect transistor (JFET) region. With the UMOS cell structure, the

specific on-resistance of power MOSFETs can be reduced to less than 0.05 pR-m2. Siliconix has lately introduced a line of power MOSFETs with low on-resistance based on the UMOS technology, Re- cently, further reductions in the specific on-resistance to the range of 0.01 pQ.ml have been demonstrated by means of an extended trench design. These developments have resulted in devices that approach the theoretically predicted limits for silicon technology.

Smart power chips and modules are already available for such applications as power supplies for computers and multi- plex bus systems for automotive electronics. Their many commercial semiconductor manufacturers include Fuji Electric, Hitachi, Mitsubishi Electric, SGS-Thom- son, and Siliconix.

AThe power MOSFET structure has evolved through three generations since the 1970s. Today most commercially available devices employ the DMOS structure [left]. However, the trench-gate structure based upon VLSl memory technology [middle] is catching on, and manufacturers expect the UMOS F E 1 approach will predominate in the future.

4 Power MOSFETs' specific on-resistance has been shrunk by higher-resolution DMOS processing (it went from 5 pm to 1 pm) and by the introduction of the trench-gate UMOS technology. The resulting devices are reaching the theoretical limits of the resistance set by the basic transport properties of silicon.

The rest of the applications fall on a trajectory of simultaneously increasing voltage and current that severely tests smart power technology. At the lowest power level lie display drives, which require integration of many high-voltage drive transistors operating at 50-500 V with currents less than 100 mA. This function can be implemented using CMOS technology integrated with multiple DMOS lateral self-isolated FETS formed in a high-resistivity substrate. Because these chips can be produced at low cost, they represent one of the earli- est examples of smart power technology.

At higher power levels lie circuits used in the subscriber-line-interface card (SLIC) and in cross-point switches found in telecommunication networks, and in monolithic lamp ballasts. These applications require operation at up to 500 V with a current-handling capability of about 1 A.

HALICA - PO\VEK ICs IN THE SADDLE 37

The most promising approach here is di- electric isolation technology. The thick- tub, V-groove process with polysilicon substrate deposition has recently been supplemented with wafer-bonding technology using an oxide layer between the wafers to obtain isolation.

This technology has also been used to devise fractional-horse-power motor control chips for a large variety of applications.The best power switch technology in this case has been found to be lateral MOSFETs and insulated-gate bipolar transistors formed using the Resurf princi- ple to obtain breakdown voltages of up to 1200 V. Using the Resurf structure to reduce the surface electric field, several companies have been developing power ICs, including Hitachi, Philips, Mit- subishi Electric, and SGS-Thomson.

When the current levels of power electronics devices exceed 1 A with operating

voltages in excess of 500 v, integrating the power switches monolithically with the rest of the smart power circuitry is no longer cost-effective. Still, in some applications, such as numerical controls used in factory automation (robotics) systems, employing discrete power switches like complementary insulated-gate bipolar transistors with a separate control chip becomes necessary.

Similarly, medium-power (10-100 kW) motor control applications, such as those to be implemented in electric vehicles, employ discrete power devices in a totem pole configuration. Here the gate drive signal is supplied by smart power chips that can perform level-shifting of the control signals to the devices connected to the high-voltage power bus-unless galvanic isolation is necessary. In this application, the insulated-gate bipolar transistor has been identified as the best of the

candidates because of its high input impedance and its superior power-handling capability.

Enter insulated-gate transistors ince its first commercial introduction in 1983, the insulated-gate S bipolar transistor has successfully

displaced the power bipolar transistor in most applications. This success has encouraged its production by numerous companies, including Fuji Electric, Harris Semiconductor, Hitachi, Mitsubishi Electric, Motorola, and Toshiba. The transistor's power-handling capability has grown rapidly from the 5-kW range of the initial discrete devices to over 200 kW for recent power modules.

At the extremely high power levels lie applications in traction (electric street- cars and locomotives) and power distribution (high-voltage dc transmission).

38

Bath n- and p-type MCTs are attrac-

Smart power technology has as yet to make inroads into this area. The power switch currently in use for these applications is a bipolar device-the gate turn- off thyristor, which requires a very large control current (usually one-third of the load current), making it impossible to integrate with the control circuits. Much research is being performed to develop an MOS-gated thyristor structure that would be a suitable substitute for the gate turn-off thyristor. The small gate drive currents needed to control devices of this kind would then allow their operation using smart power chips.

The integration of an MOS-gate region in a vertical thyristor structure was experimentally demonstrated by the author in 1979 for the first time. This technique for controlling the turn-on is now incorporated in all MOS-gated thyristors. Subsequent research has focused on

tive for applications in which conduction loss is the dominant kind of loss. But the p-type version-the first to be commercialized-has the lower safe

-

, . .--

MOS-gate structures that would enable controlled turn-off of the thyristor.

The first of these structures was the MOS-controlled thyristor [left in top diagram on p. 401. Fabricating this device requires a triple-diffused junction process that is harder to control during manufacturing than the DMOS process used for manufacturing power MOSFETs and insulated-gate bipolar transistors.

Furthermore, although the ability to turn off the thyristor current has been demonstrated, the device lacks the current-saturation capability inherent in the transistor-based structures. This capability, which is also referred to as the forward-biased safe operating area, is essential i n power electronics circuits to control the rate of change of the current and voltage during the turn-on and turn- off switching transients. These draw- backs have impeded the acceptance of

operating area by 30 percent, inherent in its p-type bipolar nature. In most hard-switched circuits, this characteris- t ic would require some form of a snub-

12

8

4 -

0 20 40 60 80 100

Turn-off current, A

MOS-controlled thyristor - First-generation p-type - Second-generation p-type - Predicted n-type

- lnsulatedgate bipolar transistor

the MOS-controlled thyristor tor commercial applications.

An MOS-gated thyristor structure that addresses these issues is the emitter- switched thyristor [middle of diagram on p. 401. In this structure, the thyristor current is constrained to flow through a lateral MOSFET integrated into the thyristor structure Not only does this design enable control over the thyristor current flow, but it in addition allows saturation of the anode current by reducing the gate bias voltage.

In demonstrations these devices have displayed an excellent forward-biased safe operating area. Their on-state voltage drop, however, IS 0.5 V higher than that for the MOS-controlled thyristor, but this value is still much lower than for the insulated-gate bipolar transistors, whose on-state voltage drop is typically 4 V, es- pecially when the devices must be de-

-_. + - - .-

ber. In contrast, the resonant capacitors ' in soft-switched circuits protect the p- type MCT from undue stress resulting from the smaller safe operating area, and, by nearly eliminating switching losses, make MCTs even more attractive.

When the first generation of p-type Mus was introduced, it was in full knowledge that a much faster version was possible. The pioneer's turn-off time of about 1 ps was much longer than in p-type diodes of the same voltage level. So, along with a new cell design that keeps cell current more uniform, a process change was made to shorten turn-off time. In fact, the second-generation p-type MCT has a lower forward drop, yet is three times faster at all turn-off current levels than the first-generation device. Because of this enhancement, one MCT user would be able to run a resonant circuit not at 100 kHz but at 300 kHz-close to the switching speed of fast and ultrafast insulated-gate bipolar transistors.

In sum, when turn-on losses (extremely low in MCTs) are included. the total measured switching losses of a prototype second-generation p-type MCT are lower than those of most ultrafast n-type insulated-gate bipolar transistors. And the switching losses of an n-type MCT, which should suit hard- switched circuits operating at frequencies approaching 200 kHz, are expected to be even lower. [See curves, left.]

The author (SM] is director of Harris Power Research and Development, Latham, N.Y., 1 an entity that is part of Harris Corp.'s ! Semiconductor Sector. 1

p-MOSFET .... .

J ... . I

p-base

I_ _ _ -. . - _ I

Anode

MOS-controlled thyristor

Floating Dual-channel n + emitter n-MOSFET Gate .....~......

Cathq

, ' Ttsource ~; 1 ; I '

n-base

Emitter-switched thyristor

n-MOSFET p MOSFET on-gate off-gate

i Dual-gate base-resistance-

controlled thyristor ~~

A For power electronics applications above 2000 V, MOS-gated thyristors are being investigated as possible replacements for the gate turn-off thyristor. The MOS-controlled thyristor [left] is the most advanced structure on the market. However, the emitter- switched thyristor [center] and the dual- gate base-resistancecontrolled structures [right] have two advantages: they can provide current saturation to high voltages and they can be manufactured using the DMOS technology presently employed for commercial production of power MOSFETs and insulated-gate bipolar transistors.

Anode

P+

Cathode

Silicon p-i-n structure

Schottky contact

Anode

I I I

Silicon merged Silicon carbide Schottky barrier diode structure p-i-n Schottky structure

A The performance of power rectifiers could seriously limit the increase in operating frequency of power electronics systems. Although the industry now relies on p-i-n rectifiers [left], the merged p-i-n Schottky (MPS) structure [center] could provide an eightfold reduction in power losses in the future. Over the longer term, the silicon carbide Schottky barrier diode (SBD) will provide another order of magnitude reduction in power losses. The thickness of the n-drift layer required for obtaining a reverse blocking voltage of 1000 V is different for each device-~nly 10 pm for the silicon carbide Schottky barrier diode [right], much smaller than for the silicon devices. Silicon carbide's higher breakdown field strength reduces the series resistance of the drift region, enabling forward current conduction with a low on-state voltage drop.

signed to block voltages above 2000 V [upper graph, p. 451.

An advantage offered by the emitter- switched thyristor is the fact that the lateral MOSFET serves as an emitter-ballast- ing resistance for the thyristor. The effect is to promote the uniform distribution of current within multicellular high-current chips and to allow their paralleling in power circuits to increase power-handling capability. Another benefit is that this thyristor can be fabricated with the same DMOS technology now used for manufacturing power MOSFETs and insulated- gate bipolar transistors.

The most recent innovation among

MOS-gated thyristor devices is the dual- gate base-resistance-controlled thyristor. This device contains a thyristor structure that is integrated with an n-channel and p-channel MOSFET [above at top right]. These MOSFETs are designed with separate gate electrodes, as implied by the name, that allow the device to operate in a thyristor-mode when a positive bias is applied to the n-channel MOSFET and none is applied to the p-channel MOS- FET Under these conditions, the device is able to cany current through the thyristor with a low on-state voltage drop just like an MOS-controlled thyristor.

When a negative bias is applied to the

p-channel MOSFET, however, it shunts the hole current entering the p-base region into the diverter. This event reduces the forward bias across the n+ emittedp- base junction, which in turn prevents any regenerative thyristor action. The device then behaves like an insulated-gate bipolar transistor, with high-voltage current saturation capability.

Although the forward-biased safe operating area of this last type of thyristor is not as wide as it is for the insulated-gate bipolar transistor, it is sufficient to perform controlled turn-on and turn-off during the switching transients. Thus the dual-gate thyristor improves the tradeoff

40 IEEE IPE<.TRIIM J U L Y 1 9 X

between the on-state voltage drop and the forward-biased safe operating area. An attractive feature of this structure is that it can be manufactured with the same process used to make power MOSFETs and insulated-gate bipolar transistors.

Better yield sought ut before MOS-gated thyristors can be developed with ratings B suitable for the high-power uses

( 1 MW and up), their yields must be enhanced. The manufacturers of MOS devices have observed a drop in yield caused by gate-source short-circuits when device area is scaled up to cope with the increased current and voltage At present, the largest chips that can be fabricated with acceptable yields are about 15 mm on the side.

These devices are already composed of millions of small cells that work in parallel to provide a current-handling capability of up to 100 A. A tenfold increase in area and cell count would be needed for the high-power application. Chances are the increase will not be achieved by incre- mental refinements of process technology aimed at lowering defect density during production. Fortunately, an alternative has recently been demonstrated-wafer repair using fusible links. With this approach, it is possible to obtain essentially 100 percent yield in such large-area MOS-gated devices along with over 95 percent usable silicon area. This success represents a paradigm shift that is essential to make smart power technology applicable to the high-power area.

Besides an understanding of the large power range of smart power technology, an appreciation is required of the wide range of frequencies that are encountered in the applications. The power rating drops as the operating frequency increases [graph at bottom right]. Thus, the extremely high power levels of 100 h4W encountered in power distribution systems are controlled at the lowest frequencies, while the power levels become limited by technological con- straints at the higher operating frequencies

Reducing acoustic noise Some recent technological approaches

in the motor control area are of interest. With a growing awareness of electronic and acoustic noise in the consumer environment, manufacturers are beginning to raise the operating frequency of devices from a few kilohertz to above 10 kHz; already medium-power industrial insulated-gate bipolar transistor inverters are operating at 15 kHz.

The primary reasons to operate at higher than acoustic frequencies are to lessen acoustic noise from the motor windings and to boost overall system effi-

10 000

100

10

1

MOS-controllc thyristor

I Device operating point

0 1 .o 2.0 3.0 4.0 On-state voltage drop

A The on-state characteristics of MOS-gated thyristors under development outshine those for the insulated-gate bipolar transistor and MOSFET already on the market. Based upon the package’s ability to remove the power dissipated within the devices, the operating point of the on-state characteristic is determined by that of the intersection of the curve of current density versus on- state voltage with that of a constant power loss. Using this criterion, the on-state current density for the insulated-gate bipolar transistor is half that for the emitter-switched thyristor and a third that for the other thyristors.

Motor drivenraction I Uninterruptible power supply

Transportation

/”” Induction heating

Fluorescent lamp Microwave oven A

10’ 1 o3 105 1 o7 109

Device operating frequency, Hz

A Because of component limitations, the higher a system’s operating frequency, the lower its operating power level. Power electronics systems for microwave ovens, for example, operate at about 1 GHz and have a rating of under 200 VA.

4 5

300

200 3 B vi - L ar 2 n

100

0

MPS = merged p-i-nhchottky

on carbide devices

I I I 1 I I

0 5 10 15 20 25 30 Switching frequency, kHz

ciency by minimizing total power losses in the devices and passive components.

It so happens that switching losses dominate in silicon power devices (insulated-gate bipolar transistors and p-i-n rectifiers) used for pulse-width modulation motor control at frequencies above a few kilohertz [graph at left]. One way of reducing these losses involves the drift region of these devices-specifically, reducing the lifetime of the minority carriers within this region, which supports high voltages. This technique is not entirely successful, because it also produces an increase in the on-state voltage drop that offsets any reduction in the switching power losses. So the most promising approach in the short term is to replace the insulated-gate bipolar transistor with

4 To grate less on the consumer's ear, motor control systems are being designed with higher operating frequencies. But power losses go up, occuring as they do at every turn-on and turn-off transient, now more numerous. Advances in power rectifier and power switches could reduce these losses; calcula- tions indicate that least loss may be achieved with silicon carbide devices presently under development.

Toward multi-kiloampere and -kilovolt devices HARSHAD MEHTA

of increasing the cur- voltage ratings of

include a novel termi- and an MO5 turn-off

thyristor with a forward blocking junction that extends over the entire device, rather than only part of it. The new tech-

uble-sided cooling in a package.

hes are being explored in three developmental devices expected t o reach the market in two years. When fully commercial, they will benefit power engineers who design utility transmission and distribution systems- such as flexible ac transmission systems

inactive silicon layer. Since the silicon

layers have equal diameter and thickness, the structure does not bow despite differential thermal expansion with molybdenum. The flatness improves yields and thermal performance.

When combined with plastic packaging, the new termination tec is expected to make it possible duce robust, highly reliable m

0-

thyristors rated for up t o 6000 V and 6500 A. Computer simulations and pre- liminary laboratory tests indicate that these new devices will be more reliable than conventional ones, and will have surge current ratings about 20 percent higher than devices with (unbonded) structure. Th weigh 80 percent less and to manufacture than the 1 tors made with heavier sten packaging. Their use improve the cost-effectiveness of transmission-level Facts systems and distribution-level Custom Power applications.

individual thyristors (islands) that provide a distributed turn-off path for the current. For a high-current GTO, good electrical contact with each island is

essential. This is hard to achieve with conventional tungsten packaging, because the different thermal expansion coefficients of tungsten and silicon cause the device to bow. This bowing tendency is eliminated in the aforementioned sandwich-like structure, allowing good electrical contact over the entire silicon surface without sacrificing the robustness or thermal advantages of an alloyed structure.

GTOs rated as high as 9kV will soon reach the market with diameters that measure 53 mm. 77 mm, and 100 mm and with symmetrical (forward and reverse blocking) configurations. Devices capable of turning off currents as large as 4000 A are at an advanced stage of development as well.

Also being investigated is a new way to create a symmetrical GTO. The silicon

switching loss in

handling capability. In the works, too, are MO5 turn-off

thyristors (MTOs), which will help designers improve the performance of bipolar

46 IEEE SPECTRUM JULU 1995

an ktOS-gated thyristor and the p-i-n rec- iifier with the merged p-i-n/Schottky (MPS) rectifier.

In the MPS rectifier, a Schottky contact i$ integrated in parallel with the p-n junction. Demonstrations in two-dimen- sional numerical simulations, corroborat- ed by experimental data, have shown that this approach is more nearly ideal: it reduces the density of injected minority carriers in the drift region by a factor of 8, while having little impact upon the on-state voltage drop. The reduced injected-carrier density translates into a fourfold reduction in the peak reverse recovery current and a halving of turn- off time. These performance enhance- ments have a marked effect on the power losses in motor control circuits. Also, the reduced reverse recovery current cuts the turn-on stress in the power switches used within the system, improving reliability into the bargain.

In systems operating at relatively low voltages, such as the output stage of switch-mode power supplies (typically I .5-5 V), silicon Schottky rectifiers are preferred over the p-i-n rectifiers for their superior on-state voltage drop and switch-

ing pertormance The current tlow in Schottky rectifiers occurs with little injection of minority carriers, which results in a small reverse recovery current flow

Extending the silicon Schottky rectiti- er technology to high voltages has not been possible The problem is a large increase in the on state voltage drop Theoretical analysis performed I5 years ago by the author, though, indicated that a low on-state voltage drop could be obtained i f wide-band-gap materials were substituted for silicon This analy 51s was validated back in 1980, when 200-V Schottky rectifiers made of galli- um arsenide were demonstrated at Cen- era1 Electric Today CaAs Schottky rectifiers with voltage ratings of up to 500 V are commercially available from Mo- torola and Sanken Electric

The silicon carbide promise s for the future, a much superior device has been projected i f sil- A icon is replaced with silicon car-

bide, which has 20 times former‘s breakdown electric field strength. Based upon these theoretical considerations, 1000-V Schottky rectifiers have been recently

demonwated that have an on-state voltage drop smaller than that tor a silicon p I n rectifier

Moreover these devices exhibit excellent switching characteristic5 and stable capability tor blocking at high tempera- tures Their characteristics are ideal tor power electronic5 circuits, but their availability commerciallv will have to wait a while The cost of silicon carbide waters must first fall to about US $100 per water and wafer size grow to about 100 mm in diameter from the 25 m m typical today, to make their production compatible with silicon processing equipment

Indeed the most impressive improvements in pertormance-that is the lowest losses over a broad range of switching frequencies-are those likely to be achieved by switches and rectifiers based on silicon carbide One result ot the analysis performed over 15 years ago on the basis of fundamental principles was a figure of merit for materials most suitable for power devices From this figure of merit can be projected a 200-told improvement in performance for silicon carbide power devices over their commerciallv available silicon counterparts That projection prompted

MO9 devices. While such devices as insulatedgate bipailar transistors and MO$controlled thyristors combine the power-han- dlin of bipolar technology A h the control ease of MO$ technology, they are e p w i a l devices that rely on Witage control (using MO$ gates) for both turn- off and turn-on. The biding voltage of the e p m i a l layers limits the ratings of these devices to about 1600 V while many emwging industrial appli- catiQns require bipolar MO$ power switches with voltage ratings up to 5 kV and high switching frequencies up to 5 kHz.

The MTO is likelv to

that could raise the voltage rating of bipolar MOS power devices from below 3000 V to about 9000 V.

Furthermore, computer simulations and laboratory tests have confirmed that a single thyristor of this type may be capable of turning off currents as high as 3000 A and may well operate a t switching frequencies up to 5 kHz. With such performance features, this high-voltage bipolar MO5 technology should gain wide- spread acceptance.

All the aforementioned technologies greatly enhance silicon device performance for high- power industrial, military, and utility applications, while con- tinuously reducing the costs of the components.

satisfy these requirements. For turn- off, it employs an MO5 gate, but it turns on like a simple thyristor, with a voltiage signal a t its ga te (on t h e device’s p region) or by some other conventional method such as light firing.

Because the MTO is a bulk silicon- based device (instead of being epitaxi-

ages as high as 8-9 kV, typical of thyristors. In addition, because the thyristor’s forward blocking junction extends across the entire device, it is possible to terminate and passivate its edges by standard high-voltage techniques, such as bevel and contour, that mechanically shape the wafer‘s edge to delimit elec-

The author (SM) is president and chief exec- utive officer of Silicon Power Corp., Mal- vern, Pa., which he formed after acquir- ing General Electric Co.’s Static Power Components Operation. For 10 years pre- viously, he served as manager of power electronics technology development at the Electric Power Research Institute in

ai-based), it can achieve blocking volt- tric fields there. The result is an MTO Palo Alto, Calif.

BALICA - POWFR IC, I N THE 141)1)1 F 47

A Discrete power semiconductors are shifting from bipolar structures to MOS-gated devices. Although extremely popular from 1950 to 1980, the power bipolar transistor has been displaced in the '90s by the power MOSFETs and insulated-gate bipolar transistors. The expectation is that silicon carbide-based power switches will replace high-voltage silicon power switches in the 2lst century.

bipolar transistor

A The power-handling capability of MOS-gated silicon power switches has grown rapidly, but is limited by the relatively small chip sizes dictated by defect density. A faster increase in the ratings of insulated-gate bipolar transistors is being achieved by increasing both chip size and blocking voltage capability.

Cree Research Inc., Durham, N.C., to develop high-quality starting material that could lead to the silicon carbide device's commercial availability. With this material, 1000-V Schottky barrier rectifiers have already been demonstrated with low on- state voltage drop (about 1 V ) and excellent switching characteristics. Although this performance benchmark validates the projections from the theoretical analysis, commercialization of this technology must await reductions in defect density (to less than licm2) and in cost as well.

System considerations The power electronics community has

long recognized that progress in reducing system size and weight and in improving system reliability a nd efficiency are paced by the availability of improved power devices that can be produced at low cost. The first impact on power electronics systems by solid-state devices was made by the marketing in the 1950s of bipolar power thyristors, which ousted the vacuum tube thyratrons by providing far superior reliability and far smaller size and weight [top left]. In the 1960s, the introduction of the bipolar power transistors allowed an increase in the operating frequency of systems, which led to further gains in small size, lightweight, and greater efficiency.

In the 1970s, the announcement of commercial power MOSFETs marked a re- alignment of power semiconductors toward the VLSl technology already taking over the semiconductor field. This device had ideal characteristics for systems that operated at frequencies above 50 kHz. Its availability played a key role in the development of compact switch-mode power supplies with efficiencies greater than 90 percent, systems that rapidly replaced the linear-mode power supplies developed ear- lier using bipolar power transistors. Designers are now using the device widely in computer peripherals (disk drives, print- er head drives, and so on) and in automotive electronic systems.

Still, the power MOSFET was unable to compete with bipolar power transistors at operating voltages above 200 V The problem was a phenomenon intrinsic to all unipolar power devices: a rapid rise in its on-state power losses as the resistance of the drift region rose. The solution ar- rived in the 1980s in the guise of the insulated-gate bipolar transistor, in which the drift region's conductivity is modulated by the injection of minority carriers. That advance slashed on-state voltage drop by 90 percent when compared with the power MOSFET, yet the latter's high input impedance was retained. The substitution of insulated-gate bipolar transistors in medium-power systems engendered a more compact and efficient product with

48 IFEE SPECTRUM !ULY 1995

built-in self-protection and diagnostic capability-which are the hallmarks of smart power technology.

MOS-gated thyristors on the move Within the decade the on-going re-

search and development on MOS-gated thyristors is expected to land them in the market. Prototypes of the MOS-controlled thyristor with modest ratings (60 A and 900 V) are already commercially available [see "MOS-controlled thyristors' several virtues," p. 381. Also, several alternative device structures based upon the DMOS process have been demonstrated. Assuming real advances in how they perform and handle power, these devices could make their mark on the size, weight, and efficiency of high-power traction systems.

Even greater improvements in power electronics systems will be realized in the 2 1 st century, once silicon carbide-based power switches and rectifiers are offered for sale. Theoretical analysis indicates that these devices could outperform the silicon insulated-gate bipolar transistor and the MOS-gated thyristor, and will surely offer solid benefits for medium- and high-power systems.

The ability of MOS-based technology to control power has expanded rapidly [see graph opposite, below]. Already the cell structure performs nearly at the limit expected from silicon unipolar devices. Any increase in power ratings therefore de- pends on a lowering of the defect density- an advance that would make larger die possible (up to a full wafer of 100 mm in diameter from the present 15-mm diameter). Industry experts tracking this trend project that the ratings of power MOSFETs will double every two years.

In marked contrast, the power ratings of insulated-gate bipolar transistors have been tripling every two years. The faster rate of increase in power ratings is due to this device's ability to scale its voltage rating without much affecting the on-state voltage drop. By now, while the transistors of this type that came on the market in 1983 had a voltage rating of only 600 V, those available today have blocking voltages above 2000 V with current-handling capability in excess of t 00 A.

By comparison, the first commercially available MOS-controlled thyristors in 1991 had modest power ratings that have yet to see much growth. This situation is due to the reluctance of the applications

market to embrace a device that lacks a forward-biased safe operating area. In fact, this device has been abandoned by Siemens AC and ABB Asea Brown Boveri Ltd. after separate investigations over a five-year period.

Based upon these projections, expectations are that the size, weight, and efficiency of power electronics systems will continue to improve. The social implica- tions are important. The greater efficiency of power supplies used in computers and telecommunications will conserve fossil fuels. The implementation of power modules based on insulated-gate bipolar transistors will enable the efficient transfer of power from the limited energy available in battery technology to new electric cars that would reduce urban pollution. And the development of MOS-gated power thyristors would make electric locomotives-including those under development for magnetically levitated (maglev) trains-more efficient and reliable.

By equipping society with an efficient means of controlling power and energy, smart power technology can be expected to complement VLSl technology in making people's lives not only more comfortable and enjoyable, but also environmentally safer. +

To probe further The most recent technical developments in

power devices, power ICs, and their applications were reported at the International Symposium on Power Semiconductor De- vices and ICs (ISPSD '95). which took place May 23-25 in Yokohama, Japan. A de- tailed description of the meeting, held annually since 1989, is included in the con- ference record, No. 4301. Contact the IEEE Service Center, 445 Hoes Lane, Pis- cataway, NJ 08855-1 331; 908-981-0060; fax, 908-981-0027,

A selected group of papers on power electronics has been compiled in a book pub- lished by the IEEE Press, Power Electronics Technology and Applications 1993, edited by Pierre A. Thollot.

"Evolution of MOS- Bipolar Power Semi- conductor Technology" was discussed by the author in April 1988 in the Pro- ceedings of the I€€€, pp. 409-18. It in- cludes an assessment of trends in technology and device ratings.

"An Overview of Smart Power Technology" is provided by the author in a special issue of the /€€E Transactions on Electron De- vices, July 1991, pp. 1568-75. Examples of smart power chips and modules developed by companies from around the world are provided.

A thorough and up-to-date understanding of underlying semiconductor physics and design considerations is provided in a text- book, Power Semiconductor Devices, by the author (PWS Publishers, Boston, 1995).

New MOS-controlled thyristors are discussed in the MCT Users Guide (Harris Corp., Semiconductor Sector, Palm Bay, Fla., Publication No. DB307A. 1992).

Switching comparisons of the first and second generations of p-type and n-type MOS-controlled thyristors are included in P. Kendal et al., "Switching Comparisons of Genl and Gen2 P-MCTs and Ultrafast N- IGBTs," Proceedings of the 1993 Industry Applications Society Annual Meeting, I E E E Publications No. CH31468.

"Power semiconductor devices for variable frequency drives" were discussed by the author in the Special Issue on Power Electronics and Motion Control, Proceedings of the I€€€,

Novel power electronics devices are discussed in "Sealed in Silicon," Electric Power Research Institute Journal, December 1986, pp. 5-1 S.

Power electronics devices and systems are addressed in the article "High-Power Electronics," by Narain G. Hingorani and Karl E. Stahlkopf, Scientific American, November 1993, pp. 78-85.

The development of thyristors for power transmission was discussed in "Thyristors: fu ture workhorses in power transmission," by Gadi Kaplan, I€€€ Spectrum, December 1982, pp. 4W5.

The applications of power semiconductor devices and systems are among topics reg- ularly covered in the /€€€ Transactions on Power flectronics, the IEEE Transactions on Industrial Electronics, and the /€E€ Transactions on Industry Applications.

August 1994, pp. 11 12-22.

About the author B. Jayant Baliga [F] is the director of an inter-

national industrial consortium called the Power Semiconductor Research Center (PSRC), which he founded at North Car- olina State University in Raleigh in 1991. Among the many technical achievements at PSRC have been the invention and experimental demonstration of t h e merged p-i-n Schottky rectifier and many improved MOS-gated thyristors-emitter- switched and base-resistance-controlled thyristors (MCTs and BRTs), and dual-gate base-resistance-controlled thyristors (DG- BRTs). Other achievements include a low- cost wafer repair technology to manufacture large MOS-gated devices, the extension of the Resurf effect to dielectri- cally isolated power ICs for the first time, and the first experimental demonstration of high-voltage silicon carbide Schottky barrier rectifiers with low on-state switching losses.

Before joining the university as a full profes- sor in 1988, 8aliga was manager of power device and IC programs at the General Electric Corporate Research and Devel- opment Center, Schenectady, N.Y., where he invented the now widely used insulated-gate bipolar transistor. During his 20 years of work on this technology, he has written six books, authored more than 375 technical papers, and been granted 75 patents and several awards. He was elect- -ed to the National Academy of Engi- neering in 1993.

BALICA - POWER ICs IN THE S A D D L E 49

power ic in the saddle

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