IEEE ELECTRON DEVICE LETTERS, VOL. 17, NO. 9, SEPTEMBER 1996 425

The MOS Inversion Layer as a Minority Carrier Injector

Florin Udrea, Member, IEEE, Gehan A. J. Amaratunga, Member, IEEE, Jean Humphrey, Jaqui Clark, and Alan G. R. Evans, Member, IEEE

Abstract-In this paper we report the first experimental demon- stration of the concept of MOS inversion layer injection (ILI) earlier proposed by us [3]-[5]. The new physical concept is based on the use of a MOS inversion layer as a minority carrier injector as part of a dynamic junction. The carrier injection of such a junction is entirely controlled by the MOS gate. Moreover, when the gate potential is reduced under the MOS threshold voltage, the junction collapses ensuring a very efficient turn- off mechanism. Based on this concept we propose two novel lateral three-terminal structures termed inversion layer diode (ILD) and inversion layer bipolar transistor (ILBT). The concept of inversion layer injection can be applied in power devices where effective MOS gate control of the active junctions is important.

I. INTRODUCTION

HE MOS inversion layedsubstrate junction can only be T forward-biased if current flow through the substrate in parallel with the channel can be established. This, unlike in a conventional MOSFET where the substrate potential is constant, results in a distributed potential across the substrate. Under these conditions the source junction can remain inactive, while due to the local potential rise in the substrate, the inver- sion layer/substrate junction can be forward-biased. Inversion layer injection (ILI) can occur in devices with bipolar-MOS action similar to insulated gate bipolar transistors (IGBT) [ 11, [2] or MOS controllable thyristor-type devices [31. The ILI effect could also be quite significant in trench IGBT’s [4]. Several other power devices based on the ILI concept [5] are possible and will be reported in the future.

This paper presents the first experimental verification of the ILI concept and demonstrates its application in two basic three terminal structures termed the inversion layer diode (ILD) and the inversion layer bipolar transistor (ILBT).

11. THE CONCEPT OF INVERSION LAYER INJECTION

The concept of inversion layer injection is based on an inversion layer acting as a minority carrier injector due to forward-biasing of the inversion layedsubstrate junction. There are two distinct phenomena which determine the physical

Manuscript received March 5, 1996; revised May 16, 1996. F. Udrea is with the Department of Engineering, Cambridge University,

G. A. J. Amaratunga is with the Department of Electrical Engineering and

J. Humphrey, J. Clark, and A. G. R. Evans are with the Southampton

Publisher Item Identifier S 0741-3 106(96)06870-X.

Cambridge CB2 IPZ, U.K.

Electronics, Liverpool University, Liverpool L69 3BX, U.K.

University Microelectronics Centre, Southampton SO9 5NH, U.K.

behavior of a forward-biased inversion layer junction. First is the presence of a strong electric field at the oxide interface which supports the electron charge in the inversion layer and second, the carrier diffusion of electrons from the inversion layer into the substrate. The use of the inversion layer as a minority carrier injector is best exemplified in the ILD structure, shown in Fig. l(a).

This structure could be regarded as a MOS-controllable resistor. If the gate contact is shorted to the cathode contact there is no inversion layer and the device acts as a high resistance. The carrier transport is based in this case on drift of holes through the resistive p-epitaxial base (it is assumed that the parasitic cathode junction is not turned on). If a positive potential is applied to the gate, an inversion layer is formed at the surface. When the anode voltage increases, the voltage drop between the p-epi substrate and inversion layer reaches the junction barrier potential. Consequently, injection of electrons from the inversion layer into the p-epi commences, as shown by the flow-lines of the simulation in Fig l(b). The injected electrons reach the inversion layer injector from the cathode via the channel formed in the p-well. High electron injection from the inversion layer leads to conductivity modulation of the p-epi and an abrupt decrease in its resistivity. The total forward voltage drop can be expressed as the sum of the voltage drop across the channel, the voltage drop across the inversion layer/p-epi junction and the voltage drop across the highly modulated p-region (between the inversion layer junction and anode).

A transistor structure operating on the same physical prin- ciple, the ILBT, can be built by adding an n-well layer at the anode side of the ILD structure, as shown in Fig. 2. The breakdown of this device is set by the doping and the geometry of the p-epi and n-well layers. In simulations and experiments breakdown voltages of 100 V to 150 V have been achieved.

Once the p+/n well anode junction turns-on, ILBT operation commences through p-n-p transistor action. However, unlike in an LIGBT [ 11, [ 2 ] , the collector (p-drift)/base (n-well) junction becomes forward biased as the hole drift current flows through the lightly doped p-base. This is equivalent to the condition for the base push-out effect in conventional power transistors and consequently electrons spread into the p-drift (collector) region. In the ILBT structure the collectorhase junction turn- on is accompanied by turn-on of the inversion layer/p-drift junction (and thus, diffusion of electrons under the gate). Since the n-well is connected to the channel, at the upper part of the n-well junction the potential difference between the

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426 IEEE ELECTRON DEVICE LETTERS, VOL. 17, NO. 9, SEPTEMBER 1996

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Fig. 1. The inversion layer injection can be clearly seen.

(a) Cross section of the inversion layer diode (ILD) and (b) numerical simulations using MEDIC1 showing the flow-lines at the cathode side.

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Fig. 2. Cross section of the inversion layer bipolar transistor (ILBT).

p-drift and the n-well (-0.6 V) is approximatively the same as that between p-drift and the inversion layer. As the anode voltage increases the point in the inversion layer from which injection into the substrate occurs (and hence the effective base of the p-n-p transistor) moves closer to the cathode end. Thus the rapid plasma extension into the p-drift is facilitated

by the effective base expansion due to additional inversion layer electron injection. Once the plasma reaches the p-well the effective channel length is ultimately reduced to that of the p-well. The ILBT, therefore allows effective and heavy modulation of the entire p-drift region from an inversion layer injector.

UDREA et al.: MOS INVERSION LAYER AS A MINORITY CARRIER INJECTOR 427

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Fig. 3. Measured output characteristics of the inversion layer diode for different gate voltages. At low anode voltages, an initial resistive region independent on the gate voltage can be observed. Once the inversion layer junction is forward-biased the resistance decreases function on the potential applied on the gate. At high anode currents the main cathode junction is latched-up. and the control on the gate is lost.

111. EXPERIMENTAL RESULTS

A 12-masks CMOS compatible process was used for both low voltage and high voltage inversion layer injection devices. An 8 pm-thick, 20 Rcm p-epi layer was grown onto a thick, 50 Rcrn n-substrate. The p-well was 2.5 pm deep while the n-well depth was varied between 2 and 4 pm. The thickness of the gate oxide was 400 A and the measured MOS threshold voltage was approximatively 0.7 V. The channel length was also varied in different designs with an average value of 3 pm.

A typical set of measured I-V characteristics for the in- version layer diode are shown in Fig. 3. The substrate was isolated. Prior to the forward-biasing of the inversion layer junction, the on-state resistance remains unchanged with in- creasing gate voltage. When the anode voltage reaches the threshold voltage associated with the forward-biasing of the inversion layer junction (the kink in the I-V characteristics in Fig. 3) the on-state resistance decreases abruptly with increasing gate voltage. The second sharp rise in the current indicates the turn on of the parasitic cathode junction (i.e., n+ cathodelp-well) and the loss of the gate control. It is important to note, that with increasing the gate potential, the critical current at which the parasitic turn-on of the cathode junction takes place, increases. This is due to a larger fraction of the total current being diverted in the form of electrons via the inversion layer thus reducing the fraction of hole current flowing under the n+ cathode diffusion in the p-well which forward-biases the cathode junction.

The experimentally observed I-V characteristics for the inversion layer bipolar transistor are shown in Fig. 4.

The initial turn-on of the device, characterized by the turn- on of the anode junction, as in an LIGBT, is seen as the rise in the current at an anode voltage of 0.5 V. The transition from the LIGBT mode into the inversion layer injection mode

“0 1 2 3 4 5 6 Anode Voltage [VI

Fig. 4. Measured output characteristics of the inversion layer bipolar tran- sistors. At low gate voltages the ILBT remains in the IGBT mode. At higher gate voltages, once the inversion layer junction is forward-biased the device switches in the inversion layer injection mode.

is seen as a kink at 0.7 V in the output characteristics in Fig. 4 when the gate voltage is equal or greater than 4 V. The latter occurs due to the concomitant turn-on of the p-drift regionln-well and p-drift regionlinversion layer junctions. As already mentioned, this is due to the rise in the p-drift potential in respect to the initial potentials of the n-well and of the n+ channel. Once the inversion layer injection spreads toward the cathode side, the conductivity of the p-base increases, thus substantially reducing the on-state resistance. At higher anode voltapes, current saturation takes place due to the pinch-off of the effective channel situated at the surface of the p-well.

IV. CONCLUSIONS It is experimentally demonstrated that the MOS inversion

layer can be utilized as a minority carrier injector. This type of injector can be used as a dynamic emitter in various configurations such as diodes, transistors or thyristors. The existence of the emitter is fully controlled by the gate electric field as opposed to the classical concept whereby the emitter is part of a permanent p-n junction. Two novel three-terminal structures based on this concept are proposed and successfully demonstrated experimentally.

REFERENCES

B. J. Baliga, Modern Power Devices. P. A. Gough, M. R. Simpson, and V. Rumennik, “Fast switching lateral insulated gate transistor,” in IEDM Tech Dig., p. 218, 1986. F. Udrea and G. A. J. Amaratunga, “The inversion layer emitter thyristor-A novel power device concept,” in Proc. 6th IEEE Znt. Symp. Power Semiconductor Devices and IC’s (ISPSD), Switzerland, May 1994, p. 190. -, “Theoretical comparison between trench and DMOS tech- nologies for insulated gate bipolar transistors,” IEEE Trans. Electron Devices, vol. 42, p. 1356, July 1995. F. Udrea “Novel MOS-gated bipolar device concepts toward a new gen- eration of power semiconductor devices,” Ph.D. dissertation, Cambridge University, 1995.

New York: Wiley, 1987.