© m.n.a. halif & s.n. sabki revision chapter 5. © m.n.a. halif & s.n. sabki the bipolar...

© M.N.A. Halif & S.N. Sabki

Revision Revision Chapter 5Chapter 5


The Bipolar Junction TransistorThe Bipolar Junction Transistor

The term Bipolar is because two type of charges (electrons and holes) are involved in the flow of electricity

The term Junction is because there are two p-n junctions

There are two configurations for this device


Differences between NPN & PNP Differences between NPN & PNP

Type of BJT

PNP-Type NPN-Type

1 PNP. If the base is at a lower voltage than the emitter, current flows from emitter to collector

NPN. If the base is at a higher voltage than the emitter, current flows from

collector to emitter.

2 PNP. Small amount of current also flows from emitter to base.

NPN. Small amount of current also flows from base to emitter.

3 Emitter is heavily p-doped compared to collector. So, emitter and collector are not interchangeable.

Emitter is heavily N-doped compared to collector. So, emitter

and collector are not interchangeable.

4 The base width is small compared to the minority carrier diffusion length. If the base is much larger, then this will behave like back-to-back diodes.

The base width is small compared to the minority carrier diffusion length. If the base is much larger, then this will behave like back-to-back diodes.

5 PNP. Voltage at base controls amount of current flow through transistor (emitter to collector).

NPN. Voltage at base controls amount of current flow through transistor

(collector to emitter).

6

7 Follow the arrow to see the direction of current flow Follow the arrow to see the direction of current flow


Figure 5-2. (a) Idealized one-dimensional schematic of a p-n-p bipolar transistor and (b) its circuit symbol. (c) Idealized one-dimensional schematic of an n-p-n bipolar transistor and (d) its circuit symbol.

Operation of NPN Transistor


Figure 5-3. (a) A p-n-p transistor with all leads grounded (at thermal equilibrium). (b) Doping profile of a transistor with abrupt impurity distributions. (c) Electric-field profile. (d) Energy band diagram at thermal equilibrium.

• The E is more heavily doped than the C

• B doping is less than the E doping but greater than the C

• At thermal equilibrium – no net I flow the Fermi level is a constant


BJT CONFIGRATIONS BJT CONFIGRATIONS


Figure 5.4. (a) The transistor shown in Fig. 3 under the active mode of operation.3 (b) Doping profiles and the depletion regions under biasing conditions. (c) Electric-field profile. (d) Energy band diagram.

Common-base configuration:

• Note: depletion layer width of the E-B junction is narrower & C-B junction is wider compared with equilibrium case

• E-B junction (forward biased) – holes injected from the p+ E into B, electron injected from the n B into E

• C-B junction (reverse biased) – if B width is narrow, holes injected from the E can diffuse thru B to reach the B-C depletion edge and the “float up” into the C

• E (emits/injects carriers) C (collects carriers from nearby junction) : C hole I E hole I

• The transistor action: carriers injected from E junction large I flow in C junction


Common-base current gain

E

Cp

I

I0

Emitter efficiency

EnEp

Ep

E

Ep

II

I

I

I

Base transport factor Ep

CpT I

I T 0

Collector current CBOEC III 0 ICBO : the leakage current between the C and B with the E-B junction open


now look at what happens to the electrons injected into the base. Because the base is made of p-type silicon, the electrons are minority carriers. The base is very thin so the electron concentration, np, will have a linear characteristic. The electron concentration wil be highest at the emitter side of the base, and will be zero at the collector side. It is zero here because the CBJ is in reverse bias, causing all minority carriers to be attached to and swept across to the collector

Carrier Carrier DistributionDistribution


Figure 5-7. Junction polarities and minority carrier distributions of a p-n-p transistor under four modes of operation.

Modes of operation of p-n-p transistor

• Active mode:

• E-B junction is forward biased, B-C junction is reverse-biased

• Saturation mode:

• both junctions are forward biased

• corresponds to small biasing V & large output I – transistor is in a conducting state & acts as a closed (or on) switch

• Cutoff mode:

• both junctions are reverse biased

• corresponds to the open (or off) switch

• Inverted mode:

• inverted active mode

• E-B junction is reverse biased, C-B junction is forward biased


I-VI-V of Common-Base of Common-Base

Figure 5-8. (a) Common-base configuration of a p-n-p transistor. (b) Its output current-voltage characteristics.

• Ic is equal to IE (i.e 01) & independent of of VBC

• Ic remains constant even down to 0V for VBC (holes are still extracted by C)

• Hole distributions (See fig. 5-9)


Figure 5.9. Minority carrier distributions in the base region of a p-n-p transistor. (a) Active mode for VBC = 0 and VBC > 0. (b) Saturation mode with both junctions forward biased.

• Hole at x=W changes only slightly from VBC>0 to VBC=0 (IC remains the same) – fig. (a)

• to reduce IC to 0 – apply a small forward bias (about 1V to B-C junction) – fig. (b)

• The forward bias will increase the hole density at x=W to make it equal to that of the emitter at x=0 (horizontal line)

• The hole gradient at x=W & IC will reduce to 0

I-VI-V of Common-Base of Common-Base


Figure 5.10. (a) Common-emitter config. of a p-n-p transistor. (b) Its output I-V characteristics.

I-V of Common-emitter

00

0

1

I

III CBO

BC

0

00 1

B

C

I

I

01 CBO

CEO

II

CEOBC III 0

Common-emitter current gain:

C-E leakage current:

0 Common-base current gain


ExampleExample

A bipolar transistor with an emitter current of 1 mA has an emitter efficiency of 0.99, a base transport factor of 0.995 and a depletion layer recombination factor of 0.998 Calculate the transport factor.

The transport factor and current gain are


The heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz. It is using mostly RF systems.Heterojunction transistors have different semiconductors for the elements of the transistor. Usually the emitter is composed of a larger bandgap material than the base. This helps reduce minority carrier injection from the base when the emitter-base junction is under forward bias and increases emitter injection efficiency. The improved injection of carriers into the base allows the base to have a higher doping level, resulting in lower resistance to access the base electrode. Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure. HBT structures are usually grown by epitaxy techniques like MOCVD and MBE.

Heterojunction bipolar Heterojunction bipolar transistortransistor


The Heterojunction Bipolar The Heterojunction Bipolar TransistorTransistor

Figure 5-17. (a) Schematic cross section of an n-p-n heterojunction bipolar transistor (HBT) structure. (b) Energy band diagram of a HBT operated under active mode.

• Emitter – wide bandgap (AlGaAs)

• Base – lower bandgap (GaAs)

• Large bandgap difference (between E-B) common-emitter current gain can be extremely large

• Homojunction: no bandgap difference – doping concentration in the E & B must be very high

EV increases the valence-band barrier height reduce injection of holes from B to E

• can use heavily doped base, maintain a high E efficiency & current gain


Figure 5-18. Current gain as a function of operating frequency for an InP-based HBT.

Advanced HBTs

• InP-based material systems

• Advantages:

• very low surface recombination

• Higher electron mobility in InGaAs than in GaAs – superior high-freq performance (in fig., cutoff freq: 254GHz)

• InP collector region has higher velocity at high fields than GaAs collector

• InP collector breakdown voltage is higher than GaAs

Cutoff freq., fT= 254GHz


Figure 5.19. (a) Device structure of an n-p-n Si/SiGe/Si HBT

(b) Collector and base current versus VEB for a HBT and bipolar junction transistor (BJT).

Si/SiGe material system

• high-speed capability – because the base is heavily doped (bandgap difference)

• Small trap density at Si surface minimizes the surface recombination current – high current gain at low Ic

• Lower cutoff freq – because lower mobilities in Si compared to GaAs- & InP- based HBTs

• Problem: E efficiency & Ic suffer (caused by EV)

• To improve: graded-layer & graded-base heterojunction


THYRISTOR & RELATED DEVICESTHYRISTOR & RELATED DEVICES

Thyristor: designed for handling high V & large IUsed for switching applications that require the device to change from an off or blocking state to an on conducting stateThyristors have much wider range of I- & V- handling capabilitiesAvailable with I ratings from a few miliamperes to over 5000A, V ratings extending above 10,000V


Figure 5-22. (a) Four-layer p-n-p-n diode. (b) Typical doping profile of a thyristor. (c) Energy band diagram of a thyristor in thermal

equilibrium.

• Called p-n-p-n diode

• Add gate electrode at the inner p-layer (p2) 3-terminal device called semiconductor-controlled rectifier (SCR) or thyristor


Figure 5-23. Current-voltage characteristics of a p-n-p-n diode.

Basic I-V characteristics of p-n-p-n diode – exhibits 5 distinct regions:

• 0-1: The device is in forward-blocking or off-state and has very high impedance. Forward breakover (switching) occurs where dV/dI=0; & at point 1 defined as forward breakover voltage VBF & switching current IS

• 1-2: The device is in a negative-resistance region – the I increases as the V decreases sharply

• 2-3: The device is in forward-conducting or on-state & has low impedance. At point 2, where dV/dI=0, the holding current Ih & holding voltage Vh

• 0-4: The device is in the reverse-blocking state

• 4-5: The device is in the reverse-breakdown region

© m.n.a. halif & s.n. sabki revision chapter 5. © m.n.a. halif & s.n. sabki the bipolar...

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