02/06/2015 8.1 Introduction to the BJT Chapter 8 Bipolar - - PDF document

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02/06/2015 8.1 Introduction to the BJT Chapter 8 Bipolar - - PDF document

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02/06/2015 8.1 Introduction to the BJT Chapter 8 Bipolar Junction Transistors NPN BJT: B E C N + P N Since 1970, the high density and low-power advantage of Emitter Base Collector the MOS technology steadily eroded the BJTs

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SLIDE 1

02/06/2015 1

Slide 8-135

Chapter 8 Bipolar Junction Transistors

  • Since 1970, the high density and low-power advantage of

the MOS technology steadily eroded the BJT’s early dominance.

  • BJTs are still preferred in some high-frequency and analog

applications because of their high speed and high power output. Question: What is the meaning of “bipolar” ?

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-136

8.1 Introduction to the BJT

IC is an exponential function of forward VBE and independent

  • f reverse VCB.

Modern Semiconductor Devices for Integrated Circuits (C. Hu)

NPN BJT:

N+ P N E C B

VBE VCB Emitter Base Collector

Slide 8-137

Common-Emitter Configuration Question: Why is IB often preferred as a parameter over VBE?

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-138 Modern Semiconductor Devices for Integrated Circuits (C. Hu)

8.2 Collector Current

B : base recombination lifetime DB : base minority carrier (electron) diffusion constant Boundary conditions :

N+ P N

emitter base collector

x W

depletion layers

B

Slide 8-139

It can be shown GB (s·cm4) is the base Gummel number

8.2 Collector Current

ni

2

NB

  • ------e

qVBE kT 

1 –  n  n  0 

  • 1

1

x/ x/WB

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-140

8.3 Base Current

Some holes are injected from the P-type base into the N+ emitter. The holes are provided by the base current, IB .

p

E' nB

'

WE WB (b)

emitter base collector contact IE

IC

electron flow

– +

hole flow IB

(a)

contact

Modern Semiconductor Devices for Integrated Circuits (C. Hu)

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Slide 8-141

Is a large IB desirable? Why?

8.3 Base Current

emitter base collector

contact IE

IC

electron flow

– +

hole flow IB

(a)

contact

Modern Semiconductor Devices for Integrated Circuits (C. Hu)

For a uniform emitter,

Slide 8-142

8.4 Current Gain

How can F be maximized? Common-emitter current gain, F : Common-base current gain: It can be shown that

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-143

EXAMPLE: Current Gain

A BJT has IC = 1 mA and IB = 10 A. What are IE, F and F? Solution: We can confirm and

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-144

A high-performance BJT typically has a layer of As-doped N+ poly-silicon film in the emitter. F is larger due to the large WE , mostly made of the N+ poly-

  • silicon. (A deep diffused emitter junction tends to cause emitter-

collector shorts.)

N-collector P-base SiO2 emitter N+-poly-Si

8.4.3 Poly-Silicon Emitter

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-145

8.5 Base-Width Modulation by Collector Voltage

Output resistance : Large VA (large ro ) is desirable for a large voltage gain

IB3 IC VCE

V

A

VA : Early Voltage

IB2 IB1

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-146

How can we reduce the base-width modulation effect?

8.5 Base-Width Modulation by Collector Voltage

N+ P N

emitter

base

collector

V

CE WB3 WB2 WB1

x n'

}VCE1< VCE2<VCE3

VBE Modern Semiconductor Devices for Integrated Circuits (C. Hu)

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SLIDE 3

02/06/2015 3

Slide 8-147

The base-width modulation effect is reduced if we (A) Increase the base width, (B) Increase the base doping concentration, NB , or (C) Decrease the collector doping concentration, NC . Which of the above is the most acceptable action?

8.5 Base-Width Modulation by Collector Voltage

N+ P N

emitter

base

collector

V

CE W

B3

W

B2

W

B1

x n'

}

V

CE 1< V CE 2<V CE 3

VBE Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-148

8.6 Ebers-Moll Model

The Ebers-Moll model describes both the active and the saturation regions of BJT operation.

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-149

IC is driven by two two forces, VBE and VBC . When only VBE is present : Now reverse the roles of emitter and collector. When only VBC is present : R : reverse current gain F : forward current gain

8.6 Ebers-Moll Model

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-150

In general, both VBE and VBC are present : In saturation, the BC junction becomes forward-biased, too. VBC causes a lot of holes to be injected into the collector. This uses up much

  • f IB. As a result, IC drops.

VCE (V)

8.6 Ebers-Moll Model

Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Ebers-Moll NPN BJT Model

Reciprocity theorem transport saturation current

BE junction BC junction Mode Reverse Reverse Cutoff (OFF) Forward Reverse Forward active (FA) Forward Forward Saturation (SAT) Reverse Forward Reverse active (RA)

BJT Modes of Operation

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02/06/2015 4

Reduced models of the operation modes

(a) Cutoff (b) Forward active (c) Saturation (d) Reverse active

Slide 8-154

8.7 Transit Time and Charge Storage

When the BE junction is forward-biased, excess holes are stored in the emitter, the base, and even in the depletion layers. QF is all the stored excess hole charge F determines the high-frequency limit of BJT operation. F is difficult to be predicted accurately but can be measured.

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-155

8.7.1 Base Charge Storage and Base Transit Time Let’s analyze the excess hole charge and transit time in the base only.

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-156

What is FB if WB = 70 nm and DB = 10 cm2/s? Answer: 2.5 ps is a very short time. Since light speed is 3108 m/s, light travels only 1.5 mm in 5 ps. EXAMPLE: Base Transit Time

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-157

The base transit time can be reduced by building into the base a drift field that aids the flow of electrons. Two methods:

  • Fixed EgB , NB decreases from emitter end to collector end.
  • Fixed NB , EgB decreases from emitter end to collector end.

dx dE q

c

1  E

8.7.2 Drift Transistor–Built-in Base Field

Modern Semiconductor Devices for Integrated Circuits (C. Hu)

  • E

B C Ec Ev Ef

  • E

B C Ec Ev Ef

Slide 8-158

8.12 Chapter Summary

  • The base-emitter junction is usually forward-biased while

the base-collector is reverse-biased. VBE determines the collector current, IC .

  • GB is the base Gummel number, which represents all the

subtleties of BJT design that affect IC.

Modern Semiconductor Devices for Integrated Circuits (C. Hu)

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02/06/2015 5

Slide 8-159

8.12 Chapter Summary

  • The base (input) current, IB , is related to IC by the

common-emitter current gain, F . This can be related to the common-base current gain, F .

  • The Gummel plot shows that F falls off in the high IC

region due to high-level injection in the base. It also falls

  • ff in the low IC region due to excess base current.
  • Base-width modulation by VCB results in a significant slope
  • f the IC vs. VCE curve in the active region (known as the

Early effect).

Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-160

8.12 Chapter Summary

  • Due to the forward bias VBE , a BJT stores a certain amount
  • f excess carrier charge QF which is proportional to IC.

F is the forward transit time. If no excess carriers are stored outside the base, then , the base transit time.

Modern Semiconductor Devices for Integrated Circuits (C. Hu)