ECE 342 – Jose Schutt-Aine 1 ECE 342 Solid-State Devices & Circuits 6. Bipolar Transistors Jose E....

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ECE 342 – Jose Schutt-Aine 1

ECE 342Solid-State Devices & Circuits

6. Bipolar Transistors

Jose E. Schutt-AineElectrical & Computer Engineering

University of Illinoisjschutt@emlab.uiuc.edu

• Bipolar Junction Transistor (BJT)– First Introduced in 1948 (Bell labs)– Consists of 2 pn junctions– Has three terminals: emitter, base, collector

Bipolar Junction Transistor

BJT – Modes of Operation

Mode EBJ CBJ

Cutoff Reverse Reverse

Forw. Active Forward Reverse

Rev. Active Reverse Forward

Saturation Forward Forward

BJT in Forward Active Mode (NPN)

Electrons are minority carriers in the base (p-type)/(0) BE TV V

p pon n e

( ) (0)p pn E n E n

dn x nI A qD A qD

iC is independent of vCB

Collector current:

Minority electrons will diffuse in the p-type base

Longitudinal Current Flow

/BE Tv VC n Si I I e

2E n i

A qD nI

AE: cross section area of BEJW: Effective width of baseNA: doping concentration baseDn: electron diffusivityq: electron charge

Base Current

BE Tv VE p i

A qD n ei

Dp: hole diffusivity in emitterLp: hole diffusion length in emitterND: doping concentration of emitter

• Base current: Two components– Hole injection into emitter iB1

– Electron recombination in base iB2

Qn: minority carrier charge in basetb: minority carrier lifetime

(0)2 2

BE Tv VE in E p

A qWnQ A q n W e

Basepnp(0)

np(ideal)

effectivebase width

From area under triangle

• Base current has two functions

BJT Operation: Longitudinal and Base Currents

• Base current is small because

• Longitudinal current

Support reverse injection

Feed recombination that occur in the base

Has large lifetime

Base is thin

Emitter is much more heavily doped than base

Depends (exponentially) on emitter junction voltage Is independent of collector junction voltage

Field due to collector-base voltage attracts carriers but has no effect on rate of attraction

BJT Operation: Current Gain

1 2B B Bi i i • Total Base current:

Define a current gain b such that

2BE Tp v VA

B Sn D p n b

D N W Wi I e

D N L D

n D p n b

D N W W

D N L D

Using previous relation for iC

b is the common-emitter current gain

In order to achieve a high gain b we need

Dn: largeLp: largeND: largeNA: smallW: small

Typically 50 < b < 200

In special transistors, b can be as high as 1000

Current Gain Temperature Dependence

BJT Operation: Emitter Current

E C Bi i i • Emitter current:

Define a such that

/1 1BE Tv V

E C Si i I e

C Ei i

Using previous relation for iC

a is the common-base current gain

a 0.99

Structure of BJT’s

Collector surrounds emitter region electrons will be collected

Ebers-Moll Model

NPN Transistor

// 1 1BC TBE T v Vv V SC S

Ii I e e

// 1 1BC TBE T v Vv VSE S

Ii e I e

// 1 1BC TBE T v Vv VS SB

I Ii e e

Describes BJT operation in all of its possible modes

Common-Emitter Large-Signal Model

Common terminal is common to input and output

Common terminal is used as reference or ground

BJT – Common-Emitter Characteristics

BJT – Voltage-Current Characteristics

Common Emitter Configuration

E B CI I I

C EI I

1C B BI I I

Common Emitter I-V Characteristics

Early Voltage

• Early Voltage VA– Dependence of collector current on collector voltage– Increasing VCE increases the width of the depletion region

Output Resistance

/ 1BE Tv V CEC S

vi I e

CE V cons t

/' BE TV VC SI I e

ro is output resistance seen from collector terminal

Alternatively, neglecting the Early effect on the collector current, we define

The output resistance then becomes

A transistor has = 100, vBE= 0.7V with IC = 1 mA. Design a circuit such that a current of 2 mA flows through the collector and a voltage of 5V appears at the collector.

20.7 ln 0.717 at 2 mA

1BE Tv V V

Problem

CBJ reversed biased FAR

Voltage drop across RC = 15-5 =10VIC = 2mA RC = 10V/2mA = 5kW

Since vBE=0.7V at IC = 1 mA

Since base is at 0V, emitter voltage is at –0.717 volts =VE

For b = 100, a = 100/101=0.99 IE = IC/a = 2/0.99 = 2.02 mA ( 15) 0.717 15

7.07 2.02

This order of accuracy is not necessary

Forward active region can be maintained for negative vCB down to about -0.4V

Operation in the Saturation ModeIV Characteristics Minority Carrier Profile

Beyond that point, the transistor enters the saturation mode and iC decreases with decreasing vCB

// 1 1BC TBE T v Vv V SC S

Ii I e e

// 1 1BC TBE T v Vv VS SB

I Ii e e

Operation in the Saturation Mode

If vBC increases, iC will decrease, as described by

The base current iB will decrease, as described by

The current gain will decrease to a value lower than bF described as:

Cforced F

B saturation

We will also have: CEsat BE BCV V V

Operation in the Saturation Mode

Blue: Gradient that gives rise to diffusion currentGray: Minority carriers driving transistor deeper into saturation

NPN in Saturation Mode

Biasing Bipolar Transistors

BJT Bias

1. Base Current Bias

CC BEBQ

0.6CCBQ

2. Emitter Bias

BJT Bias

Provides good stability with respect to changes in b with temperature

Thevenin Equivalent

BJT Emitter Bias

th th B BE E EE R I V R I

1E B C BI I I I

( 1)th BE

B BQth E

E VI I

1 2th CC

1 21 2

R RR R R

Thevenin Equivalent

1th BE th B E BE V R I R I

• Methods– First method is to find R1 & R2 from Eth and Rth and IBQ

– Second method is to select R2 to be 10 times to 20 times RE to provide good stability & then select R1 to give proper IBQ

Bipolar Biasing Approach

Remark: To keep collector voltage at the middle of the forward active region, use:

min max 12 2

C C CC ECQ

V V V RV

Stability Considerations

Objective: Minimize effect of variations in b. Circuit must be stable with respect to changes in .b

CQ CC CQ C CC BQ CV V I R V I R

– Need to examine quiescent point in variations for interchanged BJT’s

( 1)th BE

BQth E

th BE CCQ CC

E V RV V

1th BQ BQ E th BER I I R E V

Stability Considerations

CCQ CC th BE

RV V E V

Changes in b lead to significant changes in VCQ

6060 0.983

(A) If Rth>> (b+1) RE

100100 0.99

(B) If (b+1) RE >> Rth

CCQ CC th BE

RV V E V

a varies only 1% to 2% for large b variations (B) is good choice.

CCQ CC th BE

RV V E V

The circuit shown below has RC = 8.2 kW , RE = 1 kW , R2=20 kW , VCC = 12 V, b = 100, VBE = 0.7V

- Select R1 to place VCQ at midpoint of the (forward) active region.- Find maximum symmetrical peak-to-peak output voltage that

can be obtained before saturation or cutoff occurs.

Bias Example

Bias Example - Solution

Minimum: min

12 121.3043

1 8.2 9.2E

CQ CCE C

RV V V

maxCQ CCV VMaximum:

Midpoint: min max 13.30436.65

2 2CQ CQ

V VV V

12 6.650.652 CQ

0.6520.00652 mA 6.52

100BQI A

Bias Example (con’t)

0.7 0.652 0.7 1.35 VBQ E EV R I

1.35 V0.0676 mA 67.6

20 kI A

1 2 67.6 6.82 74.1 BI I I A A A

1.35 12 1.35143.6

74.1 CCV

R kI A

max 12 6.65 5.35V V 1 143.6 R k W

max 5.35V V

BJT Transistor Polarities

ECE 342 – Jose Schutt-Aine 1 ECE 342 Solid-State Devices & Circuits 6. Bipolar Transistors Jose E....

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