ECE-305: Spring 2018

Final Exam Review

Professor Peter BermelElectrical and Computer Engineering

Purdue University, West Lafayette, IN USApbermel@purdue.edu

Pierret, Semiconductor Device Fundamentals (SDF)Chapters 10 and 11 (pp. 371-385, 389-403)

4/26/2018 Bermel ECE 305 S18 1

BJT Symbols and Conventions

Bermel ECE 305 S182

Poly emitter

Low-doped base

Collector dopingoptimization

N+

P

N

Symbols

NPN PNP

Collector

Emitter

Base

Collector

Emitter

Base

IC+IB+IE=0

VEB+VBC+VCE=0

E

B

C

4/26/2018

BJT Current Gain

Bermel ECE 305 S183

P+N

P

C

E E

B

VEB (in)

ICIBCommon Emitter current gain ..

CDC

B

I

I =

Common Base current gain ..

P+

N PE

IE IC

C

BB

VEB(in) VCB(out)

CDC

E

I

Ia =

C CDC

B E C

I I

I I I = =

- 1DC

DC

a

a=

-

4/26/2018

Current Gain

Bermel ECE 305 S184

4/26/2018

5

essence of current gain

( )2, 1BEp i E q

E

BV

E

qD ne

NI

W» -

N+

N

P

( )2, 1BEi B

B

VE

qn

B

nqDe

NI

W» -

Input Response Input

Response

VBE VBC

Bermel ECE 305 S184/26/2018

Emitter Current

6

( ) ( )2 2,

,

0

,1 1=

= = - + -- BC BBE Bi B i B qV k TqV k Tnn E n

B B B Bx

nn nqdn

J qDd

D qDe e

W W Nx N

( ),

0'

2

1

=

= - = - -BEp qVi

n D

p E p

x

D ndpJ

d W ND eq

x

VBE

VBE

, ,= +p E nE EJ J J

Bermel ECE 305 S18

collector current: forward active region

n+emitter

pbase

ncollector

n+

FB RB

IE ICIEn

IEpIE = IEn + IEp

aT IEn

IC » IEn

IB = IEp

IEn = qAEni2

NAB

æ

èçö

ø÷DnWB

eqVBE /kBT

IEp = qAEni2

NDE

æ

èçö

ø÷Dp

WE

eqVBE /kBT

IC = aT IEn

IC =aFIF0 eqVBE kBT -1( )

IC =aTg F IE

aF = aTg F

Bermel ECE 305 S184/26/20187

Ebers-Moll model

n+emitter

pbase

ncollector

n+

IE ICIEn ICn

IEp

Bermel ECE 305 S18

ICp

IC VBE ,VBC( ) = aF IF0 eqVBE kBT -1( ) - IR0 eqVBC kBT -1( )

IE VBE ,VBC( ) = IF0 eqVBE kBT -1( ) -a RIR0 eqVBC kBT -1( )

4/26/2018

IB VBE ,VBC( ) = IE VBE ,VBC( ) - IC VBE ,VBC( )

8

FB RB

Ebers-Moll model

Bermel ECE 305 S18

IC VBE ,VBC( ) = aF IF0 eqVBE kBT -1( ) - IR0 eqVBC kBT -1( )

IE VBE ,VBC( ) = IF0 eqVBE kBT -1( ) -a RIR0 eqVBC kBT -1( )

aF IF0 = a RIR0

IF0 = qAEni2

NAB

æ

èçö

ø÷DnWB

+ qAEni2

NDE

æ

èçö

ø÷Dp

WE

aF = aTg F

a R = aTg R IR0 = qAEni2

NAB

æ

èçö

ø÷DnWB

+ qAEni2

NDC

æ

èçö

ø÷Dp

WC

4/26/2018 9

Gummel Plot and Output Characteristics

10

2 2, , //( 1) ( 1)BCBEi B i B q kTq kTn n

B B B B

C VVn nqD qDe e

A W N W N

I- - + -�

2, /( 1)BEp i E V kT

E E

B qqD n

eA W N

I= -

CDC

BI

I =

DCCommon emitter Current GainVBE

Bermel ECE 305 S184/26/2018

=

maximizing gains in BJTs

Bermel ECE 305 S18

2,

2,

i Bn E Edc

B p i E B

nD W N

W D n N »

NE

NB

NC

Emitter doping: As high as possible withoutband gap narrowing

Base doping: As low as possible, withoutcurrent crowding, Early effect

Collector doping: Lower than base dopingwithout Kirk Effect

Base Width: As thin as possible withoutpunch through (~1 mm in ‘50s, 200 Å now)

4/26/2018 11

ND++

How to make better Transistor

Bermel ECE 305 S1812

2,

2,

i Bn E E

B p i E B

nD W N

W D n N »

Classical Shockley Transistor

Heterojunction Bipolar Transistor

Graded Base transport

Polysilicon Emitter

4/26/2018

poly-silicon HBT emitter

Bermel ECE 305 S18

N+

P N

SiGe intrinsic base Dielectric trench

N+P+

N

P-

N-N+

CollectorEmitterBase

N+

Poly-silicon

emitter

4/26/2018 13

Bermel ECE 305 S18

SiGe HBT applications

1) optical fiber communications-40Gb/s…….160Gb/s

2) Wideband, high-resolution DA/AD convertersand digital frequency synthesizers

-military radar and communications

3) Monolithic, millimeter-wave IC’s (MMIC’s)

-front ends for receivers and transmitters

future need for transistors with 1 THz power-gain cutoff freq.

4/26/2018 14

New equations on equation sheet

4/26/2018 Bermel ECE 305 S18 15

Bipolar transistors: (assuming NPN, short emitter, base, and collector)Ebers-Moll Equations:

Practice Question 1

4/26/2018 Bermel ECE 305 S18 16

Region of Operation:A. Forward ActiveB. Inverse Active C. PinchoffD. SaturationE. Cutoff

Practice Question 2

4/26/2018 Bermel ECE 305 S18 18

Region of Operation:A. Forward ActiveB. Inverse Active C. PinchoffD. SaturationE. Cutoff

Practice Question 3

4/26/2018 Bermel ECE 305 S18 20

Region of Operation:A. Forward Active: ___B. Inverse Active: ___C. Pinchoff : ___D. Saturation : ___E. Cutoff : ___

Practice Question 4

4/26/2018 Bermel ECE 305 S18 22

Practice Question 5

4/26/2018 Bermel ECE 305 S18 24

Which of the following would reduce “base width modulation” (i.e. the Early effect) and, therefore, increase the output resistance?a) Increasing the emitter doping.b) Increasing the collector doping.c) Increasing the base width.d) Increase the emitter thickness.e) Decrease the base doping.

Practice Question 6

4/26/2018 Bermel ECE 305 S18 26

Consider the transistor circuit below

What is the region of operation for this transistor if the base voltage VA is positive?

Practice Question 7

4/26/2018 Bermel ECE 305 S18 28

Write down the Ebers-Moll equations for the transistor circuit below:

Fall 2015: Multiple Choice

4/26/2018 Bermel ECE 305 S18 30

1 (8 points). Which of the following would increase the

collector breakdown voltage in a BJT?

a. Increasing the emitter doping

b. Increasing the base doping

c. Increasing the collector doping

d. Decreasing the collector width

e. Decreasing the collector doping

Fall 2015: Multiple Choice

4/26/2018 Bermel ECE 305 S18 32

2 (8 points). How are the PN junctions biased in the forward active

region of an NPN BJT?

a. Base-collector: forward biased Emitter-base: forward biased

b. Base-collector: forward biased Emitter-base: reverse biased

c. Base-collector: reverse biased Emitter-base: forward biased

d. Base-collector: reverse biased Emitter-base: reverse biased

e. Base-collector: forward biased Emitter-base: biased in breakdown

Fall 2015: Multiple Choice

4/26/2018 Bermel ECE 305 S18 34

3 (8 points). What would be considered good values for

��� and ���, respectively, in a BJT?

a. 0.1 and 0.11

b. 0.5 and 1

c. 0.99 and 99

d. 3 and 1.5

e. 20 and 1.052

Fall 2015: Multiple Choice

4/26/2018 Bermel ECE 305 S18 36

4 (8 points). For large gain in a bipolar transistor, the emitter

doping must be …. ?

a. Larger than the base doping only

b. Smaller than the base doping only

c. Larger than the collector doping only

d. Larger than both base and collector doping

e. It does not matter

Fall 2015: Multiple Choice

4/26/2018 Bermel ECE 305 S18 38

5 (8 points). How can a heterojunction bipolar transistor be

designed to improve ���?

a. Low bandgap material in the base region

b. Low bandgap material in the emitter region

c. High electron affinity material in the base region

d. Low breakdown voltage material in the collector region

e. High bandgap material in the base region

Fall 2015: Part II

4/26/2018 Bermel ECE 305 S18 40

Consider the NPN BJT made of silicon, depicted below. Assume that it is in the forward active region, with IC=10 mA, doping concentrations ND,E=1018/cm3, NA,B=1017/cm3, and ND,C= 1016 /cm3; thicknesses WE=0.5 mm,WB=0.25 mm, and WC=2 mm; and diffusion constants Dp,E=2 cm2/s, Dn,B=20 cm2/s, and Dp,C=12 cm2/s, and Lb=1 mm . The cross-sectional area A=1 mm2. Assume that recombination within the BJT is small (i.e., diffusion lengths are much larger than the thicknesses of each layer).

a) What is the emitter injection efficiency γF?b) What is the base transport factor αT?c) What is the common base current gain αDC?d) What is the emitter gain βF (aka βdc)?e) What is the forward saturation current density IF0?

Fall 2015: Part III

4/26/2018 Bermel ECE 305 S18 46

Consider the crystalline silicon-based bipolar junction transistor (BJT) depicted below. The shaded regions represent the depletion regions, while white regions may be treated as having zero electric field (in the depletion approximation). Assume that the doping concentrations ND,E=1018/cm3, NA,B=1017/cm3, and ND,C= 1016 /cm3. AssumeLD≫WE and LD≫WC.

a) Sketch the excess minority carrier concentrations (in all white regions) in forward active mode biasing.

Fall 2015: Part III

4/26/2018 Bermel ECE 305 S18 48

Consider the crystalline silicon-based bipolar junction transistor (BJT) depicted below. The shaded regions represent the depletion regions, while white regions may be treated as having zero electric field (in the depletion approximation). Assume that the doping concentrations ND,E=1018/cm3, NA,B=1017/cm3, and ND,C= 1016 /cm3. AssumeLD≫WE and LD≫WC.

b) Sketch the excess minority carrier concentrations (in all white regions) in inverted mode biasing.

Fall 2015: Part III

4/26/2018 Bermel ECE 305 S18 50

Consider the crystalline silicon-based bipolar junction transistor (BJT) depicted below. The shaded regions represent the depletion regions, while white regions may be treated as having zero electric field (in the depletion approximation). Assume that the doping concentrations ND,E=1018/cm3, NA,B=1017/cm3, and ND,C= 1016 /cm3. AssumeLD≫WE and LD≫WC.

c) Sketch the excess minority carrier concentrations (in all white regions) in cutoff mode biasing.

Fall 2015: Part III

4/26/2018 Bermel ECE 305 S18 52

(Questions d and e are based on the following information) The minority carrier concentrations in the quasineutral regions of a pnp BJT are given in the diagram below.

d) Estimate the sign of the collector-base and emitter-base biases, and deduce the operating mode of this BJT.

e) Calculate the magnitude of the collector-base bias VCB.

Thank you for taking ECE 305

Good luck on your Final Exams!