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Chapter

8

BIPOLAR JUNCTION

TRANSISTORS

Bipolar junction transistors are important in numerous technologies|ampliers, oscillators, high

speed logic. The following pages provide an overview.

© Prof. Jasprit Singh www.eecs.umich.edu/~singh

Emitter contact

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n+

n+

n+

n+

p

B

E

C

p

p+

p+

p+

p+

p

n epitaxyn+ buried layerp-type substrate

Base contact

Collector contact

n+

n+

n epitaxy

n epitaxy

n+ n+p

n

E B C

Cross-sectional view

Base width

BIPLOAR JUNCTION TRANSISTOR: STRUCTURE

Bipolar transistors find important uses as amplifiers, drivers of other devices, and certain high speed digital circuits.

A SCHEMATIC CROSS-SECTION


+ + + + +

– – – – – –– – – – –

– – – – – –

n = Ndc~n+ = Nde

~

p = Nab~

Wb

– – – – – –– – – – –

–

– – – –

–––

–

–

–

++ + + +

IEn

Electron current

Collector current = BIEn

Hole current = Base current

n+

n

p

IEp

(a)

(b)

– – – – –

– – – – –

HOW THE BASE CURRENT (BIAS) CONTROLS THE EMITTER AND COLLECTOR CURRENT

Bipolar transistor operator on the basis of the base signal controlling the potential barrier that electrons in the emitter see.

Lowered emitter-base barrier allows injection of electrons from the emitter into the base and the collector

EQUILIBRIUM: NO BIAS

EMITTER-BASE JUNCTION IS FORWARD BIASED


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Si

Emitter contact

Basecontact

SiO2

Collector contact

A A'

p

n

n

IC

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nIEB = Emitter current injected into the base = IEn

nIBC = Electron current injected across reverse-biased base collector junction

IBE = Base current injected into the emitter = IEpp

IBE = Recombination current in the base region R

IBC = Hole current injected across reverse-biased base collector junction p

InC = Electron current coming from the emitter (= IC)~

Electron flowIEBn

n n

IBEp

p

IBER

IBCp

IBCn

InC

IB

IE

Hole flow

III

III

IV

VIBER

++BE

A

A'

CB

VI

n'

CURRENT COMPONENT IN A BIPOLAR TRANSISTOR

What is needed for a high performance device?• IEn >> IEp ; IBE ~ 0 high emitter efficiency

high base transport factor


+ + + + +

– – – – – –– – – – –

– – – – – –

VBE > 0

Minority charge density

– – – – –

+ + +

VCB < 0

Holes

ElectronsHoles

peo

nbonco

– – – – – –– – – – –

– ––

+ + ++

IE

IB

VBE > 0 VCB > 0

+

IC


Holes

ElectronsHoles

– – – – – –– – – – –

– ––

+ + ++

IE ~ 0

IB ~ 0

VBE < 0 VCB > 0

+


Holes

ElectronsHoles

IC ~ 0

SATURATION FORWARD ACTIVE CUTOFF

MODES OF A BIPLOAR JUNCTION TRANSISTOR: MINORITY CHARGE DISTRIBUTION


VEB

Common base Common emitter Common collector

E

B

CE

B

CE

B

CIE IC

VBCIB

VEC

VECVEB

IE

VCBIC

IB

VBC

IB

VEB

VEC

IC

IE

IE4

IE3

IE2

IE1IE = 0

VBC~0.7 V 0

IC

Cutoff

Saturation

Active

IB2

IB1

IB = 0

IB2

IB1

IB = 0

Reverse active

Cutoff

Saturation

Active

VBC = 0

VEC

IC

Common base Common emitter

(a)

(b)

OPERATING CONFIGURATION OF A BIPOLAR JUNCTION TRANSISTOR


PROBLEMSDEMANDS

REQUIREMENTS FOR A BIPOLAR DEVICE

• High gain• High emitter efficiency• High speed

DEMANDS AND PROBLEMS FOR A BIPOLAR JUNCTION TRANSISTOR

Heavy emitter doping

Low base doping

Narrow base width

Bandgap shrinkage causing base injection

High base resistance

OPTIMIZATION OF A BIPOLAR TRANSISTOR

ISSUES: Emitter doping, base doping, collector doping, base width, emitter thickness

EMITTER EFFICIENCY: γe ~ 1 –peoDeWbn

nboDbEmitter doping >> base doping

BASE TRANSPORT FACTOR: B ~ 1 – Wbn << LbWbn 2Lb

2

2

LOW OUTPUT CONDUCTANCE: Wbn should not change with collector bias collector doping << base doping


Base

(a)

IC

VBE

VCE|VA|

(b)

Increasingminority

carrier gradient

xb = 0 xb = Wbn

EARLY VOLTAGE AND OUTPUT CONDUCTANCE OF I-V CURVES

If collector doping is much smaller than base doping the depletion width at base collector junction will be on the collector side large early voltage, VA.

Increasing VCE causes a reduction in neutral base width collector current increases.

Collector current α ; Wbn = neutral base width1Wbn

VCE


IB

IB

IC

Log (IC, IB)

VEB (volt)0.4

Generation current

Reduced β due to Kirk effect and Auger effect

In the range VEB < 0.4~

IB exp

eVEB mkBT

In the range VEB > 0.4~

IB exp

eVEB kBT

(a)

200

100

1 µA 10 µA 100 µA 1 mA 10 mA 100 mA IC

Recombination current dominates base current

Collector current reduced due to base pushout

CU

RR

EN

T G

AIN

, β

(b)

CURRENT GAIN DEPENDENCE ON BASE (COLLECTOR) CURRENT

LOW INJECTION REGIME: Current gain is small because of non-ideal generation-recombination current.

HIGH INJECTION REGIME: Current gain drops because the effective base width is pushed out into the collector + carriers (e-h) recombine due to Auger effect.

Base and collector currents versus forward bias voltage


SWITCHING DELAYS IN A BIPOLAR JUNCTION TRANSISTOR

VBE(on) = 0.7 V τF = 0.1 ns VBE(sat) = 0.8 V τBF = 10 ns VCE(sat) = 0.1 V τS = 12 ns Cjeo = 0.5 pF Cjco = 0.2 pF φe = 0.9 V φc = 0.7 V me = 0.50 mc = 0.5

RB

5 kΩ

VCC = 5 V

–

+

RC

1 kΩ

vi(t)

vo(t)OUTPUT

INPUT

Base Base Base Base Base Base

t0 t1

t2

t3

t4

t5 t65.0 V

0

5 V

0.1 V

INPUT VOLTAGE

OUTPUT VOLTAGE

MINORITY CHARGE INJECTION

vi

Vo

Minority charge in the base

EXAMPLE PARAMETERS

SWITCHING CIRCUIT

MINORITY CHARGE REMOVAL

IMPORTANT ISSUE: Avoid going into deep saturation.


C

E

Bp

n

n+

Al

SiO2

BE

C

• Collector-Base reverse biasedSchottky diode is reversebiased

• Collector-Base forward biasedSchottky diode turns ONand collector is bypassed

Al makes an ohmic contact to the p-type base and a Schottky contact to the n-type collector

Schottky diode is turned ON at a voltage smaller than what it takes the CBJ to be in the saturated mode

Base-collector diode

Schottky diode

V

I

USE OF A SCHOTTKY JUNCTION FOR HIGH SPEED BIPOLAR DEVICES

MOTIVATION: Do not let the transistor go into deep saturation during switching.


EQUIVALENT CIRCUIT PARAMETERS

Base-Emitter Junction (forward biased)re = resistance between E and E'

Cπ = diffusion capacitance

rπ = junction resistance

Cje = junction capacitance

rb = resistance between B and B'

Collector-EmittergmVb'e' = current source

ro = output resistance

Cs = collector-substrate capacitance

Base-Collector Junction (reverse biased)rµ = junction resistance

Cµ = junction capacitance

E

E'B'C'

p-substrate

(a)

BC

gmVbe

C

E

B

rπCπVbe

Ib Ic

(c)

Cs

rcC'

ro

E'

E

re

rπCπ Cje

B'B

rb

rµ

Cµ

(b)

C

gmVb'e'

SMALL SIGNAL MODEL OF A BIPOLAR JUNCTION TRANSISTOR

Cutoff frequency

τec = τe + τt + τd + τc

τe = EBJ capacitance charging time

τc = Collector capacitance charging time

fT =1

2πτec

τt = Base transit time =Wb

2Db

2

τd = Transit time through the collector depletion region =Wdcvs


PROBLEMSDEMANDS

REQUIREMENTS FOR A BIPOLAR DEVICE

• High gain• High emitter efficiency• High speed

DEMANDS AND PROBLEMS FOR A BIPOLAR JUNCTION TRANSISTOR

Heavy emitter doping

Low base doping

Narrow base width

Bandgap shrinkage causing base injection

High base resistance

SOLUTION: HETEROJUNCTION BIPOLAR TRANSISTORS

• Emitter can be heavily doped using a semiconductor with a bandgap larger than the base semiconductor.

• Base can be heavily doped and be made narrow without increasing base resistance.

• Collector can be chosen from a material to increase breakdown voltage.

ADVANTAGES OF A HETEROJUNCTION BIPOLAR TRANSISTOR


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Base

Emittern GaAsn GaAlAs

p GaAs

n GaAs

n+ GaAs

n+ GaAs

Barrier forelectron injection

Barrier forhole injection

Homojunction transistor

Emitter(n)

Base(p)

++

Emitter(n)

Base(p)

Barrier forelectron injection

Barrier forhole injection

Ege

Egb

Heterojunction transistor

Large bandgap emitter material

Small bandgap base region

Small bandgap collector region

– – – –

+ + + +

– – – –

+ + + +

HETEROJUNCTION BIPOLAR TRANSISTOR

Current gain (HBT) = current gain (BJT) x exp

• HBT concept allows high base doping and still maintain high emitter efficiency. This allows one to make very thin base devices with low base resistance.

∆EgkBT( (


Silicon bipolar technology• Advanced fabrication techniques are allowing devices with fT ~25 GHz

Advanced fabrication techniques• Self-aligned emitter base• Trench isolation to avoid cross-talk (SiO2 fills the "trenches").• Sidewall contacts. Polysilicon is used to contact the base.• Polysilicon emitter contact provides low recombination at the contact and suppresses base injection into the emitter.

Si can be combined with • amorphous silicon (Eg = 1.5 eV)

• β-SiC (Eg = 2.2 eV)

• polysilicon (Eg = 1.5 eV)

Most promising combination is Si/SiGe, which can be fabricated by epitaxial growth.

• Excellent quality of interface allows fabrication of high-quality HBTs.• Devices can be monolithically integrated with optoelectronic devices.

• In0.53Ga0.47As is lattice-matched to InP and In0.52Al0.48As.• High-quality HBTs can be produced and integrated with optical devices.

InGaAs/InAlAs and InGaAs/InP HBTs• fT of ~175 GHz has been achieved.

GaAs/AlGaAs HBTs• fT of ~100 GHz has been demonstrated.

Si-based HBTs• Si/SiGe HBTs have shown remarkable promise. Cutoff frequencies approaching 100 GHz have been demonstrated.

AN OVERVIEW OF ADVANCES IN BIPOLAR TECHNOLOGY

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