Institute of Solid State PhysicsTechnische Universität Graz

13. Bipolar transistors

Jan. 16, 2019

Institute of Solid State Physics

bipolar transistorsTechnische Universität Graz

collector base emitter

np

n+

lightly doped p substrate

npn transistor

Used in front-end high-frequency receivers (mobile telephones), low input impedance amplifier.

p+n+

Ebers-Moll model

/ /1 1be B bc BeV k T eV k TC F ES CSI I e I e

/ /1 1be B bc BeV k T eV k TE ES R CSI I e I e

IE

IB

IC

/ 1be BeV k TF ESI I e / 1bc BeV k T

R CSI I e

R RI F FI

B E CI I I

Common base configuration

/ /1 1be B bc BeV k T eV k Tc F ES CSI I e I e

/ /1 1be B bc BeV k T eV k TE ES R CSI I e I e

solve for Vbe

saturation

active

cutoff IE < 0

http://lamp.tu-graz.ac.at/~hadley/psd/L13/commonbase/pnp_current.html

Common base configuration

Ic

IC ~ IE buffer circuit: the output current is constant over a wide range of output voltages

0 0

0

be p e be n bES

eb e bc be

be n bR CS

bc be

eA D p eA D nIW x W WeA D nIW W

Ebers - Moll Model

0

0 0

bc n bF ES

bc be

bc p c bc n bCS

c c bc be

eA D nIW W

eA D p eA D nIx W W W

Common emitter configuration

/ /1 1be B bc BeV k T eV k TE ES R CSI I e I e

/ /1 1be B bc BeV k T eV k Tc F ES CSI I e I e

B E CI I I

http

://la

mp.

tu-g

raz.

ac.a

t/~ha

dley

/psd

/L13

/com

mon

emitt

er.p

hp

current amplification ~100

Current amplification factor

Cfe

B

IhI

B E CI I I

1 1B CI I

1 1C e

B e

I BI B

~ 50 - 500

C EI I

for common emitter configuration

The Art of ElectronicsHorowitz and Hill

Transconductance

Cm

be

IgV

The transconductance can be very high.

/ /1 1be B bc BeV k T eV k Tc F ES CSI I e I e

/be BeV k TF ES C Bm

B B B

e I eI e Ig ek T k T k T

The first term depends on Vbe

Early effect

/ /1 1be B bc BeV k T eV k Tc F ES CSI I e I e

/ /1 1be B bc BeV k T eV k TE ES R CSI I e I e

B E CI I I

Ebers - Moll:

0 0be p e be n bES

eb e bc be

eA D p eA D nIW x W W

0 0bc p c bc n bCS

c c bc be

eA D p eA D nIx W W W

IES and ICS are treated as constants but the depletion widths Wbc, Wbe, Wc,and We depend on the voltages.

pe0

nb0 pc0

xe We Web WbcxcWc

0 exp beb

B

eVnk T

0 exp bcb

B

eVnk T

0 exp bee

B

eVpk T

0 exp bcc

B

eVpk T

Minority carrier concentration

pe0

pc0

xe xc

0 exp beb

B

eVnk T

0 exp bcb

B

eVnk T

0 exp bee

B

eVpk T

0 exp bcc

B

eVpk T

xn1 xp1 xn2xp2

Early effect

Common emitter configurationBase width modulation: smaller width increases the diffusion current and increases the gain.

Punchthrough: The neutral base width goes to zero and all gain is lost.Lightly dope the collector -> voltage drops in collector. Makes circuit slower.

http

://la

mpx

.tugr

az.a

t/~ha

dley

/psd

/L13

/com

mon

_e_e

arly

.php

IC ~ IB

Common emitter configuration

IC ~ IB amplifier

Small signal response

Low input impedance amplifier

Vcc

VresistorVtransistor

Small signal response

input conductance: cm

EB

igv

transconductance:

c B EB EBi i g v transistor man

BEB

EB

igv

Small signal response

01f f 0Tf f

1

Heterojunction bipolar transistors

Heterojunction bipolar transistor

small emitter-base capacitance to increase speed

HBT current gain

0

0

(npn)1

B

E

np

C BI I

Higher doping in the emitter makes the minority carrier concentration lower in the emitter.

2

0

exp( / )C V gB BiB

A A

N N E k TnnN N

2

0

exp( / )C V gE BiE

D D

N N E k TnpN N

If the emitter and the base have different band gaps

exp 100000gc vE

B c v B

EN NNN N N k T

HBT current gain

A HBT has an emitter bandgap of 1.62 and a base bandgap of 1.42.A BJT has an emitter bandgap of 1.42 and a base bandgap of 1.42.Both have an emitter doping of 1018 cm-3 and a base doping of 1015cm-3. How much larger is the gain in the HBT?

HBT 1.62 1.42exp exp 2257BJT 0.0259

g

B

Ek T

HBT

Trade off gain for higher speed

Higher base dopinglower base resistancereduced Early effectless trouble with punch throughbase can be made thinner -> faster transistors

Because of higher base doping, a higher collector doping is possible without punch through

lower collector resistance

HBT current gain

band discontinuity reduces emitter efficiencyGraded layer emitter and base improve performance

Si/SiGeAlInAs/InGaAs

Heterojunction bipolar transistors

Fastest InP/InGaAs HBT's have an fT of 710 GHz.

Higher doping in the base allows for a thinner base without punch through and lower base resistance and thus higher frequency operation

Microwave engineering

Electronics: L<< f <~ 10 GHz

Microwave: L 10 GHz < f < 1 THz

TeraHertz: L 1 THz < f < 100 THz

Optics: L 100 THz

Interdigitated contacts in power transistors

Thyristors

Reverse blocking

Forward blocking

Forward conducting

Thyristors

Used for switching high currents or voltages

1*2 > 1

Latch-up

http://www.ece.drexel.edu/courses/ECE-E431/latch-up/latch-up.html

Both BJT's conduct, creating a low resistance path between Vdd and GND. The product of the gains of the two transistors in the feedback loop, is greater than one. The result of latchup is at the minimum a circuit malfunction, and in the worst case, the destruction of the device.

Subthreshold current

If the p-concentration in the channel is low, electrons emitted into the channel by the forward biased junction diffuse across the channel without recombining.