principles of semiconductor devices-l30

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Lecture 30: Heterojunction Bipolar Transistor (I)

Mu amma As ra u A am Mar Lun [email protected]

Alam ECE‐606 S09 1



Outline

1. Introduction

2. Equilibrium solution

for

heterojunction

3. T es of hetero unctions

4. Conclusions

“Heterostructure Fundamentals,” by Mark Lundstrom, Purdue

University, 1995.

Herbert Kroemer, “Heterostructure bipolar transistors and integrated

circuits,” Proc. IEEE , 70, pp. 13‐25, 1982.




How to make a better Transistor

2n

,

, 2,i

E t

poly ballis ics E

t B N n υ

→ × ×

Heterojunction bipolar

transistor

Polysilicon Emitter

( ),

,

,, ,

2

,

g B

gg E BC B V B

E

E i B E N N ee

nβ

β

−

−

−= ≈

,

, , ,

g

C E V i E E n e




Heterojunction Bipolar Transistors

i) Wide gap Emitter HBT

n

emitter

n

collector n+

p+base

EG1>EG2EG2 EG2

ii) Double Heterojunction Bipolar Transistor

n

emitter

n

collector n+

p+base


EG1>EG2 EG2 EG3>EG2



Mesa HBTs

nemitter

ncollector n+

p+

base

EG1>EG2 EG2 EG3>EG2

p+ base

nMesa HBT

n‐co ector

n+


sem ‐ nsu a ng su s ra e



Applications

1 O tical fiber communications

‐40Gb/s…….160Gb/s

2) Wideband, high‐resolution DA/AD converters

and 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.




Background

A heterojunction bipolar transistor

Kroemer

Schokley realized that HBT is possible, but Kroemer really


.



Outline

1. Introduction

.

3. Types

of

heterojunctions4. Conclusions




Topic Map

signal

SignalDiode

Schottky

BJT/HBT

MOS




Bandgaps and Lattice Matching




Band Diagram at Equilibrium

( ) D A D q p n N N + −

∇ • = − + −

1n∂

Equilibrium

N N N r g

t q= • − +

∂

J = + ∇n E D n DC dn dt=0

1P P P

pr g

∂= ∇ • − +J

Small signal dn/dt ~ jωtn

Transient ‐‐‐ Charge control model

P P Pqp E qD pμ = − ∇J




N‐Al0.3Ga0.7As: p‐GaAs (Type‐I Heterojunction)

D A

χ2

EC

EF

χ1

E G

≈ 1.42 eV

V

E

G≈ 1.80 eV


rupt unct on



Built‐in Potential: Boundary Condition @Infinity

21 2 21 gi ,bqV E + + = − ΔΔ + χ χ

qVbi

E

χ2

χ

Δ1

Δ2

Eg,2

EV

EF

2 2 1 2 1bi g ,qV E Δ Δ= − − + − χ

N N


2

2 1

12g , B B E / k T

V , C ,

n N N e

−= −



Interface Boundary Conditions

‐

Position

xn xp

( ) ( )0 0210 0 E E κ ε κ ε − +=

0 2 0

0

1

0dx dxκ ε κ ε

− +

=




Analytical Solution for Heterojunctions

Charge D nq x N −

xpxn x , 0

0A

s E

p N q x E

k ε

+ =

E‐field, 0s B

D n A p x x

k

N N

ε

⇒ =x

( ) ( )0 0n p E x E xV

− +

= +

x

22

2 2

pn D Aq xq N

k k

x N

ε ε = +


, ,



Base Emitter Depletion Region

D A

xn xp

N x N x= ,0 ,2 s B Bs E N x V ε κ κ =

22

,, B p BE E n BE qN xqN x

=

, ,

,,

n

E B s E E Bsq N N N κ κ +

,0 ,2 s E Bs E N ε κ κ

,0 0,2 2s E s Bε κ ε κ ( ),, p bi

B B s E E Bsq N N N κ κ +




Outline

1. Introduction

.

3. Types of heterojunctions

4. Conclusions




P‐Al0.3Ga0.7As : n‐GaAs (Type I junctions)

E

0

Vacuum level

χ 1 qV ( x) qV BI

E

C V

jP

E l

E C

G≈ . e

V jn

C

E F

E V E G

≈ 1.42 eV


V V

Depletion layerDepletion

layer



(AlInAs/InP) Type II Junctions

D A

χ2

EC

EF

χ1

V




N‐Al0.3Ga0.7As : n‐GaAs Junctions

‘Isotype Heterojunction’

E C E

E V

E ≈ 1.80 eV

E G

≈ 1.42 eV

E

E

V

V

Accumulation LayerDepletion

Layer


Metal-Metal junctions have similar features …



P‐GaSb : n‐InAs (Type III)

E 0

Field‐free vacuum level

χ

1

E

C χ 2

E

V

E FP

G≈ . e

E C E

Fn E

G≈ 0.36 eV


E V



P‐GaSb : n‐InAs (Type III)

E C Δ E

C = 0.87 eV

≈

E

V

C

E

E F

G

E G

≈ 0.36 eV

Accumulation Layer!Accumulation Layer!




Conclusion

1. Heterojunction transistors offer a solution to the

‐ .

2. Equilibrium solutions for HBTs are very similar to those of

normal BJTs.

3. De endin on the ali nment there could be different

types of heterojuctions. Each has different usage.

. .


principles of semiconductor devices-l30

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