SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

268 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS

B arrier for

el ectron

injection

B arrier for

hole injection

HOMOJUNCT ION

E mi tter

(n)

Base

(p)

E mi tter

n+ GaAs

n GaAlAs

p GaAs

n GaAs

n+ GaAs

n+ GaAs

L arge bandgap

emi tter materi al

S mal l bandgap base r egi on

S mal l bandgap

collector region

E mi tter

(n)

Base

(p)

B arrier for

el ectron

injection

B arrier for

hole injection

Ege

Egb

HETEROJUNCTION

(a)

(b)

Figure 6.12: (a) A structural schematic of a heterojunction bipolar transistor made from GaAs
and AlGaAs. (b) The band profile for a homojunction and heterojunction transistor. In the HBT,
grading is used to avoid a potential “notch”.

For GaAs we have the emitter and base minority carrier concentrations

peo =

n^2 i

Nde

=

(2. 2 × 106 )^2

5 × 1017

=9. 7 × 10 −^6 cm−^3

nbo =

n^2 i

Nab

=

(2. 2 × 106 )^2

1017

=4. 84 × 10 −^5 cm−^3

The emitter efficiency is

γe=1−

peoDeWb

nboDbLe

=1−

(9. 7 × 10 −^6 )(15)(0. 5 × 10 −^4 )

(4. 84 × 10 −^5 )(100)(1. 5 × 10 −^4 )

=0. 99

In the HBT, the value ofpeois greatly suppressed. The new value is approximately

peo(Al 0. 3 Ga 0. 7 As)=

n^2 i(GaAs)

Nde

exp

(

−

ΔEg

kBT

)

= peo(GaAs)exp

(

−

ΔEg

kBT

)

=9. 4 × 10 −^12 cm−^3

In this case the emitter efficiency is essentially unity.