SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

8.7. DESIGN ISSUES IN HFETS 399

(b)

{ {

AlGaAs InGaAs

δ-doped

Layer

2DEG

• d -ds 0 Δd

{

p-InGaAs

or

GaAs

dInGaAs

EF

0

• d -ds

(c)

eΔφb

EF

0

• d -ds

(d)

dInGaAs

Is

Ipar

(a)

dbar

dbar

eΔφb

no barrier

Figure 8.26: (a) In standard MODFET structures, a small parasitic leakage currentIparflows
through the substrate. (b) MODFET structure which incorporates a back barrier to prevent cur-
rent injection through the substrate. (c) Band diagram for ap-type barrier. (d) Band diagram for
a barrier composed of a wide bandgap buffer.