SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

398 CHAPTER 8. FIELD EFFECT TRANSISTORS

(a)

{ {

i-AlGaAs GaAs

δ-doped

Layer

2DEG

n+-AlGaAs

n+-GaAs

{

EF(cap)

EF(GaAs)

eφB

(b)

Figure 8.25: (a) Layer structure and (b) band diagram of an AlGaAs/GaAs HFET with a highly
degeneraten+GaAs cap layer directly above a thinn+AlGaAs layer. The high doping in the
GaAs cap layer reduces the barriereφBthat electrons must overcome, and then+AlGaAs layer
increases the probability of electrons tunneling through a portion of the barrier.

8.7.3 Back-barriers to Substrate Injection ....................

Control of channel charge is the essence of FET operation. If electrons travel through the
path labeledIsin figure 8.26a, then they are effectively controlled by the gate. Electrons trav-
eling along the path labeledIparwithin the substrate and far from the gate are not effectively
modulated and are parasitic currents leading to both reduced output resistance (hence low power
gain) and low current gain. To keep electrons from being injected into the substrate, we need to
present a barrier to current flow, as shown in figure 8.26b. This can be done by introducing a
fully depletedp-type layer or a wider bandgap buffer. The band diagrams for each of these are
shown in figure 8.26c and figure 8.26d.
Thep-type buffer introduces negative space charge to the region immediately below the chan-
nel, thus increasing the electrostatic barrier to electron injection into the buffer by a maximum
amount

Δφb

eNad^2 bar

2 

(8.7.2)