SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

386 CHAPTER 8. FIELD EFFECT TRANSISTORS

eVG> 0

(a)

(b)

eφb

EF, m

|eVp|

EF, s

• d -ds 0

+

+

+

+

+

Ec

EF, m

• d -ds^0

+

+

+

+

EF, s

Ec

Figure 8.17: A schematic diagram of a MODFET band profile under conditions where (a) a
negative gate bias is applied to completely deplete the 2DEG, and (b) a large positive gate bias
is applied, such that the gate loses control over the 2DEG.

charge transfer in the situation where a change inVGdoes not exclusively result in a change in
the 2DEG concentration. We define the modulation efficiency(ME)to be

ME(VG)=

eΔns

ΔVG

·

1

CG

(8.5.20)

whereCG=/D(see figure 8.15a) is the gate-channel capacitance. The denominator of equa-
tion 8.5.11CG·ΔVGrepresents the ideal induced charge in the 2DEG.
In general, the change in the charge density on the gate is

|Δnm|=Δns+Δnpar+ΔNd^0 (8.5.21)

whereΔNd^0 is the change in density of occupied donors in the AlGaAs, the superscript empha-
sizing that this is the change in the concentration of neutral donor atoms. For simplicity, we will