SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

412 CHAPTER 8. FIELD EFFECT TRANSISTORS

Intrinsic model

Gate CDG Drain

RD

CDS

CGS CDC

RG

Rl

gmVGS

(a)

Source Gate Drain

Z

Rs R 1

gD–1

CDS

RD

(b)

RS

CDC

CGS CDG

gD–1

RG

Figure 8.39: (a) Equivalent circuit of a MESFET. (b) Cross-section of a MESFET indicating the
origins of the elements.

given by

gm =

∂ID

∂VG

∣∣

∣∣

VD

=

∂ID

∂Q

∣

∣∣

∣

VD

∂Q

∂VG

∣

∣∣

∣

VD

=

CG

Δt

=

CG

ttr

(8.8.2)