SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

366 CHAPTER 8. FIELD EFFECT TRANSISTORS

(a)

S G D

Semi-insulating substrate

Z

h

x

L

W(x)

EFn (drain)

EFn (source)

Quasi-linear

drop in Ec

(b)

Figure 8.8: (a) A schematic of the MESFET withVD<VDS(sat). The current flow occurs only
in the undepleted region. The channel potential at any pointxin the channel isV(x). (b) Band
diagram along the channel (dotted line in (a)).

We assume that to the first approximation, when pinch-off occurs, the drain current saturates.
What happens once saturation occurs will be discussed in the following section. The drain
voltage at which saturation occurs is

VDS(sat)=Vp−Vbi+VGS (8.3.9)