8.3. CURRENT-VOLTAGE CHARACTERISTICS 363
n-
GaAs
Ec – EF
EF
Ev
Ec
Source
(ohmic)
Gate
(Schottky)
Drain
(ohmic)
eVbi eφb
Semi-
insulating
GaAs
{Figure 8.6: A schematic of a GaAs MESFET. Also shown are the energy band profile under the
gate region and some important device parameters.
approximations:
- The mobility of the electrons is constant and independent of the electric field. Thus the
velocity increases linearly with field. - The gradual channel approximation introduced by Shockley is assumed. In the absence
of any source-drain bias, the depletion width is simply given by the one- dimensional
model we developed for thep-ndiode. However, strictly speaking, when there is a source-
drain bias, one has to solve a two-dimensional problem to find the depletion width and,
subsequently, the current flow. In the gradual channel approximation, we assume that field
in the direction from the gate to the substrate is much stronger than from the source to
the drain, i.e., the potential varies “slowly” along the channel as compared to the potential
variation in the direction from the gate to the substrate. Thus the depletion width, at a