The cross section of a MESFET is illustrated in Fig. 4.36. The basic device
parameters include the gate length , the gate width ,
and the thickness of the epitaxial layer .
Cross section of a MESFET in 4H-SiC.
MESFETs in SiC are made of n-type material because of the higher electron mobility (see
Section 3.3), and is fabricated using epitaxial layers on semi-insulating
substrates to minimize parasitic capacitances. A MESFET has three metal-semiconductor
contacts. The ohmic contacts are labeled source and drain, and the Schottky barrier is labeled
gate. A MESFET is often described in terms of the gate dimensions. If
m, the device is referred to as a
device. A microwave- or millimeter wave device typically has a gate length in the range
m. The thickness of the channel is typically one-third to one-fifth of the
. The spacing between the electrodes is one to four times that of
the gate width
. The current handling capability of a MESFET is directly
proportional to the gate width because the cross-sectional area available for the channel
current is proportional to
In operation the drain contact is biased
at a positive potential and the source is grounded. The flow of current through the conducting
channel is controlled by negative DC and superimposed RF potentials applied to the gate. The
RF signal modulates the channel current, thereby providing RF gain. The operation of the
transistor is determined by the ability of the gate signal to effectively modulate and control
the current in the conducting channel. A very detail analytical based analysis on MESFET operation
can be found in Appendix C.
Optimized device parameters for a 4H-SiC MESFET.