4.5.4.1 MESFET Structure

The cross section of a MESFET is illustrated in Fig. 4.36. The basic device parameters include the gate length $L_\mathrm{g}$, the gate width $W_\mathrm{g}$, and the thickness of the epitaxial layer $a$.

Figure 4.36: 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 $L_\mathrm{g}=0.5$ $\mu$m and $W_\mathrm{g}=500$ $\mu$m, the device is referred to as a $0.5 \times 500\,\mu$m device. A microwave- or millimeter wave device typically has a gate length in the range $0.1-1\,\mu$m. The thickness of the channel $a$ is typically one-third to one-fifth of the gate length $L_\mathrm{g}$. The spacing between the electrodes is one to four times that of the gate width $W_\mathrm{g}$. 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 $W_\mathrm{g}$.

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.

Table 4.7: Optimized device parameters for a 4H-SiC MESFET.

parameter	value
gate length $L_\mathrm{g}$	$0.5\,\mathrm{\mu}$m
gate width $W_\mathrm{g}$	1 mm
source-drain spacing $L_\mathrm{gs}+L_\mathrm{g}+L_\mathrm{gd}$	$2\,\mathrm{\mu}$m
channel doping $N_\mathrm{d}$	$5\times10^{17}$ cm$^{-3}$
channel thickness $a$	$0.15\,\mathrm{\mu}$m

T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation