The use of a Schottky barrier at the gate is the major difference from the depletion-type and enhancement-type MOSFETs, which employ an insulating barrier between the metal contact and the n-type channel. The absence of an insulating layer reduces the distance between the metal contact surface of the gate and the semiconductor layer, resulting in a lower level of stray capacitance between the two surfaces. The result of the lower capacitance level is a reduced sensitivity to high frequencies (forming a shorting effect), which further supports the high mobility of carriers in the \(GaAs\) material.
The gate terminal is connected directly to a metallic conductor lying directly against the n-channel between the source and drain terminals.
For increasing values of negative voltage at the gate terminal, the result is a reduced drain current. For positive voltages at the gate, additional electrons will be attracted into the channel and drain current will rise.
The drain and transfer characteristics of depletion-type MESFET are so similar to those of the depletion-type MOSFET results in analysis techniques similar to those applied to depletion-type MOSFETs.
There are also enhancement-type MESFETs with a construction the same as the depletion-type MESFET but without the initial channel. The response and characteristics are essentially the same as for the enhancement-type MOSFET. However, due to the Schottky barrier at the gate, the positive threshold voltage is limited to \(0\ V\) to about \(0.4\ V\) because the “turn-on” voltage for a Schottky barrier diode is about \(0.7\ V\).
The analysis techniques applied to enhancement-type MESFETs are similar to those employed for enhancement-type MOSFETs.
It is important to realize, however, that the channel must be an n-type material in a MESFET. The mobility of holes in \(GaAs\) is relatively low compared to that of the negatively charged carriers, losing the advantage of using \(GaAs\) for high-speed applications.
Depletion-type and enhancement-type MESFETs are made with an n-channel between the drain and the source, and therefore only n-type MESFETs are commercially available.