The Metal-Semiconductor FET (MESFET)

The MESFET is typically fabricated on a semi-insulating substrate (for GaAs, usually chromium-doped or undoped with deep trap states), which conducts practically no current when unbiased. A thin, lightly doped active layer is introduced into this substrate, usually by ion implantation or epitaxy – this forms the actual conduction channel. Source and drain are connected via heavily doped regions with ohmic contacts. The gate consists of a metal deposited directly on the semiconductor, forming a Schottky contact with the channel beneath it – unlike the JFET, there is therefore no pn junction here, and unlike the MOSFET, no insulating oxide layer between gate and channel.

At any metal-semiconductor junction with sufficient barrier height, a depletion region forms in the semiconductor even without an applied voltage, its width depending on the built-in Schottky barrier potential. Applying a negative gate-source voltage widens this depletion region and increasingly pinches off the underlying channel – the operating principle is thus essentially the same as that of the JFET, only with a Schottky junction instead of a pn junction as the control element. The Shockley relationship between I_D, V_GS, and a pinch-off voltage V_p applies approximately to the MESFET as well, with the exact channel geometry (thickness and doping of the active layer) determining the device parameters.