Amplifier, Threshold voltage, Diode

* ThMESFET = Metal Semiconductor Field Effect Transistor = Schottky gate FET. * e MESFET consists of a conducting channel positioned between a source and drain contact region. * The carrier flow from source to drain is controlled by a Schottky metal gate. * The control of the channel is obtained by varying the depletion layer width underneath the metal contact which modulates the thickness of the conducting channel and thereby the current.

* The key advantage of the MESFET is the higher mobility of the carriers in the channel as compared to the MOSFET. * The disadvantage of the MESFET structure is the presence of the Schottky metal gate. * It limits the forward bias voltage on the gate to the turn-on voltage of the Schottky diode. * This turn-on voltage is typically 0.7 V for GaAs Schottky diodes. * The threshold voltage therefore must be lower than this turn-on voltage. * As a result it is more difficult to fabricate circuits containing a large number of enhancement-mode MESFET. Basic structure

* GaAs MESFETs are the most commonly used and important active devices in microwave circuits. * In fact, until the late 1980s, almost all microwave integrated circuits used GaAs MESFETs. * Although more complicated devices with better performance for some applications have been introduced, the MESFET is still the dominant active device for power amplifiers and switching circuits in the microwave spectrum.

* The base material on which the transistor is fabricated is a GaAs substrate. * A buffer layer is epitaxially grown over the GaAs substrate to isolate defects in the substrate from the transistor. * The channel or the conducting layer is a thin, lightly doped (n) conducting layer of semiconducting material epitaxially grown over the buffer layer. * Since the electron mobility is approximately 20 times greater than the hole mobility for GaAs, the conducting channel is always n-type for...
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