* 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 microwave transistors. * Finally, a highly doped (n+) layer is grown on the surface to aid in the fabrication of low-resistance ohmic contacts to the transistor. * This layer is etched away in the channel region.
* Alternatively, ion implantation may be used to create the n channel and the highly doped ohmic contact regions directly in the semi-insulating substrate. * Two ohmic contacts, the source and drain, are fabricated on the highly doped layer to provide access to the external circuit. * Between the two ohmic contacts, a rectifying or Schottky contact is fabricated. * Typically, the ohmic contacts are Au–Ge based and the Schottky contact is Ti–Pt–Au. * The basic operation of the MESFET is easily understood by first considering the I–V characteristics of the device without the gate contact, as shown in figure below. * If a small voltage is applied between the source and drain, a current will flow between the two contacts. * As the voltage is increased, the current increases linearly with an associated resistance that is the sum of the two ohmic resistances, RS and RD, and the channel resistance, RDS.
* If the voltage is increased further, the applied electric field will become greater than the electric field required for saturation of electron velocity. * Under large bias conditions, an alternative expression for ID is useful; this expression relates the current directly to the channel parameters: * This expression omits the parasitic resistances, RS and RD. * The parameters in equation above are Z, the width of the channel; b(x), the effective channel depth; q, the electron charge; n(x), the electron density; and v(x), the electron velocity, which is related to the electric field across the channel. * Note that if v(x) saturates, ID will also saturate.
* This saturation current is called IDSS.
* Now consider the effect of the gate electrode placed over the channel but without any gate bias, VG = 0. * A...
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