They consists of a conducting channel positioned between a source and
drain contact region.
The carrier flow from source to drain is controlled by schottky metal
gate. The control is obtained by varying the depletion layer width underneath
the metal contact which modulates the thickness of the conducting channel &
thereby the current b/w source & drain.
The key advantage of the MESFET is the higher mobility of the carriers
in the channel as compared to the MOSFET. Since the carriers in the inversion
layer of a MOSFET have a workfunction, which extends into the oxide their
mobility also referred to as surface mobility is less than half of the mobility
of bulk material. As the depletion region separates the carrier from the
surface their mobility is close to that of bulk material. The higher mobility leads to a higher current
, transconductance and transit frequency of the device.
The disadvantages 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 0.7V for GaAS
schottky diode. 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 no. of enhancement mode MESFET.
The high transient frequency of the MESFET makes it particularly of
interest for microwave circuits. While the advantage of the MESFET provides a
superior microwave amplifier or circuit, the limitation of the diode turn ON is
easily terminated. Typically depletion mode devices are used since they provide
a larger current and larger
transconductance and the circuit contains only a few transistors, so that
threshold control is not a limiting factor. The buried channel also yields a
better noise performance as trapping and releases of carriers into and from
surface states and defect is eliminated.They consists of a conducting channel positioned between a source and
drain contact region.
The carrie flow from source to drain is controlled by schottky metal
gate. The control is obtained by varying the depletion layer width underneath
the metal contact which modulates the thickness of the conducting channel &
thereby the current b/w source & drain.
The key advantage of the MESFET is the higher mobility of the carriers
in the channel as compared to the MOSFET. Since the carriers in the inversion
layer of a MOSFET have a workfunction, which extends into the oxide their
mobility also referred to as surface mobility is less than half of the mobility
of bulk material. As the depletion region separates the carrier from the
surface their mobility is close to that of bulk material. The higher mobility leads to a higher current
, transconductance and transit frequency of the device.
The disadvantages of the MESFET structure is the presence of the
schottky metal gate. It limits the forward bias volage on the gate to the turn
on voltage of the schottky diode. This turn ON voltage is 0.7V for GaAS
schottky diode. 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 no. of enhancement mode MESFET.
The high transient frequency of the MESFET makes it particularly of
interest for microwave circuits. While the advantage of the MESFET provides a
superior microwave amplifier or circuit, the limitation of the diode turn ON is
easily terminated. Typically depletion mode devices are used since they provide
a larger current and larger
transconductance and the circuit contains only a few transistors, so that
threshold control is not a limiting factor. The buried channel also yields a
better noise performance as trapping and releases of carriers into and from
surface states and defect is eliminated.
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