Like other forms of field effect transistor the GaAs Fet or MESFET has two forms that can be used:
Depletion mode MESFET: If the depletion region does not extend all the way to the p-type
substrate, the MESFET is a depletion-mode MESFET. A depletion-mode MESFET is
conductive or "ON" when no gate-to-source voltage is applied and is turned "OFF" upon the
application of a negative gate-to-source voltage, which increases the width of the depletion
region such that it "pinches off" the channel.
Enhancement mode MESFET: In an enhancement-mode MESFET, the depletion region is
wide enough to pinch off the channel without applied voltage. Therefore the enhancementmode MESFET is naturally "OFF". When a positive voltage is applied between the gate and
source, the depletion region shrinks, and the channel becomes conductive. Unfortunately, a
positive gate-to-source voltage puts the Schottky diode in forward bias, where a large current
The MESFET is used in many RF amplifier applications. The MESFET semiconductor technology
provides for higher electron mobility, and in addition to this the semi-insulating substrate there are
lower levels of stray capacitance. This combination makes the MESFET ideal as an RF amplifier. In
this role MESFETs may be used as microwave power amplifiers, high frequency low noise RF
amplifiers, oscillators, and within mixers. MESFET semiconductor technology has enabled amplifiers
using these devices that can operate up to 50 GHz and more, and some to frequencies of 100 GHz.
The GaAS FET / MESFET has a number of differences and advantages when compared to bipolar
transistors. The MESFET has a very much higher input as a result of the non-conducting diode
junction. In addition to this it also has a negative temperature co-efficient which inhibits some of the
thermal problems experienced with other transistors.
When compared to the more common silicon MOSFET, the GaAs Fet or MESFET does not have the
problems associated with oxide traps. Also a MESFET has better channel length control than a
JFET. The reason for this is that the JFET requires a diffusion process to create the gate and this
process is far from well defined. The more exact geometries of the GaAS FET / MESFET provide a
much better and more repeatable product, and this enables very small geometries suited to RF
microwave frequencies to catered for.
In many respects GaAs technology is less well developed than silicon. The huge ongoing investment
in silicon technology means that silicon technology is much cheaper. However GaAs technology is
able to benefit from many of the developments and it is easy to use in integrated circuit fabrication
GaAs FET / MESFET in use
The GaAs Fet / MESFET is widely used as an RF amplifier device. The small geoemtries and other
aspects of the device make it ideal in this application. Typically a supply voltage of around 10 volts
will be used. However care must be taken when designing the bias arrangements because if current
flows in the gate junction, it will destroy the GaAS FET. Similarly care must be taken when handling
the devices as they are static sensitive. In addition to this, when used as an RF amplifier connected
to an antenna, the device must be protected against static received during electrical storms.
If these precautions are observed, the GaAs FET or MESFET will perform exceedingly well. The
MESFET or GaAs FET is an electronics component that is relatively cheap, and will perform well.