The MESFET is a high performance form of field effect transistor that is used mainly for exacting microwave applications both as a low noise signal amplifier and in higher power RF circuits.
The abbreviation MESFET stands for MEtal-Semiconductor Field Effect Transistor and the most widely used form is the GaAsFET fabricated using the III-IV semiconductor material gallium arsenide.
GaAs FET / MESFET structure
The MESFET structure is very similar to a junction FET or JFET. As the name of the MESFET indicates, it has a metal contact directly onto the silicon, and this forms a Schottky barrier diode junction. As such the Schottky diode is used as a reverse biased diode in the same was that a JFET does. The main difference is that the Schottky diode forms a very much smaller diode.
The material that is used can be silicon or other forms of semiconductor. However the material that is most widely used is gallium arsenide GaAs. Gallium arsenide is normally chosen because of the very superior electron mobility it provides that enables superior high frequency operation to be achieved.
The substrate for the semiconductor device is semi-insulating for low parasitic capacitance, and then the active layer is deposited epitaxially. The resulting channel is typically less than 0.2 microns thick.
The doping profile is normally non-uniform in a direction perpendicular to the gate. This makes for a device which has good linearity and low noise. Most devices are required for high speed operation, and therefore an n-channel is used because electrons have a much greater mobility than holes that would be present in a p-channel.
The gate contacts can be made from a variety of materials including Aluminium, a Titanium-Platinum-Gold layered structure, Platinum itself, or Tungsten. These provide a high barrier height and this in turn reduces the leakage current. This is particularly important for enhancement mode devices which require a forward biased junction.
The gate length to depth ratio is an important as this determines a number of the performance parameters. Typically it is kept at around four as there is a trade-off between parasitic responses, speed, and short channel effects.
The source and drain regions are formed by ion-implantation. The drain contacts for GaAs MESFETs are normally AuGe - a Gold-Germanium alloy.
There are two main structures that are used for MESFETs:
- Non-self aligned source and drain: For this form of MESFET, the gate is placed on a section of the channel. The gate contact does not cover the whole of the length of the channel. This arises because the source and drain contacts are normally formed before the gate.
- Self aligned source and drain: This form of structure reduces the length of the channel and the gate contact covers the whole length. This can be done because the gate is formed first, but in order that the annealing process required after the formation of the source and drain areas by ion implantation, the gate contact must be able to withstand the high temperatures and this results in the use of a limited number of materials being suitable.
Like other forms of field effect transistor the GaAs Fet or MESFET has two forms that can be used:
- Enhancement mode MESFET: In an enhancement-mode MESFET, the depletion region is wide enough to pinch off the channel without applied voltage. Therefore the enhancement-mode 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 can flow.
- 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.
MESFET / GaAsFET characteristics
The MESFET is used in many RF amplifier applications. It is used in many RF and microwave applications where its characteristics give it an edge over other technologies.
Some of the key characteristics include:
- High electron mobility: The use of Gallium Arsenide or other high performance semiconductor materials provides for a high level of electron mobility which is required for high performance RF applications. 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.
- Low capacitance levels: The Schottky diode gate structure results in very low stray capacitance levels which lend themselves to excellent RF and microwave performance.
- High input impedance: The MESFET has a very much higher input when compared to bipolar transistors as a result of the non-conducting diode junction.
- Negative temperature coefficient: The MESFET / GaAs FET has a negative temperature co-efficient which inhibits some of the thermal problems experienced with other transistors.
- Lack of oxide traps: When compared to the more common silicon MOSFET, the GaAs FET or MESFET does not have the problems associated with oxide traps.
- High level of geometry control: The 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 on-going 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 processes.
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.
The circuits normally used supply voltages of the order of around 10 volts. However great care must be taken when designing the bias arrangements because if current flows in the gate junction, it will destroy the GaAS FET.
Similarly great care must be taken when handling the devices as they are very sensitive to static. Even static voltages of under 100 volts can destroy the junction.
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, providing high frequency performance combined with low noise and a high level of efficiency.
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