MESFET The MESFET is a high performance form of field effect transistor that is used mainly for high performance microwave applications and in semiconductor RF amplifiers. The abbreviation MESFET stands for MEtal-Semiconductor Field Effect Transistor. In its most popular form it is made from gallium arsenide and is called a GaAsFET. The GaAs FET or MESFET shares many features with the standard junction FET or JFET, although the MESFET is able to offer superior performance, especially in the region of RF microwave operation, especially for use within RF amplifiers. The key differentiator between the MOSFET and the MESFET is that the MESFET uses a Schottky barrier diode rather than an oxide layer to isolate the gate from the channel. MESFET history and development The MESFET is a form of electronics component that can trace its roots back to the beginnings of semiconductor technology. When the first bipolar transistor was developed, it was actually the principle of a field effect transistor that was being investigated. Only when this idea did not appear to work, did they turn their sights to a bipolar device with the result that the first major milestone in semiconductor technology was the discovery of the bipolar transistor in 1949. The next major step in field effect semiconductor technology was the development of the first practical form of field effect transistor in 1953. As semiconductor technology developed and it became easier and more cost effective to produce the highly pure forms of semiconductor that were needed for field effect electronics components. In addition to this it also became possible to produce oxide layers of the quality required. With these developments in semiconductor technology the first MOSFETs were produced in 1960. Further developments in semiconductor technology occurred and other devices such as GaASFETs, gallium arsenide FETs followed. These devices offered a very low noise and high frequency capability for their day. The first MESFETs were developed in 1966, and a year later their extraordinary high frequency / RF microwave performance was demonstrated. GaAs FET / MESFET structure The MESFET is a form of semiconductor technology which 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. 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 TitaniumPlatinum-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 parasitics, 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: 1. 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. 2- 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.
MESFET operation 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 can flow. MESFET applications 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 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. 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.
Characteristics of Schottky Barriers When a metal is brought into contact with a semiconductor, an electrostatic potential barrier (referred to as Schottky barrier) is created at the interface as the result of the difference in the work function of the two materials. To appreciate the physical nature of the barrier we can model the interface by visualizing a situation whereby the metal is gradually brought toward the semiconductor surface until the separation becomes zero. As this separation between the metal-semiconductor surface is reduced, the induced charge in the semiconductor increases, while at the same time the space charge layer widens. A greater part of the contact potential difference begins to appear across the space charge layer within the semiconductor. Because the carrier concentration in the metal is several orders of magnitude larger than that in the semiconductor when the separation is brought to zero, the entire potential drop then appears within the semiconductor itself. This is in the form of a depletion layer situated adjacent to the metal and extending into the semiconductor.