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The name HEMT stands for High Electron Mobility Transistor. The device is a form of field effect transistor, FET, that utilises an unusual properly of a very narrow channel enabling it to operate at exceedingly high frequencies.
In addition to the very high frequency performance, the HEMT also offers a very attractive low noise performance.
Essentially the device is a field-effect transistor that incorporates a junction between two materials with different band gaps (i.e. a heterojunction) as the channel instead of a doped region which is used in the standard MOSFET.
As a result of its structure, the HEMT may also be referred to as a heterojunction FET, HFET or modulation doped FET, MODFET on some occasions.
Even though the HEMT is basically a form of field effect transistor it utilises an unusual mode of electron movement.
This mode of carrier transport was first investigated in 1969, but it was not until 1980 that the first experimental devices were available investigation and initial use.
During the 1980s they started to be used, but in view of their initial very high cost their use was very limited.
Now with their cost somewhat less, they are more widely used, even finding uses in the mobile telecommunications as well as a variety of microwave radio communications links, and many other RF design applications.
HEMT structure & fabrication
The key element within a HEMT is the specialised PN junction that it uses. It is known as a hetero-junction and consists of a junction that uses different materials either side of the junction. The most common materials used aluminium gallium arsenide (AlGaAs) and gallium arsenide (GaAs).
Gallium arsenide is generally used because it provides a high level of basic electron mobility which is crucial to the operation of the device. Silicon is not used because it has a much lower level of electron mobility.
There is a variety of different structures that can be used within a HEMT, but all use basically the same manufacturing processes.
In the manufacture of a HEMT, first an intrinsic layer of gallium arsenide is set down on the semi-insulating gallium arsenide layer. This is only about one micron thick.
Next a very thin layer (between 30 and 60 Angstroms) of intrinsic aluminium gallium arsenide is set down on top of this. Its purpose is to ensure the separation of the hetero-junction interface from the doped aluminium gallium arsenide region. This is critical if the high electron mobility is to be achieved.
The doped layer of aluminium gallium arsenide about 500 Angstroms thick is set down above this. Precise control of the thickness of this layer is required and special techniques are required for the control of this.
There are two main structures that are used. These are the self-aligned ion implanted structure and the recess gate structure. In the case of the self-aligned ion implanted structure the gate, drain and source are set down and are generally metallic contacts, although source and drain contacts may sometimes be made from germanium. The gate is generally made from titanium, and it forms a minute reverse biased junction similar to that of the GaAsFET.
For the recess gate structure another layer of n-type gallium arsenide is set down to enable the drain and source contacts to be made. Areas are etched as shown in the diagram. The thickness under the gate is also very critical since the threshold voltage of the FET is determined by this. The size of the gate, and hence the channel is very small. Typically the gate is only 0.25 microns or less, enabling the device to have a very good high frequency performance.
The operation of the HEMT is somewhat different to other types of FET.
Electrons from the n-type region move through the crystal lattice and many remain close to the hetero-junction. These electrons form a layer that is only one electron thick forming what is known as a two dimensional electron gas. Within this region the electrons are able to move freely because there are no other donor electrons or other items with which electrons will collide and the mobility of the electrons in the gas is very high.
A bias applied to the gate formed as a Schottky barrier diode is used to modulate the number of electrons in the channel formed from the 2 D electron gas and in turn this controls the conductivity of the device. This can be compared to the more traditional types of FET where the width of the channel is changed by the gate bias.
There are several advantages to using HEMT devices:
- High gain: HEMTs have a high gain at microwave frequencies because the charge carriers are almost exclusively the majority carriers and the minority carriers are not significantly involved.
- Low noise: HEMTs provide very low noise operation because the current variation in the devices is low when compared to other field effect devices.
The HEMT was originally developed for high speed applications. It was only when the first devices were fabricated that it was discovered they exhibited a very low noise figure. This is related to the nature of the two dimensional electron gas and the fact that there are less electron collisions.
As a result of their noise performance they are widely used in low noise small signal amplifiers, power amplifiers, oscillators and mixers operating at frequencies up to 60 GHz and more and it is anticipated that ultimately devices will be widely available for frequencies up to about 100 GHz. In fact HEMT devices are used in a wide range of RF design applications including cellular telecommunications, Direct broadcast receivers - DBS, radar, radio astronomy, and any RF design application that requires a combination of low noise and very high frequency performance
HEMTs are manufactured by many semiconductor device manufacturers around the globe. They may be in the form of discrete transistors, but nowadays they are more usually incorporated into integrated circuits. These Monolithic Microwave Integrated Circuit chips, or MMICs are widely used for RF design applications, and HEMT based MMICs are widely used to provide the required level of performance in many areas.
Other HEMT based devices
There are a number of variants of the basic HEMT device. These other devices provide additional performance in some areas.
- pHEMT : The PHEMTs gains its name because it is a Pseudomorphic High Electron Mobility Transistor. These devices are extensively used in wireless communications and LNA applications.
PHEMT transistors offer a high power added efficiency combined with excellent low noise figures and performance. As a result, PHEMTs are widely used in satellite communication systems of all forms including direct broadcast satellite television, DBS-TV, where they are used in the low noise boxes, LNBs used with the satellite antennas. They are also used in general satellite communication systems as well as radar and microwave radio communications systems. PHEMT technology is also used in high-speed analogue and digital IC technology where exceedingly high speed is required.
- mHEMT : The mHEMT or metamorphic HEMT is a further development of the pHEMT. The buffer layer is made of AlInAs, with the indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the GaInAs channel. This brings the advantage that practically any Indium concentration in the channel can be utilised, so the devices can be optimized for different applications. It is found that a low indium concentration provides better low noise performance whilst a high indium concentration provides more gain.
These HEMT variants are less widely known, but able to provide some characteristics needed in some niche applications.