The high electron mobility transistor (HEMT)
The High Electron Mobility Transistor or HEMT is a form of field effect
transistor (FET) that is used to provide very high levels of performance at
microwave frequencies. It offers a combination of low noise figure combined with
the ability to operate at microwave frequencies.
The development of the HEMT took many years and it was not until many years
after the basic FET was established that the HEMT appeared on the market. The
specific mode of carrier transport used in HEMTs was first investigated in 1969,
but it was not until 1980 that the first experimental devices were available.
During the 1980s they started to be used, but in view of their initial very high
cost their use was considerably limited. Now with their cost somewhat less, they
are more widely used, even finding uses in the mobile handset markets.
Construction
The key element within a HEMT is thespecialised pn junction that it uses. It is
known as a hetero-junction and consists of a junction that uses different
materials either side fot he 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 and
this is crucial to the operation of the device. Silicon has a much lower level
of electron mobility and as a result it is never used in a HEMT.
There is a variety of different structures that can be used within a HEMT,
but all use basically the same manufacturing processes.
First an intrinsic layer of gallium arsenide is set down on the
semi-insulating gallium arsenide layer. This is only about one micron thick.
about one micron thick is set down. 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 as shown in the
diagrams. 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.,/p>
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.
Operation
The operation of the HEMT is somewhat different to other types of FET and as a
result it is able to give a very much improved performance over the standard
junction or MOS FETs.
Electrons from the n-type region move through the crystal lattice and many
remain close to the hetero-junction. These electrons for a layer that is only
one layer 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.
Applications
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.
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