Updated July 9,
here to go to our main page on wafer processing
here to go to our page on microwave semiconductor tradeoffs
to go to our page on growing semiconductor boules
to go to our main page on MMICs
to go to our main page on FETs
Fresh from the boule, GaAs wafers
need a whole ton of processing to become useful bits in your cell
phone or airborne radar. We are microwave guys, so we are gonna
give you the short version of this. The first thing that needs to
be done is to "grow" some specific layers onto the wafers.
These layers form the basis for microwave semiconductors; wafers
that have these layers are called "starting material".
At this point your material must be kept in a clean room, preferably
Here's a picture of six-inch
GaAs starting material given to us by Xpert Semiconductor of Taiwan.
Xpert is a supplier of epitaxially-grown GaAs wafers, in both four
and six inch diameters. Thanks, Angela!
In almost all microwave applications,
N-type doping is used. The principal N-type dopant that is used
in the GaAs industry is silicon (group IV on the periodic
chart). N-type dopant atoms are referred to as donors, because
they have an extra electron that is free to move about the lattice.
If P-type doping is required (as in PIN diodes), beryllium (group
II on the periodic chart) is often used. P-type dopant atoms are
referred to as acceptors, because they are missing an electron,
so they provide a "hole" that can also move about (thanks,
The semiconductor area of a FET
is built on top of the semi-insulating substrate. Layers of N-type
material are grown on top of the substrate. The lowest layer is
called the buffer layer, it forms the transition from the semi-insulating
to the semiconducting layers. The channel layer (doped with N) is
where the FETs are realized. A top layer of N+ is added above the
channel, which is used as the contact area for the ohmic source
and drain contacts. The N+ must later be removed where the FET gates
make Schottky contacts to the channel. Saying that FETs need only
two layers of semiconductor (N and N+) is a huge over-simplification.
A lot of time and money has been spent by GaAs IC foundries perfecting
different profiles within the channel, and different RF requirements
such as low noise figure or high power require different starting
The doped layers are grown or
implanted on top of the semi-insulating GaAs using epitaxy or ion
implantation. Epitaxy involves building up additional crystalline
layers on top of the starting material, either through metal-organic
chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
Ion implantation is a high-volume
method most often used in the simplest MESFETs. In this process
no new material is grown, but the starting material is bombarded
with atoms of the required dopants to create the N and N+ layers.
Different energy doses are used to slam the dopants to different
depths, which provides the versatility of different layers. Ion
implantation tends to add stress to the GaAs wafer which must be
considered; wafer annealing is often required after implant to reduce
the stress. Implantation is a cheap, high-volume process that originated
in the silicon world.
The active channel of PHEMT is
a mixture of indium and gallium in arsenide. In28Ga72As means that
28 percent of the GaAs atoms have been replaced with indium.
Binary and ternary compounds
are further discussed on this