Electro-Optics - Applied Optics Fundamentals and Device Applications - page 14

Source Gate Drain

n channel

n + n +

Proton
isolation

Semi-insulating
GaAs substrate

Single
GASFET

FIGURE 2.8
Integrated GASFET structure cross section. (Adapted from K. Wang and S. Wang, “State-of-the-
art ion-implanted low-noise GaAs MESEFETs and high-performance monolithic amplifiers.”)

are implanted into the source and drain contact regions to help reduce the
source resistance. The FET source and drain ohmic contacts are formed next
using gold alloys. High temperatures are used to bond the contacts and to
ensure smooth edges. The contact photolithography process continues with
the formation of the gate metallization. This is the most critical step of the
process since the gate metallization is typically 0.5 μm long by 300 μm wide
and forms the space charge, or depletion, region. The gate metallization is
generally an aluminum alloy.
The MESFET in Figure 2.8 has a gate made of titanium and aluminum
and then overlayered with a titanium and gold mixture for a low-resistance
bonding. In MMIC fabrication, device isolation is an important factor in
reducing RF losses. Good isolation and a reduction in pad capacitance are
achieved, as shown in Figure 2.8, by direct proton bombardment [17].
The basic operation of the MESFET has been described by Liao [18]: “A volt-
age is applied in the direction to reverse bias the in + in junction between the
source and gate, while the source and drain electrodes are forward biased.
Under this bias condition the majority carrier electrons flow in the n-type
layer from the source electrode, through the channel beneath the gate, to
the drain electrode. The current in the channel causes a voltage drop along
its length so that the Schottky barrier gate electrode becomes progressively
more reverse biased toward the drain electrode. As a result, a charge deple-
tion region is set up in the channel and gradually pinches off the channel
against the semi-insulating substrate toward the gate end. As the reverse
bias between the source and the gate region increases, so does the height
of the charge depletion region. The decrease of the channel height in the
nonpinched-off region will increase the channel resistance. Consequently,
the drain current I D will be modulated by the gate voltage.”
In the microwave domain, the MESFET design has the advantage of a very
short gate length that in conjunction with the high electron mobility of GaAs