Electro-Optics - Applied Optics Fundamentals and Device Applications - page 15

results in short transit times beneath the gate region, thereby increasing the
available frequency of operation for the device. Standard low-noise discrete
GASFETs have gate lengths as short as 0.25 μm. Gate lengths as short as
0.1 μm have been reported in the literature, but MMIC devices are generally
limited to 0.5 μm gate lengths due to process limitations and low yields.
The simple structure of the GASFET along with its superior frequency
response compared to JFETs and bipolar transistors have made it the
fundamental building block of MMIC technology. Heterostructure and
superlattice devices may eventually replace the MESFET, but not before
processing technology matures considerably. In addition to the ion implan-
tation fabrication technology, molecular beam epitaxy (MBE) and metal-
organic chemical vapor deposition (MOCVD) are used in many MMIC
processes.
2.2.3 Optical Control of Microwave Devices
The control of microwave devices and circuits using optical rather than
electronic signals has gained much attention in the past few years [19].
Experimental studies have been ongoing since the 1960s, but only in the
past several years has the experimentation come to fruition in devices and
systems. New high-speed electro-optic devices and fiber-optic distribution
systems are the main reason for bringing the results of these earlier experi-
mental studies to the applications arena, thus increasing the interest in con-
trolling microwave devices by optical means.
The reasons for optically controlling microwave devices and circuits are
many. First, microwave devices and systems are becoming more and more
complicated and sophisticated. They require faster control and higher mod-
ulation rates. Using optical illumination as a source of control is one way
to fulfill these requirements [20]. Optical control is faster because an opti-
cal signal does not encounter the inherent delays that an electrical signal
encounters such as rise-time RC time constants in control circuitry and
cabling. Second, optical control yields greater isolation of the control signal
from the microwave signal. It is also much simpler to design the optical con-
trol circuitry than the electronic control circuitry because the electronic cir-
cuitry couples unwanted signals into the output if great care is not taken in
the design of the optical control circuitry itself. Greater isolation improves
the spurious response of the output signal as well as decreases the design
complexity of the control circuitry. Both of these factors result in optically
controlled devices having a better output noise specification at a lower cost.
Another benefit of using optical control is that electro-optic and microwave
devices can be fabricated on a monolithic integrated circuit because of the
similarity in material and fabrication techniques. This results in smaller
lighter weight components at a potentially lower cost once the fabrica-
tion processes are developed. Compatibility with optical fibers is another
advantage of optical control. Optical fibers are becoming less costly, and are