Integrated Optics - Applied Optics Fundamentals and Device Applications - page 265

impurities adversely affect the energy gap of the AlGAAs and the epitaxial
match lattice parameters for single crystal growth and interface require-
ments, as well as adversely altering doping carrier concentrations and estab-
lishing unwanted localized or delocalized energy band states.
Growth of optoelectronic waveguides and submicron devices in the ultra-
high vacuum conditions (10 −14 Torr) behind the wake shield of the NASA
Space Shuttle (a working volume of about 50 m 3 ) or a space-based manufac-
turing facility will greatly enhance device performance. Improved perfor-
mance factors include superior confinement of the exciton cloud in the layer
of lower E g for ultra-fast switching and modulation of optical quantum well
devices; better homogeneity of layers and waveguides, and constancy in the
mismatch of refractive indices throughout the waveguide boundary inter-
face of an optical coupler of interferometric device; and improved gain char-
acteristics of high electron mobility transistors.
5.11 GaAsFoundryCapabilities
About 25 years ago the first GaAs ICs were introduced in the form of micro-
wave prescalers. A parallel may be seen between the development of silicon
based industries and the growth pattern of the GaAs industry. First came
the transistors, then simple logic applications such as flip-flops, and from
this arose full multifunction monolithic capability. At this point, high qual-
ity GaAs substrate material is being achieved and several foundries are in
mass production of MMICs for the microwave industry. A major application
of this MMIC technology centers on the need for low cost highly reliable
microwave components for Phased Array Radar Transmit/Receive Modules.
Optical applications of MMIC technology are discussed in a later section.
MMIC processing and foundry capabilities are directly applicable to the
requirements and problems of optical device fabrication.
Several wide bandwidth functional building blocks are available for
design use, including switches, phase shifters, and medium performance
amplifiers, with the GASFET being the key building block. Applications
and the GASFET technology are discussed further in a later section. The
designer will save valuable development time and money by adjusting
the circuit architecture to take advantage of available functional blocks.
While the discrete GASFET manufacturers are producing devices for low
noise applications with gate lengths of less than 0.3 μm. MMIC foundries
are capable of routinely producing only 0.5 μm FETs, restricting the micro-
wave designer to a “medium low noise amplifier” (implying a noise figure
of 3 dB at 12 GHz, for example). Heterostructure devices, such as the high
electron mobility transistor (HEMT), are currently in production at MMIC
foundries.