Integrated Optics - Applied Optics Fundamentals and Device Applications - page 236

the MBE process results in a better interface than the growth of the inverted
structure, that is, GaAs on AlGaAs. This has been observed in a compari-
son of direct and inverted HEMT devices. Such a difference has not been
reported for structures grown by the MOCVD technique. One final compari-
son between the two growth techniques involves monolithic integration of
optical and electronic devices. This will probably require selective epitaxial
growth, that is, growing well-controlled AlGaAs/GaAs layers on selected
portions of the wafer. To date MOCVD has been more promising than MBE
for selective epitaxy.
5.5.5 Microwave and Electronic Circuit Compatibility
The device structures and fabrication processes for AlGaAs waveguide
devices are inherently compatible with those for microwave and electronic
integrated circuits because the same materials and processing techniques are
used. Both field effect transistors (FET) and bipolar transistors can be fabri-
cated in AlGaAs structures on a GaAs substrate. In fact, high performance
microwave transistors and high speed electronic switching transistors such
as the modulation-doped FET (MODFET), the high electron mobility transis-
tor (HEMT), and the heterojunction bipolar transistor (HBT), are all based on
AlGaAs system heterojunctions.
Most electronic integrated circuits are currently fabricated in silicon
because of the well-developed fabrication technology for that material.
Research and development, however, has clearly demonstrated that GaAs
integrated circuits are capable of much higher frequency operation because
of the greater electron mobility and scattering limited velocity in GaAs rela-
tive to silicon. The fabrication processes used to make these GaAs integrated
circuits are essentially the same as those used to make optical devices on
GaAs substrates. Additional details regarding the monolithic integration of
optical, microwave, and electronic devices on a single GaAs substrate are
contained in the next section.
5.5.6 Integratability
When hybrid integration offers the advantage of expediency, fully mono-
lithic integration of all microwave and optical elements on the same sub-
strate to form an optomicrowave integrated circuit (OMMIC) is the ultimate
preferred approach, because it offers the greatest reliability and the lowest
cost once a high-volume production line is established. There are, of course,
some problems of integratability that must be dealt with. Optical integrated
circuits require low-loss transmission of optical signals, efficient genera-
tion and detection of optical power at the desired wavelength, and effi-
cient modulation and switching of optical signals with a wide bandwidth.
Microwave integrated circuits require low-loss transmission of microwave
signals and minimum electromagnetic coupling (crosstalk) between various