Electro-Optics - Applied Optics Fundamentals and Device Applications - page 45

commercial vendors. Space and power limitations become unacceptable,
however, when considering these devices for use in smaller and faster com-
puting environments. Such environments necessitate the development of
lower power and high-density devices and packages [63]. In particular, pack-
aging methods must be developed to allow high-speed silicon and GaAs
components to be integrated with optoelectronic components and wave-
guides [64]. Direct integration of optoelectronic devices with conventional
logic devices will increase density and reliability of the interconnect because
the electrical-optical interface occurs on chip [65], eliminating the need for
hybrid or separate packaging techniques for the optics.
If a photodetector is to be integrated on a circuit with more than a few
electronic components, it must meet a number of criteria [66]. First, the detec-
tor must be compatible with electronics processing. It must therefore be
processed on a production line and be compatible with the substrates used
for the electronics. For GaAs, this means at least a 3 in. semi-insulating sub-
strate. Second, the material for the detector cannot interfere with the elec-
tronics. Thus, if epitaxial material is required, it must be excluded from the
regions in which the electronics will be fabricated and the transition to the
epitaxial region must be smooth enough to permit fine-line photolithogra-
phy. Third, any process needed to fabricate the detector cannot degrade the
performance of the electronics. For example, a very high temperature step
may cause unacceptable surface damage. Fourth, the detector and electron-
ics must be adequately isolated on the substrate.
Finally, the integrated photodetector must meet the overall receiver system
specifications. The receiver's function is to convert the optical signal to an
electrical signal compatible with digital electronics. This requires coupling
the input fiber to the detector, designing and fabricating a detector with suffi-
cient bandwidth and sensitivity, interfacing the detector with a preamplifier,
and converting the analog signal from the preamplifier to a digital signal.
The sensitivity of the receiver depends strongly on the node capacitance,
with the improvement being most significant for a reduction in capacitance
at the highest bit rates. For example, a reduction in mean detectable optical
power of nearly 5 dB is possible if the front-end capacitance is reduced from
a relatively good hybrid receiver value of 1 F to a value of 0.2 pF at B = 1 Gb/s.
This additional margin might then be used to permit a less-expensive cou-
pling scheme or to power split from a laser to multiple detectors in an opti-
cal bus. Alternatively, the fivefold decrease in capacitance would permit the
detector amplifier to be operated at nearly five times the bit rate with no deg-
radation in accuracy. This is a strong argument for integration.
While the integration of the detector with the amplifier may prove to be
the limiting factor on the speed of the optical link due to circuit limitations,
the integration of the laser with associated electronics will be more diffi-
cult from a materials and processing compatibility standpoint. There are
three reasons for this. Lasers require multilayered heterostructures up to
7 μm thick. They also need two parallel mirrors separated by on the order