148 CHAPTER 4. OPTICAL RECEIVERS
quantum efficiency can be maintained. Two approaches have been used to meet these
somewhat conflicting design requirements. In one design, a FP cavity is formed to
enhance the absorption within a thin layer through multiple round trips. An external
quantum efficiency of∼70% and a gain–bandwidth product of 270 GHz were realized
in such a 1.55-μm APD using a 60-nm-thick absorbing layer with a 200-nm-thick
multiplication layer [40]. In another approach, an optical waveguide is used into which
the incident light is edge coupled [41]. Both of these approaches reduce the bias voltage
to near 10 V, maintain high efficiency, and reduce the transit time to∼1 ps. Such APDs
are suitable for making 10-Gb/s optical receivers.
4.2.4 MSM Photodetectors
In metal–semiconductor–metal (MSM) photodetectors, a semiconductor absorbing layer
is sandwiched between two metals, forming a Schottky barrier at each metal–semicon-
ductor interface that prevents flow of electrons from the metal to the semiconductor.
Similar to ap–i–nphotodiode, electron–hole pairs generated through photoabsorption
flow toward the metal contacts, resulting in a photocurrent that is a measure of the in-
cident optical power, as indicated in Eq. (4.1.1). For practical reasons, the two metal
contacts are made on the same (top) side of the epitaxially grown absorbing layer by
using aninterdigitedelectrode structure with a finger spacing of about 1μm [42]. This
scheme results in a planar structure with an inherently low parasitic capacitance that
allows high-speed operation (up to 300 GHz) of MSM photodetectors. If the light is
incident from the electrode side, the responsivity of a MSM photodetector is reduced
because of its blockage by the opaque electrodes. This problem can be solved by back
illumination if the substrate is transparent to the incident light.
GaAs-based MSM photodetectors were developed throughout the 1980s and ex-
hibit excellent operating characteristics [42]. The development of InGaAs-based MSM
photodetectors, suitable for lightwave systems operating in the range 1.3–1.6μm,
started in the late 1980s, with most progress made during the 1990s [43]–[52]. The
major problem with InGaAs is its relatively lowSchottky-barrier height(about 0.2 eV).
This problem was solved by introducing a thin layer of InP or InAlAs between the In-
GaAs layer and the metal contact. Such a layer, called thebarrier-enhancement layer,
improves the performance of InGaAs MSM photodetectors drastically. The use of a
20-nm-thick InAlAs barrier-enhancement layer resulted in 1992 in 1.3-μm MSM pho-
todetectors exhibiting 92% quantum efficiency (through back illumination) with a low
dark current [44]. A packaged device had a bandwidth of 4 GHz despite a large 150
μm diameter. If top illumination is desirable for processing or packaging reasons, the
responsivity can be enhanced by using semitransparent metal contacts. In one experi-
ment, the responsivity at 1.55μm increased from 0.4 to 0.7 A/W when the thickness of
gold contact was reduced from 100 to 10 nm [45]. In another approach, the structure
is separated from the host substrate and bonded to a silicon substrate with the inter-
digited contact on bottom. Such an “inverted” MSM photodetector then exhibits high
responsivity when illuminated from the top [46].
The temporal response of MSM photodetectors is generally different under back
and top illuminations [47]. In particular, the bandwidth∆fis larger by about a factor
of 2 for top illumination, although the responsivity is reduced because of metal shad-