There are many different types of high speed photodetectors possible with the most appropriate kind of detector being determined by the different application. With architecture concerned, p-i-n, Metal-Semiconductor-Metal (MSM), and Avalanche Photodiodes [44, 23] are generally referred to and are discussed below.
1It is noted that avalanche photodiode has a much bigger noise current than that calculated from
(2.6) because of the excess noise factor introduced by randomness of avalanche multiplication process [41].
2This would be another case in the application of CMOS photodetector as APS (Active Pixel
2.3.1
p-i-nPhotodiode
A p-i-n photodiode consists of a p-n junction with a layer of intrinsic or lightly doped semiconductor sandwiched between the p and n layers. The intrinsic region has a small number of carriers and is easily depleted of any charge. Therefore, the depletion region is almost entirely contained in the intrinsic region. Fig. 2.3 shows a p-i-n photodiode together with its electric field [25]. Light absorbed in the semiconductor produces electron-hole pairs, however, due to the substantial electric field in the depletion (intrinsic) region, pairs produced in the depletion region or within a diffusion length of it, will be separated by the electric field leading to current flow in the external circuit.
Figure 2.3: p-i-n photodiode
Very high speed and high sensitivity compound semiconductor p-i-n photodiodes have been reported in literature. For example, a waveguide integrated p-i-n photodiode (WG PIN PD) with a 3 dB bandwidth of > 40 GHz at 1550 nm was demonstrated by Wang et al [26]. In [27], 1550 nm InP/GaInAs/InP p-i-n photodiodes have been fabricated with speeds of up to 60 GHz. However, despite the high bandwidth and high sensitivity clearly achievable with compound semiconductors, there is an eagerness to produce silicon long wavelength photodetectors. This is, again, due to the low cost, large scale integration achievable with silicon. A possible means of extending the spectral response of silicon into the long wavelength region has been to use heterostructures composed of silicon (Si) and germanium (Ge). Many successful reports of p-i-n SiGe/Si superlattice photodetectors have been made
for wavelengths of 850-1300 nm. For instance, Temkin et al developed a 1300 nm GeSi/Si waveguide p-i-n photodiode that operated at speed > 1 GHz and had an internal quantum efficiency of 40% [29], while Tashiro et al demonstrated a 10.5 GHz, 980 nm planar p-i-n SiGe/Si diode with an external quantum efficiency of 25-29% [30].
2.3.2
MSM Photodiode
a MSM PD is comprised of back-to-back Schottky diodes that use an interdigitated electrode configuration on an undopped semiconductor layer, as shown in Fig. 2.4. When light with energy hv> Egis incident, the light that hits the semiconductor
surface is absorbed and creates electron-hole pairs (EHs) within the active region, and then one set of electrodes acts as a cathode and the other as an anode. The holes drift toward the negative electrodes, and electrons travel to the positive electrodes under the influence of an electric field by an applied reverse bias voltage [32, 33].
The metal electrode fingers have finger width w and are separated by a distance s.
High photodiode quantum efficiency requires s≫w, or low electrode shadowing as
it is termed. Absorbed photons generate electron-hole pairs in the semiconductor, the holes drift with the applied electric field to the negative contacts while the
electrons drift to the positive contacts forming a current (iphoto). The bandwidth
of a MSM diode is similar to that of a p-i-n detector in that it is both RC time constant and transit time limited.
Figure 2.4: MSM photodiode cross section and top view
MSM photodiodes have simple, planar structures and can easily be fabricated with FET (field effect transistor) processes provided that the substrate is highly resistive or intrinsic epitaxy layer is available. They have also been shown to operate to very high speeds with bandwidths of 510 GHz on low temperature (LT) GaAs and 110 GHz on bulk silicon reported [34, 35]. The silicon photodiode, however, achieved this speed at very short wavelengths (400 nm) and at longer infrared wavelengths (800 nm) this speed became diffusion limited.
Although superior for its least complexity and high speed performance, MSM photodiodes are always associated with problems of quite low responsivity because of the reflection from the surface metals and semiconductor surface; the finite carrier lifetime as the carriers traverse the gap between the electrodes before being collected; absorption of incident light outside the region in which photogenerated carriers can be collected by the electrodes; and surface recombination currents and deep traps within the semiconductor material which may lower the detected optical signal. Furthermore, similar to p-i-n photodiode, it is not compatible with CMOS process.
2.3.3
Avalanche Photodiode
Avalanche photodiodes differ from p-i-n and MSM diodes in that they incorporate a high field region that multiplies the photocurrent through the avalanche generation
of additional electron-hole pairs. The operation of PIN diodes is based on the generation of one electron-hole pair for each photon entering the lattice. avalanche photodiodes (APDs), on the other hand, the generated electrons and holes carry so much energy that they themselves can stimulate other electrons and holes, creating an avalanche effect.
APDs operate with controlled avalanche, that is, with a multiplication factor, M, of several hundred. Thus, each photon entering the device may create hundreds of of electron-hole pairs, providing a large output current. Shown in Fig. 2.5 is the structure of a typical APD, consisting of a sandwich of n+, p, and i layers atop a p+ substrate. As in PIN diodes, the intrinsic region enables generation of electron-hole pairs. Grown as a very uniform and thin layer, the p region supports a high electric field to create avalanche.
Besides the internal gain and high responsivity, APDs suffer from ”gain-bandwidth” trade off. That is, the higher the multiplication factor is, the longer the avalanche persists, limiting the changes of the current and the response to high frequency signal. Secondly, the reverse bias voltage applied to APDs must be controlled precisely so as to achieve avalanche while avoiding break down. Furthermore, it is incorporated with significant noise because of the noisy avalanche process [25].