Schematic wavevector diagrams depicting (a) electron-initiated and (b) hole-initiated impact ionisation events.ĭue to conservation of energy and momentum, a threshold energy, E th, prerequisite has to be satisfied by the primary carrier. A straightforward comparison between InP and InAlAs APDs will then be presented with an analysis on the difference. The results of the BER calculations on receiver systems using InP APDs will be presented, followed by a discussion on the competing effects of performance-determining factors. A comprehensive assessment of the measurement systems reported in the literature is also provided followed by two suggestions for an improved design. In this chapter, we will describe the model used to investigate the receiver-sensitivity-optimisation of InP and InAlAs APDs, which include dark current contributions from tunnelling current. Several efforts have been made to systematically characterise promising detector material systems including InP and InAlAs. Characterisation of the APD excess noise factor in test structures is also necessary in order to model the BER of an APD-based receiver system. Such models have been developed but none included some form of dark current mechanism, which can significantly affect the receiver’s sensitivity. It is, therefore, very useful and interesting to model the sensitivity of an APD-based receiver system accurately. Thus, careful attention is required when determining the multiplication layer thickness for an optimum APD design. On the other hand, the increase in the field in thin layers accentuates tunnelling currents at exponential rates ( Forrest et al., 1980a). Reducing the thickness of the multiplication layer serves to reduce the excess noise factor, due to the dead space effect, ( Li et al., 1998) and minimise ISI via reducing carrier transit times across the avalanche region. More importantly, changing the thickness of the multiplication layer strongly affects the receiver sensitivity, as the aforementioned three factors change. Thus, for a fixed multiplication layer thickness, there is a sensitivity-optimised gain that offers a balance between SNR while keeping the degrading contributions from the excess noise factor and intersymbol-interference (ISI) at a minimum. Generally, the excess noise and avalanche-buildup time increases with APD gain. The sensitivity of APD-based high speed optical receivers is governed by three main competing factors, namely the excess noise, avalanche-buildup time and dark current of the APD. The receiver sensitivity is defined as the minimum average optical power to operate at a certain BER 10 -12 being a common standard for digital optical receivers. The sensitivity performance criterion for digital receivers is its bit-error rate (BER), which is the probability of an error in the bit-identification by the receiver. Studies have also shown that the breakdown voltage of InAlAs APDs is less temperature dependent compared to InP ( Tan et al., 2010), which would be useful in temperature sensitive applications, thus making temperature control less critical. While holes ionise more readily than electrons in InP, the opposite holds true for InAlAs and InGaAs, as electrons ionise more readily than holes thus making the InGaAs/InAlAs combination superior to InGaAs/InP in a SAM APD, in terms of lower excess noise, higher gain-bandwidth product, and improved sensitivity. In comparison to InP, tunnelling currents remain lower in InAlAs due to its larger bandgap. It has been predicted that Indium Alluminium Arsenide (In 0.52Al 0.48As) will replace InP, as a more favourable multiplication layer material due to its lower excess noise characteristics ( Kinsey et al., 2000). ![]() Indium Phosphide (InP) is widely used as the multiplication layer material in commercially available APDs for applications in the 0.9–1.7µm wavelength region with In 0.53Ga 0.47As grown lattice-matched to it as the absorption layer. This increases the signal-to-noise ratio (SNR) and ultimately improves the receiver sensitivity as the gain increases until the APD noise rises to become dominant. In an optical receiver system, the advantage of internal gain, in the APD, is experienced when the amplifier noise dominates that of a unity-gain photodiode. Its internal mechanism of gain or avalanche multiplication is a result of successive impact ionisation events. ![]() The avalanche photodiode (APD) is widely used in optical fibre communications ( Campbell, 2007) due to its ability to achieve high internal gain at relatively high speeds and low excess noise ( Wei et al., 2002), thus improving the system signal-to-noise ratio.
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