technology photodiodes
To create such a system, groups around the world have been developing of high-power, ultra-fast photodiodes. This effort had yielded several photodiode architectures, which have been trialled in MOF wireless communication systems deliver data rates exceeding 10 Gbit/s.
Photodiodes serving this application ideally excel in both speed and output power. One way to enhance the photodiode’s speed is to reduce its parasitic capacitance, which is possible by trimming the device’s absorption volume. However, when the absorption volume falls below about 1 µm3
, the density of photo-
generated free carriers is very high, and this increases the space-charge field and photocurrent density, which screen the external applied bias field. The upshot: A severe degradation in electrical bandwidth due to a reduction in the drift velocity of the photo-generated carriers (see Figure 2 (a) for an illustration of this point).
Researchers have developed two approaches that can produce an increase in power and electrical bandwidth. These methods are based on replacing a single photodiode with several smaller ones, or turning to a superior epitaxial structure.
With the first option, high power of the optical input can be shared between several miniaturized ultra-high- speed photodiodes. The power generated by each of them can then be combined with a low-loss electrical transmission line. This approach, known as the distributed or traveling-wave photodetector (TWPD),
was first used in the era of the vacuum tube to improve the bandwidth performance of the vacuum tube amplifier. Parallel connections are the simplest way to combine the photocurrent from these small photodiodes. The major downside of this approach is a serious degradation of the RC-limited bandwidth, which stems from the hike in junction capacitance. But this can be avoided with a distributed structure (Figure 2 c). In this case, the MMW signal generated from each photodiode is coherently combined, leading to minimal broadening and distortion of the resultant signal. In an ideal case, the maximum bandwidth is as high as the bandwidth of each single photodiode.
Modifying the traditional epi-layer structure of the p-i-n photodiode is the other popular approach to improving the device’s power and speed. This can combat saturation of the device’s speed by quickening the rate at which photo-generated carriers are drained from the photodiode’s active layers. With this approach, space-charge density falls, driving down the induced electric field.
One of the best ways to do this is to speed up the carrier drift-velocity inside the photodiode. This is possible by switching to a uni-traveling carrier photodiode (UTC-PD) design (see Figure 3). This swaps an intrinsic photo-absorption layer with a p-type doped epilayer, and modifies carrier transport: Photo-generated holes relax directly into the p-contact metal without drift, diffusion or accumulation in the photo-absorption layer, and only electrons remain in the active carrier. This promises to greatly enhance carrier velocity, by
Figure 2.(a) With a single photodiode,intense light injection produces a high-output,photocurrent-induced voltage on the load,which has the opposite polarity to the bias
voltage.This reduces the net electric field in the active layer of the device, thereby slowing down the carrier drift-velocity and speed performance.(b) Splitting the optical power between an array of photodiodes that are connected in parallel increases the output saturation current,but severely degrades the RC-limited bandwidth.(c) This restriction is lifted with distributed (travelling-wave) connections,which ensure matching of the velocity and phase of the injected optical wave and the photo-generated carrier wave
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www.compoundsemiconductor.net March 2012
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