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The avalanche photodiode (APD), is also reverse-biased. The difference with the PIN diode is that the absorption of a photon of incoming light may set off an electron-hole pair avalanche breakdown, creating up to 100 more electron-hole pairs. This feature gives the APD high sensitivity (much greater than the PIN diode).
A PIN diode (p-type, intrinsic, n-type diode) is a diode with a wide, undoped intrinsic semiconductor region between p-type semiconductor and n-type semiconductor regions.
PIN diodes act as near perfect reistors at RF and microwave frequencies. The resistivity is depenedent on the DC current applied to the diode.
A PIN diode exhibits an increase in its electrical conductivity as a function of the intensity, wavelength, and modulation rate of the incident radiation.
The benefit of a PIN diode is that the depletion region exists almost completely within the intrinsic region, which is a constant width (or almost constant) regardless of the reverse bias applied to the diode. This intrinsic region can be made large, increasing the area where electron-hole pairs can be generated. For these reasons many photodetector devices include at least one PIN diode in their construction, for example PIN photodiodes and phototransistors (in which the base-collector junction is a PIN diode).
They are not limited in speed by the capacitance between n and p region anymore, but by the time the electrons need to drift across the undoped region.
Avalanche photodiodes (APDs) are photodetectors that can be regarded as the semiconductor analog to photomultipliers. By applying a high reverse bias voltage (typically 100-200 V in silicon), APDs show an internal current gain effect (around 100) due to impact ionization (avalanche effect). However, some silicon APDs employ alternative doping and beveling techniques compared to traditional APDs that allow greater voltage to be applied (> 1500 V) before breakdown is reached and hence a greater operating gain (> 1000). In general, the higher the reverse voltage the higher the gain. Among the various expressions for the APD multiplication factor (M), an instructive expression is given by the formula
M = \frac{1}{1 - \int_0^L\alpha(x)\, dx}
where L is the space charge boundary for electrons and \alpha\, is the multiplication coefficient for electrons (and holes). This coefficient has a strong dependence on the applied electric field strength, temperature, and doping profile. Since APD gain varies strongly with the applied reverse bias and temperature, it is necessary to control the reverse voltage in order to keep a stable gain. Avalanche photodiodes therefore are more sensitive compared to other semiconductor photodiodes.
If very high gain is needed (105 to 106), certain APDs can be operated with a reverse voltage above the APD's breakdown voltage. In this case, the APD needs to have its signal current limited and quickly diminshed. Active and passive current quenching techniques have been used for this purpose. APDs that operate in this high-gain regime are in Geiger mode. This mode is particularly useful for single photon detection provided that the dark count event rate is sufficiently low.
A typical application for APDs is laser range finders and long range fiber optic telecommunication. New applications include positron emission tomography and particle physics [1]. APD arrays are becoming commercially available.
APD applicability and usefulness depends on many parameters. Some of the larger factors are: quantum efficiency which is an indication of how well incident optical photons are absorbed and then used to generate primary charge carriers, total leakage current which is the sum of the dark current and photocurrent and noise. Electronic dark noise components are series and parallel noise. Series noise, which is the effect of shot noise, is basically proportional to the APD capacitance while the parallel noise is associated with the fluctuations of the APD bulk and surface dark currents. Another noise source is the excess noise factor (F). It describes the statisitical noise that is inherent with the stochastic APD multiplication process.
A photodiode is a semiconductor diode that functions as a photodetector. Photodiodes are packaged with either a window or optical fibre connection, in order to let in the light to the sensitive part of the device. They may also be used without a window to detect vacuum UV or X-rays.
A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent case so that light can reach the base-collector junction. The phototransistor works like a photodiode, but with a much higher sensitivity for light, because the electrons that are generated by photons in base-collector junction are injected into the base, this current is then amplified by the transistor operation. A phototransistor has a slower response time than a photodiode however.
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