Avalanche photodiode

Avalanche photodiode.

An avalanche photodiode (APD) is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche multiplication. From a functional standpoint, they 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 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 (single-photon avalanche diodes) 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 diminished. 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 rangefinders and long range fiber optic telecommunication. New applications include positron emission tomography and particle physics. APD arrays are becoming commercially available.

APD applicability and usefulness depends on many parameters. Two of the larger factors are: quantum efficiency, which indicates how well incident optical photons are absorbed and then used to generate primary charge carriers; and 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, ENF. It describes the statistical noise that is inherent with the stochastic APD multiplication process. This is not to be confused with the fano noise (F), which describes the fluctuation of the total electric charge collected in the APD.

Materials

In principle any semiconductor material can be used as a multiplication region:

Excess noise

Excess noise refers to the noise due to the multiplication process at a gain, M and is denoted by ENF(M) and can often be expressed as:

\text{ENF} = \kappa M + \left(2 - \frac{1}{M}\right)\left(1 - \kappa\right)

where \kappa\, is the ratio of the hole impact ionization rate to that of electrons. For an electron multiplication device it is given by the hole impact ionization rate divided by the electron impact ionization rate. It is desirable to have a large asymmetry between these rates to minimize ENF(M), since ENF(M) is one of the main factors that limit, among other things, the best possible energy resolution obtainable.

Performance limits

In addition to excess noise, there are limits to device performance associated with the capacitance, transit times and avalanche multiplication time.[1] The capacitance increases with increasing device area and decreasing thickness. The transit times (both electrons and holes) increase with increasing thickness, implying a tradeoff between capacitance and transit time for performance. The avalanche multiplication time times the gain is given to first order by the gain-bandwidth product, which is a function of the device structure and most especially \kappa\,.

See also

References

  1. 1 2 3 Tsang, W.T., ed. (1985). Semiconductors and Semimetals. Vol. 22, Part D "Photodetectors". Academic Press.
  2. Tarof, L.E. (1991). "Planar InP/GaAs Avalanche Photodetector with Gain-Bandwidth Product in Excess of 100 GHz". Electronics Letters 27: 34–36. doi:10.1049/el:19910023.
  3. Wu, W.; Hawkins, A.R.; Bowers, J.E. (1997). "Design of InGaAs/Si avalanche photodetectors for 400-GHz gain-bandwidth product". Proceedings of SPIE 3006: 36–47. doi:10.1117/12.264251.
  4. Campbell, J. C. (2007). "Recent advances in Telecommunications Avalanche Photodiodes". IEEE Journal of Lightwave Technology 25: 109–121. doi:10.1109/JLT.2006.888481.

Further reading

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