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02 Nov 2005
Measuring low light levels can be a challenge, especially at high speeds. Tim Stokes explains why avalanche photodiodes are often an attractive solution to the problem.
Avalanche Photodiodes (APDs) are semiconductor detectors of light that exploit a built-in gain region to achieve an optical sensitivity beyond that of conventional PIN-photodiodes. The result is a very sensitive light sensor that is capable of high-speed operation and ideal for applications such as optical communications, scientific apparatus and industrial inspection.
Although a wide range of optical detectors such as conventional photodiodes, photomultiplier tubes (PMTs) and pyroelectric detectors are available on the market, APDs are the preferred choice if light levels are limited (microwatts or nanowatts) and a fast response (up to gigahertz) is required.
Internal amplification
An APD's internal gain is generated by an electron multiplication process which gives the device its avalanche title. As in the case of conventional photodiodes, incident photons striking the detector create electron-hole pairs (charge carriers) in the device's depletion layer (see figure 1).
However, with an APD a large external voltage (typically from 100 to 300 V) is applied to create a strong electric field across the device. This field causes these light-generated carriers to move towards the N and P sections of a semiconductor P-N junction at a speed of up to 100 km/s.
En route, these carriers collide with atoms in the crystal lattice and if the electric field is strong enough (around 105 V/cm) then the carriers gain enough kinetic energy to ionize the atoms creating more electron-hole pairs. The effect repeats itself like an avalanche, resulting in a gain in the number of carriers generated for a single incident photon.
APD gain is typically in the range from ×10 to ×300 for most commercial devices, but there are APDs available from specialist manufacturers with gains of thousands.
Most commonly available APDs are fabricated from silicon and employ a so-called "reach through" structure where light is incident from the N-side of the silicon. These devices are sensitive to light in the visible and near-infrared (450-1000 nm wavelength range). By designing devices where light is incident from the P-side, the sensitivity to UV and blue light can be enhanced and operation can stretch down to wavelengths as short as 200 nm.
Wavelength response
As with regular photodiodes the longest wavelength that can be detected is determined by the bandgap of the detector material - the smaller the bandgap energy, the longer the detectable wavelength. Silicon has a bandgap energy of 1.12 eV at room temperature, which translates to a cut-off wavelength of around 1100 nm.
As the layer within the APD structure that gives rise to the "gain" is thinner than a regular PIN photodiode, the wavelength of peak response for silicon APDs tends to be between 600 and 800 nm, somewhat shorter than the 900-1000 nm for a regular silicon photodiode.
Deep depletion silicon APDs, which are highly sensitive in the 900 to 1100 nm waveband range, are available but these generally have the disadvantage of requiring a much higher bias voltage which leads to more noise (larger dark current).
To detect longer wavelengths an alternative semiconductor material with a smaller bandgap is required and germanium (Ge), or indium gallium arsenide (InGaAs), are two popular options. APDs fabricated from these materials operate in the 900-1700 nm wavelength range and tend to have lower gains (×10).
A wide selection of silicon APDs can be found on the market today. Sizes range from <100 μm to several centimetres in diameter with a variety of packages, including TO metal cans, ceramic chip carriers and, more recently, surface-mounted devices. In contrast, the range of commercial Ge and InGaAs APDs is much smaller, since they are used predominantly for specific applications such as optical communications.Noise
All semiconductor detectors suffer from noise. This manifests itself as an unwanted variation in the electrical current (electrons) that is measured.
Dark current One such source of noise is "dark current" which is current that the detector produces even in the absence of a light signal due to thermal generation of electron-hole pairs. In practice, this dark current determines the minimum amount of light that can be detected.
In an APD, dark current is generated both from leakage at the surface of the diode and from electron-holes which are thermally generated and then multiplied in the gain region. Consequently, increasing the gain of the APD by increasing the external bias also increases this dark current.
Excess noise The APD multiplication process also produces additional noise, known as "excess noise", which is due to the statistical nature of the ionization in the avalanche region. Each ionization event has a certain probability of occurring and the overall gain from the device is the statistical average of all of these individual events.
The consequence of this is twofold. As the APD gain increases the output signal increases linearly, but the noise increases as shown in figure 2. This means that for any APD there is an optimum operating gain where the maximum signal to noise performance can be obtained. This point is usually well below the actual maximum gain for that APD. Manufacturers usually optimize the set-up of the APD at the factory prior to supplying the customer with a complete module that is ready to use.
Even though an APD gives an amplified output, its signal-to-noise performance (SNR) is not necessarily better than that of conventional photodiodes, as APDs suffer from more shot noise (random fluctuations in the current flow). This is a consequence of the excess noise factor, which does not appear when regular photodiodes are used. In practical systems the SNR tends to be improved in higher bandwidth applications by use of an APD over a conventional photodiode, whereas in lower speed applications use a low noise photodiode with a high performance op-amp, generally gives superior SNR.
Response speed
In order for a regular photodiode to detect lower light levels, the gain in its operating circuit is usually increased by using a larger feedback resistor. Unfortunately, this has the consequence of reducing the speed of response and increasing the thermal noise associated with the operating circuit.
In contrast, an APD allows the gain to be increased while maintaining the speed of response. For example, a 0.5 mm diameter silicon APD will operate at close to 1 Ghz, while a 30 μm InGaAs APD can work up to 10 Gbit/s. This performance advantage is the main reason why APDs are popular in applications such as optical data transmission, rangefinding, high-speed industrial inspection and medical and scientific instrumentation.
Single photon counting
Provided that the noise of the APD is low enough, it is also possible to operate an APD in "Geiger mode" to perform single photon counting. In this case, the APD has to be operated at a few volts above its breakdown voltage, which is typically around 150 to 300 V. In order to operate in this regime extremely stable operating conditions are required, which necessitates a carefully controlled temperature and a special power supply. If this is not the case, the noise of the detector will simply "run away".
Such photon counting APDs are starting to challenge more established photomultiplier tube (PMT) technology, due to the higher quantum efficiencies of the semiconductor device. However, we should add a note of caution here as such highly stable, highly sensitive APD systems are often more expensive than a comparable PMT.
What's more, such low noise APDs are generally only hundreds of micrometres (or smaller) in diameter and thus more light can be lost in the optical collection system than may be gained from the higher quantum efficiency of the detector itself. For the majority of applications, the larger detection area, higher gain and superior SNR of the PMT will still make it the detector of choice for this kind of application.
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