Gain, Bandwidth, and Signal-to-Noise Ratio of Photodetectors
In optical communication systems, the sensitivity (which depends on gain) of the photodetector and its response time (bandwidth) are of critical importance. It is common to express the gain-bandwidth product as a figure of merit for detectors.
- Typical gain-bandwidth characteristics of such a SACM APD are shown in Fig. and are limited by the transit time of carriers through the structure.
- Unfortunately, designs that increase gain tend to decrease bandwidth and vice versa. It is common to express the gain-bandwidth product as a figure of merit for detectors.
- In a p-i-n diode, there is no gain mechanism, since at most one electron-hole pair is collected by the junction for each photon absorbed.
- Thus, the gain is essentially unity, and the gain-bandwidth product is determined by the bandwidth, or frequency response.
- In a p-i-n, the response time is dependent on the width of the depletion region.
- Another important property of detectors is the signal-to-noise ratio, which is the amount of usable information compared with the background noise in the detector.
- In the case of photoconductors, the major source of noise is random thermal motion of the carriers, leading to fluctuations in the dark current (called Johnson noise).
- The noise current increases with temperature (~kT) and with the conductance of the material in the dark.
- Therefore, the photoconductor noise at a given temperature can be reduced by increasing the dark resistance. Yet another source of noise at low frequencies is 1/f or flicker noise, due to carrier trapping and detrapping at defects.
- In a p-i-n diode, the dark current is smaller and the dark resistance much higher than in a photoconductor, and the main source of noise is random thermal generation-recombination of EHPs within this region (called shot noise).
- The shot noise is ultimately due to the quantization of the charge on electrons and holes.
- The noise in a p-i-n device is considerably lower than that in a photoconductor, as well as in APDs.
- The various sources of noise determine the signal-to-noise (S/N) ratio in a photodetector.
- One quantifies it as a noise-equivalent power (NEP), which is the minimum detectable signal that would produce the same rms output as the noise.
- The detectivity of the photodetector is then defined as D = 1/NEP.
- The NEP depends on the area of the photodetector as well as the bandwidth.
- The specific detectivity, D*, is then defined as that for a detector of unit area and a bandwidth of 1 Hz.
- Clearly, once the bandwidth requirements are met, it is desirable to choose a photodetector with the highest D*.