The most important properties of photodiodes are:
the responsivity, i.e., the photocurrent divided by optical power – related to the quantum efficiency, dependent on the wavelength
the active area, i.e., the light-sensitive area
the maximum allowed photocurrent (usually limited by saturation)
the dark current (in photoconductive mode, important for the detection of low light levels)
the speed, i.e. the bandwidth, related to the rise and fall time, often influenced by the capacitance
The speed (bandwidth) of a photodiode is typically limited either by electrical parameters (capacitance and external resistor) or by internal effects such as the limited speed of the generated carriers. The highest bandwidths of tens of gigahertz are usually achieved with small active areas (diameters well below 1 mm) and small absorption volumes. Such small active areas are still practical particularly for fiber-coupled devices, but they limit the photocurrents achievable to the order of 1 mA or less, corresponding to optical powers of ≈ 2 mW or less. Higher photocurrents are actually desirable for suppression of shot noise and thermal noise. (Higher photocurrents increase shot noise in absolute terms, but decrease it relatively to the signal.) Larger active areas (with diameters up to the order of 1 cm) allow for handling of larger beams and for much higher photocurrents, but at the expense of lower speed.
The combination of high bandwidth (tens of gigahertz) and high photocurrents (tens of milliamperes) is achieved in velocity-matched photodetectors, containing several small-area photodetectors, which are weakly coupled to an optical waveguide and deliver their photocurrents into a common RF waveguide structure.
The quantum efficiency of a photodiode is the fraction of the incident (or absorbed) photons which contribute to the photocurrent. For photodiodes without an avalanche effect, it is directly related to the responsivity S: the photocurrent is
with the quantum efficiency η, the electron charge e and the photon energy hν. The quantum efficiency of a photodiode can be very high – in some cases more than 95% – but varies significantly with wavelength. Apart from a high internal efficiency, a high quantum efficiency requires the suppression of reflections e.g. with an anti-reflection coating.
In some cases, additional properties of photodiodes have to be observed, such as linearity of response over a wide dynamic range, the spatial uniformity of response, or the shape of the dynamic response (e.g. optimized for time domain or frequency domain), or the noise performance.
The noise performance of photodiodes can be very good. For high photocurrents, it can be limited by shot noise, although thermal noise in the electronics is often stronger than that. For the detection of very low light levels (e.g. for photon counting), the dark current can also play a role.
A higher responsivity (although sometimes at the cost of lower quantum efficiency) can be achieved with avalanche photodiodes. These are operated with a relatively high reverse bias voltage so that secondary electrons can be generated (as in photomultipliers). The avalanche process increases the responsivity, so that noise influences of subsequent electronic amplifiers are minimized, whereas quantum noise becomes more important and multiplication noise is also introduced.
A photodiode is sometimes integrated into the package of a laser diode. It may detect some light getting through the highly reflecting back facet, the power of which is proportional to the output power. The signal obtained can be used, e.g., to stabilize the output power, or to detect device degradation.
The electronics used in a photodiode-based photodetector can strongly influence the performance in terms of speed, linearity, and noise. As mentioned above, current amplifiers (transimpedance amplifiers) are often a good choice.