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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-05-09

Definition: semiconductor devices with a p-n or p-i-n structure for the detection of light

Photodiodes are frequently used photodetectors. They are semiconductor devices which contain a p-n junction, and often an intrinsic (undoped) layer between n and p layers. Devices with an intrinsic layer are called p-i-n or PIN photodiodes. Light absorbed in the depletion region or the intrinsic region generates electron-hole pairs, most of which contribute to a photocurrent.

Operation Modes

Photodiodes can be operated in two very different modes:

• Photovoltaic mode: like a solar cell, the illuminated photodiode generates a voltage which can be measured. However, the dependence of this voltage on the light power is rather nonlinear, and the dynamic range is quite small. Also, the maximum speed is not achieved.
• Photoconductive mode: here, a reverse voltage is applied to the diode (i.e., a voltage in the direction where the diode is not conducting without incident light) and measures the resulting photocurrent. (It may also suffice to keep the applied voltage close to zero.) The dependence of the photocurrent on the light power can be very linear over six or more orders of magnitude of the light power, e.g. in a range from a few nanowatts to tens of milliwatts for a silicon p-i-n photodiode with an active area of a few mm². The magnitude of the reverse voltage has nearly no influence on the photocurrent and only a weak influence on the (typically rather small) dark current (obtained without light), but a higher voltage tends to make the response faster and also increases the heating of the device.

Current amplifiers (also called transimpedance amplifiers) are often used as preamplifiers for photodiodes. Such amplifiers keep the voltage nearly constant (e.g. near zero, or at some possibly adjustable negative bias), so that the photodiode is operated in the photoconductive mode. Current amplifiers can also have very good noise properties, and a better tradeoff for sensitivity versus bandwidth, compared e.g. to simple circuits with a resistor and a voltage amplifier. Some commercially available amplifier devices help to make power measurements in the laboratory very flexible by providing many different sensitivity settings, thus a huge dynamic range with low-noise performance, and also possibly a built-in display, adjustable bias voltage and signal offset, adjustable filters, etc.

Figure 1: Current-voltage characteristics of a photodiode for different optical powers. In photovoltaic mode (see the line for a 1-kΩ load resistor), the response is rather nonlinear. In photoconductive mode, here shown for a simple circuit with a reverse bias applied through a load resistor, a very linear response is achieved. The same holds for a constant reverse bias (not shown).

Semiconductor Materials

Typical photodiode materials are:

• silicon (Si): low dark current, high speed, good sensitivity roughly between 400 nm and 1000 nm (best around 800-900 nm)
• germanium (Ge): high dark current, slow speed due to large parasitic capacity, good sensitivity roughly between 600 nm and 1800 nm (best around 1400-1500 nm)
• indium gallium arsenide (InGaAs): expensive, low dark current, high speed, good sensitivity roughly between 800 nm and 1700 nm (best around 1300-1600 nm)

The indicated wavelength ranges can sometimes be substantially exceeded by models with extended spectral response.

Key Properties

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 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, → RC-limited performance) or by internal effects such as the limited speed of the generated carriers. 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 achievable photocurrents 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. Larger active areas (with diameters up to the order of 1 cm) allow for handling larger beams and for much higher photocurrents, but at the expense of much lower speed.

The combination of high bandwidth (tens of GHz) and high photocurrents (tens of mA) 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 and the electron charge e. The quantum efficiency of a photodiode can be very high – in some cases more than 95% – but significantly varies with wavelength. Apart from a good internal efficiency, a good quantum efficiency requires good 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 introduced as well.

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 a 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.

See also: p-i-n photodiodes, photodetectors, velocity-matched photodetectors, avalanche photodiodes, metal-semiconductor-metal photodetectors, shot noise, bandwidth, Spotlight article 2006-10-16

Categories: metrology, photonic devices

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!