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Photodiodes are frequently used photodetectors, which have largely replaced the formerly used phototubes. 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. The photocurrent can be quite precisely proportional to the absorbed (or incident) light intensity over a wide range of optical powers.
Figure 1 schematically shows the typical design of the photodiode on p–i–n type. Here, one has an intrinsic region between an n-doped and a p-doped region, where most of the electric carriers are generated. Through the electrical contacts (anode and cathode), the generated photocurrent can be obtained. The anode may have a ring shape, so that the light can be injected through the hole. A large active area can be obtained with a correspondingly large ring, but that tends to increase the capacitance, thus reducing the detection bandwidth and the dark current; also, the efficiency may drop if carriers are generated too far from the electrodes.
For a high responsivity, one should have a material with a strong absorption for the optical wavelength of interest. When using a thicker layer for obtaining efficient absorption, one may lose a lot of the generated carriers.
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 nonlinear, and the dynamic range is fairly 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 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 trade-off for sensitivity versus bandwidth, compared with, e.g., 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, and 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.
Typical photodiode materials are:
- silicon (Si): low dark current, high speed, good sensitivity between roughly 400 and 1000 nm (best around 800–900 nm)
- germanium (Ge): high dark current, slow speed due to large parasitic capacity, good sensitivity between roughly 900 and 1600 nm (best around 1400–1500 nm)
- indium gallium arsenide phosphide (InGaAsP): expensive, low dark current, high speed, good sensitivity roughly between 1000 and 1350 nm (best around 1100–1300 nm)
- indium gallium arsenide (InGaAs): expensive, low dark current, high speed, good sensitivity roughly between 900 and 1700 nm (best around 1300–1600 nm)
The indicated wavelength ranges can sometimes be substantially exceeded by models with extended spectral response.
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.
For a particularly high detection bandwidth, advanced photodiode designs are used. For example, some devices contain an optical resonator around the thin absorbing section. In that way, one can achieve efficient absorption and thus a high quantum efficiency despite a rather small thickness of the intrinsic region, as is chosen for reducing the drift time.
So-called waveguide photodiodes contain an optical waveguide which leads light along the absorbing region. That absorbing region can then again be very thin, and nevertheless one can obtain efficient absorption in a short length. By minimizing the length of the active region, one can also minimize the electrical capacitance and reach a very high bandwidth.
In some cases, the electrode structure is made such that it forms an electrical waveguide, where the electric wave can propagate in parallel with the optical wave in the optical waveguide. Such traveling-wave photodiodes can reach a bandwidth well above 100 GHz.
Some semiconductor materials are intrinsically better suited than others for fast photodiodes. For example, indium gallium arsenide (InGaAs) is particularly suitable, because that direct bandgap material (in contrast to silicon, for example) exhibits a rather short absorption length, allowing the realization of very thin absorbing layers, in which the photocarriers can be quickly collected. For fast avalanche photodiodes, it is also important to have a low ratio of the impact ionization coefficients for holes and electrons.
See also: p–i–n photodiodes, photodetectors, velocity-matched photodetectors, avalanche photodiodes, metal–semiconductor–metal photodetectors, phototubes, shot noise, bandwidth, Spotlight article 2006-10-16
and other articles in the categories optical 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) and software could become very valuable for your business!
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