Photodiodesensor

In the photovoltaic mode (see the line for a 1-kΩ load resistor), the response is nonlinear. In the photoconductive mode, shown here 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).

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 increases the dark current; also, the efficiency may drop if carriers are generated too far from the electrodes.

Menlo Systems offers a series of photodetectors for lowest light level signals. From avalanche to PIN photodiodes, you can find the detector that is best for your specific application.

Photodiodecircuit

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.

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Photodiodeworking principle

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.

Photodiodes are available not only as single-segment detectors. There are dual and quadrant photodiodes, which can be used for precision sensing, and also one-dimensional and two-dimensional photodiode arrays. For more details, see the article on position-sensitive detectors.

Photodiodeapplication

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.

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Ultrafast photodetectors from ALPHALAS for measurement of optical waveforms with rise times starting from 10 ps and total spectral coverage from 170 to 2600 nm (VUV to IR) have bandwidths from DC up to 30 GHz. Configurations include free-space, fiber receptacle or SM-fiber-pigtailed options and have compact metal housings for noise immunity. The UV-extended versions of the Si photodiodes are the only commercial products that cover the spectral range from 170 to 1100 nm with a rise time < 50 ps. For maximum flexibility, most models are not internally terminated. A 50 Ohm external termination supports the highest speed operation, while a high impedance load generates large amplitude signals. Applications include pulse form and duration measurement, mode beating monitoring and heterodyne measurements. Balanced photodiodes complement the large selection of more than 70 unique models.

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with the quantum efficiency <$\eta$>, the electron charge <$e$> and the photon energy <$h\nu$>. 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.

For a particularly high detection bandwidth in the gigahertz region, 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.

The speed (bandwidth) of a photodiode is typically limited either by electrical parameters (capacitance and external resistor) or by internal effects such as carrier transit time in the depletion region. (In some cases, relatively slow diffusion of carriers generated outside the depletion region limits the bandwidth.) 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.

Some semiconductor materials are intrinsically better suited than others for fast photodiodes. For example, indium gallium arsenide (InGaAs) is particularly suitable because that direct band gap 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.

Photodiodediagram

There are so-called sandwich detectors or two-color photodiodes consisting of two (or more) photodiodes in a sequence. The top photodiode is made from the materials with the largest band gap energy and absorbs short-wavelength light while transmitting much of the light with longer wavelengths, which cannot be absorbed. That transmitted light then hits a further photodiode. The ratio of powers detected by the photodiodes depends on the wavelength.

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For substantially increased responsivity, one may either use avalanche photodiodes (see below) or phototransistors; these are based on quite different operation principles.

For a high responsivity of a photodiode, 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 and therefore still not substantially improve the responsivity.

Even when used in photoconductive mode, photodiodes are usually not understood to be photoconductive detectors, which have a significantly different operation principle.

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

Photodiodecharacteristics

Photodiodesymbol

In order to avoid that trade-off, one often uses a current amplifier (also called transimpedance amplifier). Such an amplifier, which is typically realized with an operational amplifier (op-amp), keeps the voltage at the diode nearly constant (e.g. near zero, or at some possibly adjustable reverse bias), so that the photodiode's capacitance loses much of its relevance. The residual voltage variations at the photodiode are inversely proportional to the gain of the used operational amplifier. Still, it is good to minimize the input capacitance when requiring a high detection bandwidth; for example, it is better to directly connect a photodiode to the current amplifier, instead of using a long cable connection.

Sandwich detectors may be used for remote temperature measurements, for example, where you uses the ratio of signals from two photodiodes: the higher the temperature, the higher is the relative amount of radiation at shorter wavelengths.

CSRayzer offers different kinds of photodiodes used in high speed, ultra-low light detection, and laser range finding, LIDAR and free space communications.

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Gentec Electro-Optics offers a great range of power detectors based on silicon or germanium photodiodes for powers up to 750 mW.

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.

Photodiodes are frequently used photodetectors, which have largely replaced the formerly used vacuum 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.

The same principle may also be applied with photodiodes made of the same material because at longer wavelengths (closer to the bandgap) the top photodiode will not absorb all light. One again gets a wavelength-dependent ratio of signals from two photodiodes.

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PINphotodiode

So-called waveguide photodiodes contain an optical waveguide which confines light along its path through the absorbing region. That 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.

AMS Techno­logies carries an exceptionally broad range of photodiodes (PDs), based on various materials, cooled or uncooled, featuring single devices as well as photodiode arrays or assemblies:

More specific terms: avalanche photodiodes, Geiger mode photodiodes, lateral effect photodiodes, quadrant photodiodes, p–i–n photodiodes, silicon photodiodes, germanium photodiodes, InGaAs and GaAs photodiodes

Commercially available laboratory current amplifiers help to make power measurements 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.

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.

Some photodiodes are available in the form of photodiode arrays of one-dimensional or two-dimensional kind. Two-dimensional detector arrays, e.g. for use as image sensors, may be realized with photodiodes or with other kinds of photodetectors.

Current amplifiers, which are also available as OEM devices, can also have very good noise properties. The relevant figure is the noise-equivalent input current, which can be well below 1 pA/Hz1/2.

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.

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In the simple circuit according to Figure 3, the magnitude of the bias voltage drops with increasing photocurrent due to the voltage drop at the load resistor. While that has little influence on the linearity, it leads to a charging or discharging of the photodiode's capacitance whenever the incident light intensity changes, so that the detection bandwidth is reduced; it may become RC-limited. That introduces a trade-off between detection bandwidth and responsivity: a high bandwidth requires a small load resistor, which leads to a low responsivity and also a higher noise-equivalent power, which is often limited by thermal noise (Johnson noise) of the load resistor.

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.

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