The basic structure of the photodiode is a reverse biased PN junction. When light of the proper wavelength is directed toward the junction, hole electron pairs are created and swept across the junction by the field developed across the depletion region. The result is a current flow, photocurrent, in the external circuit, proportional to the effective irradiance on the device. It behaves basically as a constant current generator up to its avalanche voltage, shown in Figure 1. It has a low temperature coefficient and the response times are in the submicrosecond range. Spectral response and speed can be tailored by geometry and doping of the junction. Increasing the junction area increases the sensitivity (photocurrent per unit irradiance) of the photodiode by collecting more photons, but also increases junction capacitance, which can increase the response time.
The absorption coefficient of light in silicon decreases with increasing radiation wavelength. Therefore, as the radiation wavelength decreases, a larger percentage of the hole-electron pairs are created closer to the silicon surface. This results in the photodiode exhibiting a peak response point at some radiation wavelength. At this wavelength a maximum number of hole-electron pairs are created near the junction. For wavelengths longer than this, more hole-electron pairs are created deeper in the structure beyond the photodiode PN junction. For shorter wavelength more of the incident radiation is absorbed closer to the device surface, and does not penetrate to the junction. In this manner, spectral response characteristics of the silicon photodiode are modified by the junction depth.
All common silicon light detectors consist of a photodiode junction and an amplifier. In most commercial devices, the photodiode current is in the submicroampere to tens of microamperes range, and an amplifier can be added to the chip at minimal cost. Total device response to bias, temperature and switching waveforms becomes a combination of photodiode and amplifier system response.
All semiconductor junction diodes are photosensitive to some degree over some range of wavelengths of light. The response of a diode to a particular wavelength depends on the semiconductor material used and the junction depth of the diode. In some cases, light emitting diodes can be set to detect their own wavelength of light. Whether or not a particular device is photosensitive to its emission wavelength depends upon how well the bulk material absorbs this wavelength to create hole electron pairs. GaAlAs, which has high output efficiency due to decreased bulk absorption at 880 nm , exhibits virtually no photosensitivity at 880nm for the same reason. The GaAs emitters, however, tend to be reasonable detectors of light generated at the 940nm GaAs emission wavelength. This phenomenon can be very useful in some applications, such as half-duplex communication links.
A Photodiode can be operated in a photovoltaic mode (no bias) or a photoconductive mode (reversed bias).
In this configuration, the output voltage developed in the load resistor RL is a linear function of the incident light energy, as long as RL is small compared to the shunt resistance of the photodiode. As a result, the output voltage is limited. One way to achieve sufficiently low load resistance, and an amplified output voltage, is by feeding the photocurrent to an operational amplifier virtual ground as shown below. The circuit has a linear response and has low noise due to the almost complete elimination of leakage current.
In the photoconductive mode, the generated photocurrent produces a voltage across a load resistor in parallel with the shunt resistance Rd. Since, in the reverse biased mode Rd is substantially constant, large values of Rf may be used still giving a linear response between output voltage and applied radiation intensity. This form of circuit is required for high speed of response. The main disadvantage of this mode of operation is the increased leakage current due to the bias voltage, giving higher noise than the other circuit modes already described. Practical photoconductive mode circuits are shown below (Note that in both circuits the photodiode is reverse-biased).
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