- There are a number of applications for devices which change their resistance when exposed to light.
- For example, such light detectors can be used in the home to control automatic night lights which turn on at dusk and turn off at dawn.
- They can also be used to measure illumination levels, as in exposure meters for cameras.
- Many systems include a light beam aimed at the photoconductor, which signals the presence of an object between the source and detector.
- Such systems are useful in moving-object counters, burglar alarms, and many other applications.
- Detectors are used in optical signaling systems in which information is transmitted by a light beam and is received at a photoconductive cell.
Considerations in choosing a photoconductor for a given application include the sensitive wavelength range, time response, and optical sensitivity of
- In general, semiconductors are most sensitive to photons with energies equal to the band gap or slightly more energetic than band gap.
- Less energetic photons are not absorbed, and photons with hv > Eg are absorbed at the surface and contribute little to the bulk conductivity.
- Therefore, the table of band gaps indicates the photon energies to which most semiconductor photodetectors respond.
- For example, CdS (Eg = 2.42 eV) is commonly used as a photoconductor in the visible range, and narrow-gap materials such as Ge (0.67 eV) and InSb (0.18 eV) are useful in the infrared portion of the spectrum.
- Some photoconductors respond to excitations of carriers from impurity levels within the band gap and therefore are sensitive to photons of less than band gap energy.
The optical sensitivity of a photoconductor can be evaluated by examining the steady state excess carrier concentrations generated by an optical generation rate gop. If the mean time each carrier spends in its respective band before capture is and , we have
and the photoconductivity change is
For simple recombination, and will be equal. If trapping is present, however, one of the carriers may spend little time in its band before being trapped. From the above equ it is obvious that for maximum photoconductive response, we want high mobilities and long lifetimes. Some semiconductors are especially good candidates for photoconductive devices because of their high mobility; for example, InSb has an electron mobility of about 105 cm2/ v-s and therefore is used as a sensitive infrared detector in many applications.
The time response of a photoconductive cell is limited by the recombination times, the degree of carrier trapping, and the time required for carriers to drift through the device in an electric field. Often these properties can be adjusted by proper choice of material and device geometry, but in some cases improvements in response time are made at the expense of sensitivity.
For example, the drift time can be reduced by making the device short, but this substantially reduces the responsive area of the device. In addition, it is often
desirable that the device have a large dark resistance, and for this reason, shortening the length may not be practical. There is usually a compromise between
sensitivity, response time, dark resistance, and other requirements in choosing a device for a particular application.