Photoconductive detectors decrease their terminal resistance when exposed to light. Figure 5.38a shows a practical representation of a photoconductive detector. It consists of a thin-film trace of a semiconductor such as cadmium sulfide deposited on a ceramic substrate. The length of the meandering trace is L, the width is W, and t is the thickness. There are two metallizations on either side of the trace to which electrical contacts can be made.
|Fig 5.38 (a) Photoconductive detector. (b) Energy level diagram for an intrinsic semiconductor.|
When a voltage V is applied, a current flows across the width of the trace,allalongthetrace.
The light input modulates ρc and consequently the resistance and the current
across the trace.
The transduction mechanism can be qualitatively explained with the aid of the energy level diagram for an intrinsic semiconductor shown in Fig. 5.38b.
An intrinsic semiconductor absorbs a photon when its energy barely exceeds the energy of the forbidden gap and simultaneously creates an electron-hole pair. The electron makes a single transition across the bandgap into the conduction band. This electron and the corresponding hole in the valence band contribute to the increase in total current. As a consequence, the conductivity increases and the resistance of the photoconductor decreases with increased photon input.
In extrinsic semiconductors, owing to the added impurities, donor and acceptor ionized states are created in the forbidden gap, which also contribute additional carriers to the conduction and valence band, which increase the conductivity even further. The addition of impurities increases the quantum efficiency and the long wavelength cutoff35 of photoconductive detectors.
Photoconductors made from lead sulfide (PbS) and lead selenide (PbSe) are used as infrared detectors in the range of optical wavelengths from 1 to 3 μm and 1 to 6 μm, respectively. Cadmium sulfide (CdS) cells are used in exposure meters, light dimmers, photoelectric relays, and other consumer-type applications.
Cadmium sulfide cells can be optimized in the range of 515 to 730 nm, which is close to the spectral response of the human eye.