Reexamination of Gain Theory for Photoconductive Devices

Photoconductive detectors are semiconductor optoelectronic devices that absorb optical energy and convert it to electrical signal. However, photoconductive gain or quantum efficiency (QE) theory of photodetector exhibits considerable controversy in optoelectronics literature. Gain is generally defined as the ratio of the number of photogenerated charge carriers collected by the electrodes and the number of photons absorbed in the semiconducting photoconductor. This gain is often expressed as the ratio of the carrier lifetime over the carrier transit time. The lifetime is the average time before an electron recombines with a hole, and the transit time is the time needed for photogenerated carriers to travel from one electrode to another under an applied voltage. This simple theory implies that it is possible to obtain high gain by reducing the transit time.
In this dissertation, the gain theory of photoconductive detector with an intrinsic (undoped) semiconductor is reexamined by assuming primary photoconductivity. In contrast to the widely adopted gain formula as a ratio of the carrier lifetime to transit time, allowing for a value much greater than unity, it is shown that this ratio can only be used as QE under the low-drift limit, but has been inappropriately generalized in the literature. The analytic results for photocarrier density, photocurrent, and QE in terms of normalized drift and diffusion lengths are obtained, which indicates that QE is limited to unity for arbitrary drift and diffusion parameters. A distinction between the two QE definitions used in the literature, but not explicitly distinguished, is discussed. The accumulative quantum efficiency (QEacc) includes the contributions of the flow of all photocarriers, regardless of whether they reach the electrodes, whilst the apparent quantum efficiency (QEapp) is based on the photocurrent at the electrodes. In general, QEacc > QEapp; however, they approach the same unity limit for the strong drift. Furthermore, it is shown that the photocurrent in the photoconductive channel is in general spatially nonuniform and that the presence of diffusion tends to reduce the photocurrent. As one form of secondary photoconductivity, it is confirmed that doping in a photoconductive device can yield a gain, limited by the ratio of the mobilities of majority and minority carriers. Based on the simulation results, new analytic results that show good agreement with simulated results are proposed.
This work lays the ground for understanding mechanisms of experimentally observed, above-unity photoconductivity gains. Moreover, these findings should offer new insights into photoconductivity and semiconductor device physics and may potentially lead to novel applications.