Measurement of Minority Carrier Lifetime by the Phase Shift of Photoconductivity

Abstract

The reasons for undertaking the measurement of minority carrier lifetime in semiconductors may be classified into three broad groups: —

• I. Control of the production process. It is well known that the carrier lifetime is very sensitive to certain impurities and defects, but it is so far not possible to relate absolute values to a given degree of purity. The lifetime can, nevertheless, be used as a very sensitive detector of departures from a normal, and hence its usefulness in production control. Some samples of germanium and silicon show a very strong dependence of lifetime on injection level so that, to check the magnitude of this effect, measurements should be made at high and low injection levels.

• II. As part of a fundamental investigation of electron levels in semiconductors. For this purpose, more detailed measurements must be made over a wide range of injection levels, and this effect Observed over a wide range of temperatures.

• III. As a measure of diffusion length for device design. The value required here is the actual value to be expected in the device after its various heat treatments, and includes a knowledge of the mobility. In practice, this cannot be accurately predicted and has to be measured in the device by a specialised technique which is usually not applicable to requirements I. and II.