Minority Carrier Extraction in Germanium

Abstract

The concentration of minority carriers in a sample can be effectively decreased below its equilibrium value by an extraction process in which, (a) the minority carriers in the sample are swept out by an electric field in a time ≪lifetime, while (b) the minority carrier flow into the sample at the entry contact, is suppressed. Procedures are discussed for making contacts satsifying condition (b). Such contacts have low resistance, and give photovoltages of opposite polarity to that of injecting contacts. With increasing field during extraction, there is a saturation increase of sample resistance, corresponding to approximately complete minority carrier depletion. Saturation extraction suppresses the intrinsic decrease in resistivity for a range of some 50°C for a group of $p$-type samples with resistivities of 7 to 40 ohm-cm at 300°K. Limitations on the range of effectiveness of extraction are imposed by failure of conditions (a) and (b) to hold at higher temperatures and/or deeper in the intrinsic range. Transport and transient effects associated with extraction can be used, as with injection, for measurement of minority carrier mobility and lifetime. Extraction is further useful for controlling collector contact properties, and for determining the composition of reverse current of point contact rectifiers, in terms of hole and electron components.