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A study of carrier generation in, and interaction of, space-charge regions in germanium Mitchell, Ronald Reid

Abstract

Germanium space-charge regions have been studied under conditions of double-depletion achieved by applying a reverse bias to both junctions of a transistor structure. The generation and distribution of carriers between the two junctions are described in terms of models for the thermal generation of carrier pairs and for potential distribution in a cylindrical system. Measurements over a wide range of temperature reveal that generation of carriers occurs through the medium of one or more sets of recombination centers and not by direct transition between the valence and conduction bands even at elevated temperatures. The Shockley-Read recombination-generation theory is applied to obtain the activation energies associated with the recombination centers. The impurity density, base width and junction areas are estimated from measurements of punchthrough voltage and junction capacitance. For some specimens the capacitance measurements made with one junction floating confirm the sharp increase in capacitance at punchthrough noted by Barker. The distribution of current between the junctions when both are equally reverse biased is found to be roughly proportional to their areas. It is also shown that control of the reverse current across one junction may be achieved by variation of the reverse-bias on the other junction. The mechanism of this interaction is considered in terms of the diffusion of carriers between the two space-charge regions. The phenomen of slightly non-saturating reverse current is explained in terms of the expansion with reverse bias of the space-charge region within the base. An expression relating the base current and reverse bias in a cylindrical system shown to give good agreement with experiment for one specimen. For the units in which the increase in current with voltage is appreciable, space-charge expansion cannot account for the increase and two other mechanisms are considered: avalanche multiplication in the bulk and along the surface, and a high generation rate on the surface. The surface conduction channels on the base region are investigated in two ways. Maximum floating junction potentials are calculated from measured values of the common emitter amplification factor (using the Shockley 1949 theory) and these are compared with directly measured potentials. A second method involves direct measurement of a.c. surface conductance between collector and emitter when both junctions are reverse-biased to prevent bulk conduction. Both tests reveal very small degrees of surface conductance on all specimens.

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