The Effect of Temperature on the Ionization Coefficient in Silicon†

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Abstract
The avalanche breakdown of p-n junction diodes across their space charge regions is known to be analogous to the Townsend mechanism for gases. Electric charge carriers, which gain sufficient energy from the field, are able to produce secondary electron-hole pairs. Both the holes and the electrons can themselves have ionizing collisions and thus the process leads to an avalanche. An important factor, controlling the breakdown, is the ionization coefficient a, defined as the number of electron-hole pairs produced by a carrier moving unit distance in the direction of the field. This paper presents the results of nn investigation into the effect of lattice temperature on the ionization coefficient. This has been achieved by observing the breakdown voltage of a range of silicon diodes with either step or linear graded junctions, and applying simple and well-known relationships between ionization coefficient and breakdown voltage. Measurements have been made over the temperature range 77 to 400°K, and for field strengths from 4 to 9 × 105 volts/cm. Results show the ionization to become more efficient with decrease in temperature over this range of field strength. Temperature is found to have a greater effect at the lower field strength. This is shown to be consistent with modern theory.