Thermodynamical analysis of optimal recombination centers in thyristors

Abstract

The main criterion for an optimal recombination center in thyristors has been reformulated, starting from a basic thermodynamical level. From an extended grand canonical ensemble, statistical expressions for the thermal emission rates of an impurity atom are obtained in terms of changes in enthalpy, electronic and vibrational entropy. The ratio between lifetimes at high and low injection levels is evaluated for single and double level recombination centers and calculated for different enthalpy positions and entropy changes of the center and for different resistivities of the thyristor middle region. It is shown that the expected changes in entropy, when a charge carrier is emitted or captured by the center influences the optimal enthalpy position in silicon by as much as 0.1 eV. Also, a complete change in the influence of injection level on lifetime at standard resistivities and temperature for high power devices is noted. The optimization criterion is compared with experimental data pertinent to thyristor optimization. The paper demonstrates the necessity of making a profound thermodynamical analysis of the charge carrier traffic at a recombination center when using experimental data for deep impurities in the optimization of thyristor lifetime ratings.