The influence of heavy doping effects on the reverse recovery storage time of a diode

During the reverse recovery process in a modern Si p-n junction diode, the value of J_EO/J_BO (the ratio of emitter to base dark saturation currents) increases and the recombination of carriers in the emitter becomes important due to heavy doping effects. A theory is developed to take these effects into account. The emitter and the base components of the current during the reverse recovery phase are found to vary with time. However, their sum remains equal to the constant reverse current J_R, which flows in the external circuit. The ratio of the total quantity of charge present in the base to that present in the emitter is found to increase rapidly with time. Values of the storage time t_s for different values of J_EO/J_BO are calculated. In a typical case, the storage time is reduced by a factor 5 in a diode with J_EO/J_BO = 2. In such cases, the values of lifetime τ_B calculated using measured t_s values and the Kingston's formula, become inaccurate. Theoretical expression for the total charge Q_BS left in the base at t = t_s in a base dominated diode is derived.An earlier semi-empirical formula known as Kuno's formula is derived theoretically. It is found that the formula is valid both for the base dominated diode as well as in a diode with large contribution of the emitter but only when J_R/J_F is small. According to this formula t_s vs 1n(1 + J_F/J_R) plot is approximately a straight line with slope approximately equal to τ_B in both cases. For large values of J_R/J_F when t_s values are small, the correct formula shows that the plot is highly curved. An analysis of this part of the curve yields a value of J_EO/J_BO.