Deviation From Proportional Relationship Between Emitter Charging Time and Inverse Current of Heterojunction Bipolar Transistors Operating at High Current Density

We investigated the relationship between the emitter charging time and the inverse current of heterojunction bipolar transistors. A proportional relationship assuming the Boltzmann approximation and constant capacitances is often used to separate the components of the total delay time; however, without the aforementioned assumption, the theoretical calculations show that the relationship is not proportional but has an intercept. This deviation causes a nonnegligible error in the results obtained at high current densities. Additionally, we show the possibility of reducing the total delay time beyond that inferred from the conventional linear characteristics by a high-current-density operation.