Expansion Limitation of Current Channel in Avalanche Transistors Under Voltage Ramp Triggering Conditions

To explain the frequent thermal failure of avalanche transistors (ATs) in Marx-bank circuits (MBCs), the process of current channel formation under voltage ramp triggering conditions is investigated. The current channel expansion is obvious when the transistor is triggered through the base. However, the channel expansion is limited in faster switching when the transistor is triggered by a voltage ramp with the base-emitter shorted. The position and width properties of current channel are observed by optical visualization and verified by corresponding physical models, respectively. The variations of dynamic electric field and current channel are analyzed. The results show that the excessive carriers in the initial period of switching accelerate the process of the voltage drop and cause the reconstruction of transverse electric field, which limits the current channel expansion. The temperature of the device rises in the current channel with limited conductive width and the irreversible thermal failure is formed. The consistency between the simulations and experimental results verifies the validity of the explanation and provides a reference for the optimization design of device and improvement of MBCs operation reliability.