Characterizing charge trapping in microwave transistors

This paper looks at the problems associated with pulsed testing of GaN and GaAs HEMTs and its use in examining the charging and discharging times of the various traps that affect conduction. A particular problem is that the RF behaviour of these transistors varies with the state of charge of the traps. This is a concern for a large class of applications where the usage pattern is comparable to the time constants of the traps. Such classes include the intermittently-switched front ends of 802.11 and mobile telephone circuits. The conventional approach with pulsed testing is to sit at a bias point for a sufficiently long time and then to pulse to characteristic voltages very quickly before returning to the bias. If the pulsing to the applied characteristic voltages is much faster than the time constants of the traps of the transistor, then the characteristic measured will reflect the state of charge of the traps for the bias point. Our approach here is to perform a series of characteristic measurements as the bias and trap charge-state change. Each characteristic is measured too quickly to affect the trapped charge significantly. The set of characteristics then reflects the changing nature of the transistor's bias and state of charge of its traps.