Microwave power performance of MBE-grown AlGaN/GaN HEMTs on HVPE GaN substrates
Common-emitter I-V measurements were performed at elevated temperatures. Measurements at 300 degrees C are shown in Fig. 2 for a device with a 50 x 50 pill 2 emitter, using base Current steps of 250 mu A. Note that the data was obtained for a device with slightly different I-V characteristics from those measured in Fig. I (e.g. h(fe), at 25 degrees C was measured to be 20. along with an offset voltage of approximately 3V). At 300 degrees C, the HBT functions well and exhibits an improved offset voltage of approximately 1.5-2.0 V a knee voltage of 8 V along with a drop in the value of h(fe) to 10. The improvement in offset voltage is attributed to the enhanced ionisation efficiency of the Mg acceptors at higher temperature which at 300 degrees C has reduced the base sheet resistance by approximately a factor of four, as measured from on-wafer transfer length method (TLM) patterns. The increase in knee voltage is attributed to ail increase in the extrinsic emitter and collector resistances. The reduction of h(fe), with increasing temperature is shown more clearly in Fig. 3. It is attributed to a reduction in the diffusion length for electrons in the base, together with a possible increase in the hole current back-injected into the emitter.