Modeling and characterization of gallium nitride based metal-oxide-semiconductor heterostructure field-effect transistors for RF power amplifiers

GaN-based heterojunction field-effect transistors (HFETs) based on wide-bandgap semiconductor technology exhibit high-power, high-frequency performance at elevated temperature. However, the high metal-semiconductor gate current exhibited by and HFETs under forward bias can cause reliability problems such as breakdown walkout, current collapse, and power drift. Having a metal-oxide-semiconductor gate structure, metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) exhibit the gate leakage current orders of magnitude lower than that of their HFET counterparts, thereby improving the reliability of GaN HFETs.;Unfortunately, adding the insulating layer creates many challenges in device characterization and modeling. First, the role of oxide layer to the current collapse is not well understood. Traps are commonly observed at the oxide-semiconductor interface, which can promote current collapse. On the other hand, the oxide layer is an effective block to the source of trapping due to the fact that the oxide layer reduces the leakage current from the gate. Second, by addition of the oxide layer, the separation between the gate and the channel is increased, which one would expect to lower transconductance and therefore decrease gain and frequency performance for power amplifiers compared to HFETs. Surprisingly, MOSHFETs exhibiting comparable or even higher transconductance to HFETs have been recently reported. While the higher mobility of MOSHFETs is believed to be the origin of higher transconductance, the mechanisms behind the enhancement of mobility are debatable. Third, to facilitate the circuit design, a compact model for the active device is essential for accurate prediction of circuit performance. There are many large-signal compact models for GaN HFETs, whereas a proper model does not yet exist for MOSHFETs.;In this work, the trapping effects of the GaN MOSHFETs will be studied in detail. The access region trapping effects, namely current collapse, is studied by On and Off voltage stress and illumination. The oxide layer effectively blocks electrons from injecting into the access region surface under the Off-state stress condition, but does not alleviate the On-state stress current collapse, due to the hot electron injection from the channel. Effects of illumination on the recovery of the current collapse are also discussed.;The mobility of the 2DEG channel in GaN MOSHFETs is also studied in comparison with HFETs. Significant mobility enhancement is seen in the MOSHFET under pulsed condition and at low temperatures, which can be attributed to donor-like surface states. The mobility enhancement helps compensate for the lower electron density of the MOSHFET, providing transconductance comparable to but more linear than that of the HFET. This implies that the MOSHFET can deliver both performance and reliability advantage over HFET.;Insight gained from the study of current collapse and mobility enhancement has lead to an improved GaN MOSHFET model that is modified from the Angelov model. Compared to the EEHEMT-based model, the modified Angelov-based model is an electro-thermal model with a built-in thermal sub-circuit to account for the self-heating effect. Several model parameters are varied with temperature, including those for threshold voltage, drain current, gate current and gate capacitance. Additional parameters allow the present model to provide better fitting of drain current in the linear region and gate capacitance near the cutoff region. As the result, the nonlinear performance of both Class-A and Class-AB amplifiers are accurately simulated over a wide range of input powers, matching impedances, and ambient temperatures.