Large-signal modeling of GaN-based microwave power transistors

During the past decade, the commercial outlook for GaN-based devices has grown considerably to meet the future need for wireless communications. Although record high output power densities at microwave frequencies have been achieved, factors limiting output power and reliability of the devices under high power operation still remain uncertain. Drain current collapse has been identified as the major obstacle for development of highly reliable GaN-based devices. Traps, introduced during the growth of the material, have been associated with current collapse. However, the physical mechanism of carrier trapping/detrapping under the influence of device bias was not clearly understood. This dissertation is focused on the efforts to develop large-signal models of GaN-based field-effect transistors (FETs) after identifying the possible mechanism for current collapse. It is found that impact ionization plays a key role in the generation of the carriers being captured by the trapping centers. Trap characterization is carried out using drain-lag measurement data. Calculated trap energy levels from detrapping time constants are in agreement with those reported by other groups using deep-level transient spectroscopy (DLTS). Detrapping time constants are correlated to the dispersion frequencies of transconductance and output resistance and are found to be temperature dependent. Temperature dependences of the trapping and transport parameters have been incorporated in the physics-based large-signal model. ^ The model is applied to analyze temperature, frequency and size dependences of nonlinearities and microwave power performances using Voletrra series and time-domain techniques. Improved linearities are observed in shorter gate length devices, however, better thermal stability is obtained in larger structures. An AlGaN/GaN HEMT based class-E microwave power amplifier is designed incorporating thermal and trapping effects. Although the performance of the amplifier is significantly degraded by the thermal and trapping effects, GaN-based class-E amplifier gives improved efficiencies at elevated frequencies compared to Si- and GaAs-based counterparts.