5. Simulation Studies

VARIOUS hydrodynamic simulation studies of different HEMT structures are presented in this chapter, which can be divided into two main parts: the first part deals with normally-on devices and their characteristics, while the second one focuses on normally-off structures. At the beginning, three different generations of normally-on devices are analyzed by using the models discussed in Chapter 4. Both the DC and AC characteristics are studied. As several application areas require the devices to operate at elevated temperatures, simulations of AlGaN/GaN HEMTs at high temperatures are presented, too. The simulator delivers good predictive results for the DC and RF characteristics of various devices after initial calibration. The temperature dependence of the maximum current and cut-off frequency of submicron devices is further studied.

One of the drawbacks of GaN-based HEMTs is the pronounced decrease of the transconductance g $_\ensuremath {\mathrm {m}}$ at higher gate bias, due to increasing source-gate and gate-drain resistances. Via simulations it is proved that the electric field distribution and the resulting carrier velocity quasi-saturation are the main source for the transconductance collapse, consequently also for the resistance rise. A shorter source-gate distance leads to a higher g $_\ensuremath {\mathrm {m}}$ peak value, but a more abrupt collapse at high gate bias. These effects are further discussed with respect to device linearity.

An investigation on the field plate technique in AlGaN/GaN power HEMTs follows. The critical geometrical variables controlling the field distribution in the channel are determined and optimized for improved device reliability. The results are implemented in the actual design of later device generations.

InAlN/GaN HEMTs have been recently proposed to provide higher polarization charges without the drawback of high strain. Relying on experimental work a simulation study of InAlN HEMTs is conducted. Using the calibrated setup, specific device effects are explored and the AC device performance is estimated.

Only a few approaches to obtain normally-off device characteristics exist, as described in Chapter 2. First, the trade-off between high-frequency performance and threshold voltage achieved by a gate recess technique is analyzed. Another approach is a thin InGaN cap layer, which introduces a polarization field which raises the conduction band of the AlGaN/GaN interface. Relying on the experimental work of Mizutani et al. [18] a simulation study of the proposed devices is conducted. The characteristics of the devices are compared to those of structures featuring the gate recess technique.

All of the presented simulation studies are based on experimental data from real devices. The AlGaN/GaN structures were produced and measured at the Fraunhofer-Institute for Solid-State Physics (IAF), while the InAlN/GaN structures were fabricated at the Institute for Solid-State Electronics, TU Wien.