Qualitative and Quantative Characterization of Trapping Effects in AlGaN/GaN High Electron Mobility Transistors

GaN-based high electron mobility transistors (HEMTs) have been considered excellent candidates for high power, high speed and high temperature applications due to high breakdown voltage, high electron saturation velocity and high operation temperature. Such superiority of AlGaN/GaN HEMTs leads to power density several times higher than commercially available devices. AlGaN/GaN HEMT power amplifiers have applications for wireless base-station, and communication and military radar systems. Recently, impressive device performances in AlGaN/GaN HEMTs have been reported such as high output power of 40 W/mm at 4 GHz, current gain cut-off frequency (fT) of 190 GHz and power-gain cut-off frequency (f_MAX) of 230 GHz. However, critical issues including current dispersion and device reliability have limited AlGaN/GaN HEMTs for practical applications. Current dispersion causing decrease in the actual output drain current and voltage swing at RF operations is attributed to surface/interface states in the AlGaN/GaN heterostructures created during material growth and device processing. Even though there have been remarkable improvements in growth/device fabrication technologies, trapping effects in AlGaN/GaN HEMTs cannot be removed perfectly, indicating that an accurate HEMT model including trapping effects is still needed for development of both novel HEMT processing/material growth techniques and implementation of AlGaN/GaN HEMT-based circuits.

In this Ph.D research, we investigated distinct electrical behavior of AlGaN/GaN HEMTs through study on PGA effects using DC and pulsed I-V characterization under different pulse widths and quiescent biases to understand electron capture/emission phenomena. Through the electrical characterization at different measurement conditions Current collapse in AlGaN/GaN was analyzed in terms of trap activities, and impact of PGA effects was qualitatively evaluated as a method for device processing monitoring.

Based on temperature-dependent drain current transients, it was demonstrated that PGA modifies trap activity in AlGaN/GaN HEMTs. The PGA process removes shallow traps with an activation energy of ~ 38 meV and t_E of ~ 0.5 µs at 295 K and induces deeper traps at least with an activation energy of ~ 0.31 eV and t_E of ~ 21.6 µs at 295 K. Shallow traps result in fast drain current transient and high reverse gate leakage current while deep traps lead to slow current recovery but a small leakage current.

Electrical behaviors of interface states at metal/AlGaN interface in Ni/AlGaN/GaN Schottky diodes was investigated to discover PGA effects on Ni/AlGaN interface states by comparing EBIC analysis results and electrical characteristics of Ni/AlGaN/GaN diodes. It was showed that the post-annealing reduced the density of the electrically active states at the Schottky metal/AlGaN surface, leading to decrease of reverse leakage current, and J_S, and increase of Schottky barrier height. In addition, XPS analyses was performed to investigate if unintentional reaction on free AlGaN surface was examined using during the PGA process since unexpected oxidation on free AlGaN surface by PGA can prevent trapping of electrons injected from reversely-biased gate as the same role of the normal passivation layer. XPS analyses showed that unintentional oxide layer was formed by PGA.

Base on the qualitative trap study, trap characterization methods were developed in terms of device parameters since evaluation of the trapping effect can be easily performed using these parameters. Current dispersion due to electron trapping was characterized in terms of device parameters such as threshold voltage, effective gate length, and parasitic series resistances based on two proposed models. One is zero-drain-bias output conductance method using output conductance with gate bias near zero drain voltage. The other is low-drain-voltage field method using drain current with respect to gate bias at a fixed low drain voltage. It was demonstrated that a higher negative electric field between gate and drain contacts cause a larger current dispersion due to trapping of more electrons, leading to a higher V_T shift in the positive direction, a longer effective gate length, and smaller parasitic resistances. The HEMT modeling was performed only in the linear region of I-V characteristics, so further comprehensive modeling including the saturation region is required.

In summary, we have systematically investigated trapping effects in AlGaN/GaN HEMTs. Current dispersion between DC and pulsed I-V characteristics was analyzed in terms of trap activity at different quiescent bias points with different pulse width showing that PGA modifies electrical properties of active traps. Optimized PGA removes deep-level traps which are responsible for slow drain current transient. Based on a better understanding of electron capture/emission mechanism, behavioral device models for trap characterization were developed. Current dispersion due to electron trapping was characterized in terms of device parameters in AlGaN/GaN HEMTs. Finally, it is suggested that our methodologies can be applied for AlGaN/GaN HEMT modeling in presence of trapping effects for evaluation of newly-introduced device/growth techniques, monitoring of device fabrication process, and, potentially, circuit applications while further systematic modeling including the saturation region is required.