Studies of traps in AlGaN/GaN high electron mobility transistors on silicon
Anand Mulagumoottil Jesudas
Date of Issue2016
School of Electrical and Electronic Engineering
AlGaN/GaN-based high-electron-mobility transistors (HEMTs) are promising for high-voltage, high-frequency and high-power applications such as DC-DC convertors, cellular base stations, radar and wireless communication systems. The impressive intrinsic material parameters such as large bandgap, high saturation velocity and high critical electric field favours their operation under extreme conditions. Also, the ability to form a 2-dimensional electron gas channel at the AlGaN/GaN heterojunction interface leads to a remarkably high carrier mobility making GaN HEMTs a very fast and versatile device. In spite of these advantages, GaN HEMT technology still faces various reliability issues. One such major reliability issue that is detrimental to the device operation is the electron trapping phenomenon. Trapping of electrons in these devices leads to current dispersion or current collapse or increase of dynamic ON-resistance (dyn-RDS[ON]) when the devices are operating at high voltages. These traps are found on the surface of the HEMT structure or distributed within AlGaN barrier or GaN buffer regions. Though significant progress has been achieved in the growth and fabrication technology, the occurrence of traps in GaN HEMTs still persist. Various reports suggest that traps could also be created when the devices are operated at high temperature and high voltage conditions over a long period of time. For realizing a reliable AlGaN/GaN HEMT technology, it is necessary to have a systematic understanding of trapping effects in GaN HEMTs. In this thesis, a comprehensive trap analysis pertaining to the trapping mechanisms present in the AlGaN barrier and GaN buffer of an AlGaN/GaN HEMT on Si substrate has been carried out. Using pulsed ID-VD characterization under different pulse-widths and quiescent bias conditions, we have investigated the current collapse caused by the electron capture/emission phenomena. By treating the current dispersion in the linear and the saturation regions of the pulsed ID-VD characteristics as two distinct events, we have succeeded in explaining the electric field influence on dyn-RDS[ON] and Vth by electron trapping analysis. Using different ON-state and OFF-state stress conditions as filling pulses, we have also investigated the trapping phenomenon in the AlGaN barrier and GaN buffer layers of the AlGaN/GaN HEMTs. The OFF-state stress condition that favours the tunnelling of gate electrons into the AlGaN barrier was used to study the trapping phenomenon in the barrier layer of AlGaN/GaN HEMTs. The actual location of the electron trapping was also identified by varying the electric field in the gate-drain region of AlGaN/GaN HEMTs. Through the ON-state stress based studies, we have realized the presence of a distribution of trap energy levels in the GaN buffer layer. Moreover, the linear dependency of effective trap activation energies with applied stress voltage has also been identified. The trap properties such as activation energies, emission time constants and the capture cross-sections have been obtained by temperature dependent transient measurements. With the help of applied stress conditions and the obtained trap information, we have succeeded in identifying the actual location of the traps in AlGaN/GaN HEMTs. By incorporating all the experimentally obtained trap information into 2D numerical simulations, we have also validated our experimental findings. A multi-trap energy level model was adopted to account for the study of the distribution of trap energy levels in the GaN buffer layer. In order to further improve the GaN HEMT device reliability, we have introduced a dielectric gate stack under the gate of AlGaN HEMTs and device encapsulation with benzo-cyclobutane (BCB). Using a bilayer gate dielectric insulation, we have suppressed the current collapse by ~55% and also reduced the gate leakage currents by at least an order of magnitude. Surface leakage current that prevailed due to the surface passivation layer was reduced by at least two orders of magnitude by the BCB encapsulation. Subsequently, we have enhanced the device breakdown voltage by at least two times as compared to the non BCB encapsulated HEMTs. In summary, trapping related reliability issue still remains as a major obstacle for AlGaN/GaN HEMTs on silicon substrates, thus preventing them from being more widely adopted in applications particularly under more extreme operating conditions such as high-voltage and high-temperature. In this thesis, detailed experimental and theoretical studies of the origins and trapping mechanisms of traps in AlGaN/GaN HEMTs on silicon substrates are reported. The findings in this work provide new insight of traps in AlGaN/GaN HEMTs and will facilitate the optimisation of growth and fabrication process to enhance the device performance and reliability.