Physics-based Approach For Efficient & Reliable Enhancement-mode AlGaN/GaN High Electron Mobility Transistor (HEMT) Technology
Power semiconductor devices have been the key to growth of power electronics market, and cover application areas ranging from mobile stations (commercial) to missile seekers (military). Continuously increasing demand for improving the power handling capacities and need for higher bandwidth capable devices has impelled the need for search of novel device structures and materials other than Silicon (Si). High electron mobility transistors (HEMT) based on Gallium Nitride (GaN) have emerged as one of the most promising candidates to replace Si based power semiconductor devices. AlGaN/GaN HEMTs offer several advantages over their Si counterpart, such as high electron mobility (~2000 cm^2/Vs), high sheet carrier density (~ 1E13 cm^-2), and higher critical electric field (~ 3MV/cm). These properties result in devices based on GaN to have superior and efficient ON-state performance, while achieving voltage handling capacities better than their Si counterparts. The theoretical material limits for GaN shows promising applications in power as well as RF domain. These application areas, combined with the widespread LED market, is expected to bring down the cost of development of GaN based devices to be competitive to that of Si-based devices. However, despite very high-power density and operational frequency figures already demonstrated for GaN HEMTs, there is yet to be wide-scale deployment of these devices. One of the major challenges in GaN HEMTs is the development of high performance yet reliable and fail-safe normally-OFF or enhancement-mode (e-mode) devices. Normally-OFF devices are the devices that conduct current only when a positive voltage is applied at one of its terminals, referred to as gate terminal. These devices have a positive threshold voltage (Vth) beyond which they conduct current or become ON. Such devices are desirable in power applications to improve reliability of the power electronic system. Further, extensive application of the GaN based devices is also limited by reliability challenges unique to this material system, such as dynamic ON resistance (Ron), ambient light dependent behavior, and hot electron induced degradation. This work follows a physics-based approach to demonstrate high performance and reliable e-mode devices using a device performance-reliability co-design approach. Physical insights gained into the mechanism governing e-mode operation, besides the reliability challenges, allowed the development of novel methods to demonstrate reliable normally-OFF AlGaN/GaN HEMTs in this work. Firstly, to demonstrate high-performance e-mode AlGaN/GaN HEMTs, we have designed and developed a novel p-type and high-κ ternary oxide AlxTi1-xOy -. The p-type nature of the oxide resulted in an increase in Vth of the device, demonstrating a possibility to achieve positive Vth and hence e-mode operation. The oxide was developed in a thermal Atomic Layer Deposition system by depositing alternate films of Al2O3 and TiO2. By changing the deposition cycles of these films, the Al% and thickness of AlTiO could be precisely controlled. The p-type and high-κ nature of the resulting oxide was found to be a strong function of Al%. This allowed complete control over the p-type nature of developed AlTiO and hence threshold voltage of AlTiO gate oxide-based devices. Using the developed high-κ (25) and p-type Al0.5Ti0.5Oy as gate oxide, in conjunction with a thinner AlGaN barrier under gate, 600-V e-mode AlGaN/GaN HEMTs were demonstrated with performance metrices comparable to the best in literature. The HEMTs showed superior ON-state performance (ON current ~400 mA/mm and ON resistance = 8.9 ohm-mm) and gate control over channel (Ion / Ioff= 1E7, subthreshold slope = 73 mV/dec, and gate leakage < 200 nA/mm). Given that the developed p-type AlTiO was first of its kind, the next area of focus was to probe into the physical mechanism governing the 2-Dimensional Electron Gas (2-DEG) depletion or positive Vth shift achieved by the integration of these oxides in the gate stack. Given the wide band gap nature of AlTiO and AlGaN/GaN system, an electro-optical experiment-based method was used to probe the underlying mechanism , . Experiments were carried out on devices with various gate oxides (Al2O3,TiO2, AlTiO) and GaN buffer stacks (varied carbon doping) with 365 nm UV exposure. These experiments revealed maximum negative Vth shift with UV exposure in AlTiO-gated devices. Moreover, the negative Vth shift was a function of Al% in AlTiO. Further, the negative Vth shift was established to be due to deionization of deep-level negative states in AlTiO, induced due to presence of Al at Ti sites ([Al]'Ti), at/near the oxide/nitride interface under the gate metal. The study thus identified the presence of negatively ionized deep-level states at room temperature to result in p-type doping of AlTiO, thereby leading to the positive Vth shift in AlTiO-based HEMTs. Post demonstration of high-performance e-mode GaN HEMTs and the related physical mechanisms, the next part of the work dealt with gaining physical insights into reliability challenges plaguing AlGaN/GaN HEMTs with a view to achieving robust devices. Measure-Stress-Measure routines were followed to evaluate the dynamic ON resistance (dynamic Ron) of AlGaN/GaN HEMTs on carbon (C)-doped GaN buffer. The experiments revealed a unique stress time-dependent OFF-state drain-to-source critical stress voltage (Vcr), above which the dynamic Ron of AlGaN/GaN HEMTs increased significantly  – . The Vcr was found to be a strong function of device design parameters, such as, gate-drain distance, field plate length, and passivation thickness. Moreover, the Vcr was observed for both Schottky and Metal-Insulator-Semiconductor (MIS)-HEMTs. With the help of detailed experiments with varying substrate bias, temperature and C-doping in GaN buffer, electron trapping in C-doping induced buffer acceptor traps is proposed and validated to be the source of the dynamic Ron degradation in these devices. This electron trapping is shown to be controlled by the electric field near gate connected field plate of the devices, as it modulates the trap ionization probability and injection of carriers into the GaN buffer. Thus, the HEMTs exhibit a Vcr beyond which high dynamic Ron is observed. Further experimentation on the gate bias dependence of the dynamic Ron of devices with different gate stacks revealed the carrier density in the GaN buffer to be a function of gate control over the channel . This results in an OFF-state gate bias-dependent dynamic Ron degradation in AlGaN/GaN MISHEMTs. This is attributed to the MISHEMTs having an inferior channel control due to the insertion of the gate dielectric. Discovery of an electron trapping controlled dynamic Ron in GaN HEMTs encouraged us to examine the same under semi-ON state stressing as well . Semi-ON state, where channel is in semi-ON condition, stresses the device in a condition where significant electron density exists in the presence of high electric field. This condition results in the generation of highly energized electrons, known as hot electrons. Dynamic Ron experimentations on GaN HEMTs under semi-ON conditions revealed that the interaction of hot electrons with the GaN buffer results in significant self-heating in the GaN buffer near the field plate edge and enhances electron de-trapping from these traps. On the other hand, trapping was found to be determined by field conditions near the field plate edge. This trapping-de-trapping process determines the net electron density trapped in the GaN buffer traps and thus determines the dynamic Ron of the HEMTs under semi-ON state stressing. Furthermore, the study revealed Schottky-HEMTs to have a better dynamic Ron behavior under semi-ON state stressing, as compared to the MISHEMTs. This was due to higher hot electron-induced self-heating and related de-trapping in the Schottky HEMTs. Besides analysing dynamic Ron behavior, reliability aspects related to device breakdown were also analysed . Experiments revealed a slew rate dependent dynamic breakdown voltage in AlGaN/GaN HEMTs on a C-doped buffer. Detailed analysis revealed the role of electron transport through the C-doped GaN buffer in the observed HEMT breakdown behavior. Further, besides gaining insights into the mechanisms governing dynamic Ron in GaN HEMTs, this work also demonstrates a methodology to improve the dynamic Ron of GaN HEMTs even in the presence of C-doping induced acceptor traps, which are introduced to improve the device breakdown. The developed methodology is based on the understanding gained from this work that electron trapping in the GaN buffer can be controlled by relaxing electric field magnitude near the field plate edge. The same was achieved in this work by incorporation of p-type Al0.5Ti0.5Oy as surface protection layer over SiNx passivation . The proposed approach successfully relaxed electric field near field plate edge and resulted in mitigation of dynamic Ron in AlGaN/GaN HEMTs, even in the presence of C-doping induced buffer traps. This work thus resulted in the development of high performance and reliable AlGaN/GaN HEMTs on C-doped GaN-on-Si epi-stack, which was achieved through detailed physical insights into the governing mechanisms. Moreover, by solving the fundamental reliability challenge of dynamic Ron and demonstrating a novel AlGaN/GaN HEMT technology based on AlTiO, this work paves the way towards commercial deployment of a novel technology for high performance and reliable e-mode power HEMTs.