Self-heating and trapping effect in AlGaN/GaN high electron mobility transistors on CVD-diamond

• Main Author: Kumud Ranjan
• Other Authors: Ng Geok Ing
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering::Microelectronics
• Online Access: https://hdl.handle.net/10356/137690
• Institution: Nanyang Technological University

GaN-based high-electron-mobility Transistors (HEMTs) are widely used for high frequency, high voltage and high-power applications. However, there are still many major challenges facing these devices such as thermal management, size reduction and long-term reliable operation, especially when they are operating at very high voltage (e.g., VD≥48 V). In such an operating condition, the device suffers substantial self-heating and encounters enhanced phonon scattering, which degrades device current due to the decrease of 2-DEG mobility and electron velocity as a result of the increased junction temperature. These problems are predominant particularly when GaN HEMTs are fabricated on conventional low thermal conductive substrates (e.g., Si (KSi=130 W/m-K), Sapphire (KSa=46 W/m-K) and SiC (KSiC= 450 W/m-K)). The usage of high thermal conductivity substrates such as diamond (1000-2000 W/m-K) is now emerging as a viable technique to extract the heat under high-power operations for GaN-based high-electron-mobility transistors (HEMTs). Typically, for GaN-on-Diamond process development, both the host Si or SiC substrate and the growth-defect-rich stress mitigation transition layers are first removed. The remaining HEMT layers are then bonded onto CVD-diamond, which then acts as the new substrate. Thus, the HEMTs fabricated on transferred GaN-on-Diamond will have different behaviour of self-heating and trapping than that of conventional HEMTs fabricated on GaN-on-Si. In this thesis, the main objectives are to investigate the self-heating effect on DC and RF performance, and the trapping behaviour in GaN HEMTs-on-CVD Diamond. The major contributions of this thesis are summarized below: (1) The quantitative investigation of the self-heating effect on DC and RF performances was carried out for AlGaN/GaN HEMTs on CVD-Diamond (GaN/Dia) and Si (GaN/Si) substrates. The GaN/Dia HEMTs were found to exhibit ~5.7-times lower rate of ID reduction than GaN/Si HEMTs. This behaviour was also confirmed by 2D device simulation. The fT reduction rate was ~6.75-times lower in the case of GaN/Dia than GaN/Si HEMTs whereas no significant reduction of fmax was observed in GaN/Dia HEMTs. Small signal measurements and equivalent circuit parameter extraction were done to analyze the variation in the performance of the devices. The comparatively lower reduction rate of µeff and veff in GaN/Dia HEMTs made its performance less degraded (~15%) as compared to (~50%) GaN/Si HEMTs. These results show that GaN/Dia HEMTs can be operated even at higher VD as well as at higher PD which are paramount features for developing compact high power SSPAs for CW application. (2) The hetero-interface trapping characteristics in AlGaN/GaN HEMT on CVD-diamond were investigated at different temperatures (25 °C to 200 °C) using the conductance method. The fast traps (0.16 to 10.01 µs) were identified as the dominating traps in our HEMT structure. The density of fast traps (DTf) increases with temperature from 6.7×1010 cm-2 eV-1 at 25 °C to 1.4×1012 cm-2eV-1 at 200 °C. For 25 °C to 200 °C, the interface trap state energy (ET) was obtained to be between 0.27 and 0.51 eV below the conduction band. The observation of increased DT with temperature could be due to the excitation of additional traps deeper in the bandgap. A lower value of DTfmin for GaN-on-CVD Diamond compared to GaN on Silicon devices is attributed to the removal of the defect-rich GaN transition layer during the substrate transfer process. Finally, the temperature-dependent pulsed IDS-VDS measurements revealed good agreement with the behaviour of DT with temperature obtained using the conductance method. Further to improve the device performance, GaN-MISHEMTs-on-CVD diamond was fabricated and analyzed the gm linearity, gate leakage, and interface trap behavior. (3) For the first time, the investigation of interface traps in AlGaN/GaN metal-insulator-semiconductor (MIS) HEMTs on CVD-diamond was reported in this work. The critical issue in MIS-HEMTs is the dielectric interface trap behaviour. The detailed investigation of dielectric interface and hetero-interface is carried out in AlGaN/GaN MISHEMTs on CVD-diamond and compared with the conventional Schottky AlGaN/GaN HEMTs on CVD-diamond. The finding of slow and fast type of traps in MISHEMTs on the contrary to only fast traps in conventional HEMTs indicated the source of traps. Such a study of interface trap behaviour in the device will help to identify and improve the reliability of AlGaN/GaN HEMTs and MISHEMTs-on-CVD diamond.