A Study on the High Breakdown Voltage AlGaN/GaN HEMTs Employing H2O Annealing and HfO2 Gate Insulator

Abstract

AlGaN/GaN high-electron-mobility transistors (HEMTs) have received a considerable amount of attention for high-power application such as RF and power electronic systems due to their wide bandgap properties, such as their high critical field (3 MV/cm) and low intrinsic carrier concentration (1010 cm–3). Also, AlGaN/GaN heterostructure offers a high density (1013 cm–2) and a high mobility (2000 cm2/V•s) for a two-dimensional electron gas such that AlGaN/GaN HEMTs exhibit a high breakdown voltage with a low on-resistance. However, the soft breakdown characteristic of AlGaN/GaN HEMTs caused by surface leakage current is a critical issue. AlGaN/GaN HEMTs have a considerable amount of surface states such as that related to dislocation from the GaN buffer/substrate interface, arranged charges caused by spontaneous polarization charges, and nitrogen vacancies induced by plasma and thermal processes. At a high breakdown voltage, the electron trapping at shallow traps on the surface should be suppressed. In order to improve the reverse blocking characteristics of AlGaN/GaN HEMTs, I fabricated AlGaN/GaN HEMTs employing the proposed H2O annealing treatment and compared electrical characteristics with those of the HEMT by widely used O2 annealing method. The breakdown voltage and the drain leakage current of AlGaN/GaN HEMT (LGD= 20 µm) employing the H2O annealing treatment were 1674 V and 13.1 nA/mm (VDS= 100 V, VGS= –10 V), respectively, while those of HEMT with the O2 treatment were 1512 V and 60.1 nA/mm. Not only relatively degraded reverse blocking characteristic but also a normally-on characteristic of AlGaN/GaN HEMTs has been dealt as a serious issue. Most of previously reported AlGaN/GaN HEMTs have a normally-on characteristic with threshold voltage (VTH) of –3 to –6 V due to the polarization-induced 2DEG between AlGaN and GaN layer. In order to prevent Burn-out phenomenon and designing additional circuit because of the normally-on devices, normally-off characteristic is strongly desirable for an integrated circuits application. In this thesis, by applying simple KOH wet etch and RF-sputtered HfO2 as a gate insulator, normally-off AlGaN/GaN MOS HEMTs were successfully fabricated and investigated. The proposed KOH wet etch resulted in an adequate recess-depth and smooth etched surface. The gate-recessed HEMT exhibits threshold voltage (VTH) shifts from –3 to 1.5 V after 150 s KOH-wet etch. The breakdown voltage of 1580 V and specific on-resistance (Ron,sp) of 8.09 mΩ•cm2 was measured in AlGaN/GaN HEMT with the gate-drain distance of 20 μm-long. The high figure of merit (FOM) of 308 MW•cm–2 was achieved. My experimental results indicate that the proposed simple KOH wet etching and RF sputtered HfO2-gate insulator may be promising for the normally-off AlGaN/GaN MOS HEMTs fabrication. Lastly, another recent issue of AlGaN/GaN HEMT is dealt in the Appendix chapter which is a gold free issue. Conventional conducting metal Ni/Au was compared with Ni/Cu and Ni/W contact. My experimental result shows that Cu also may be a good conducting metal for AlGaN/GaN HEMTs and has a great potential for power device application. Moreover, I have proposed and fabricated high performance of AlGaN/GaN Schottky Barrier Diodes (SBDs) with various Schottky contact by sputtering method. The reverse blocking characteristics such as the leakage current and breakdown voltage was improved by TaN and ITO Schottky contact due to its high SBH. However, forward current of TaN and ITO Schottky contact was less than that of Ni/Au Schottky contact. The TaN Schottky SBDs achieved the highest breakdown voltage of 605 V and ITO Schottky SBDs achieved 472 V. On the contrary, the breakdown voltage of Ni/Au Schottky SBDs exhibited 335 V.