Study On Leakage Current Mechanism And Frequency Dispersion Properties Of GaN-based Electronic Devices

GaN and its alloys have shown attractive physical properties, such as wide direct band gap, high breakdown electrical field, high electron saturation velocity, high thermal conductivity and high thermal stability. Due to the strong spontaneous and piezoelectric polarization effects, high density two-dimensional electron gas with sheet carrier density up to1013cm-2could exist at the AlGaN/GaN interface, which has shown excellent electronic transport properties. As a result, GaN-based materials have great prospect in applications of high frequency, high temperature and high-power semiconductor devices. However, high-density defects commonly exist in heteroepitaxial GaN, have been considered as the major cause of excessive leakage current and frequency dispersion effects observed in GaN-based devices. In this work, the impact of structural defects on the performance of GaN-based electronic devices was investigated, utilizing material/device characterization techniques, such as capacitance-voltage (C-V) and current-voltage-temperature (I-V-T) measurements as well as numerical simulation method. The main conclusions are listed as follows:1) high-quality vertical Schottky barrier diodes (SBDs) on low-defect-density GaN homo-epilayer were fabricated. A leakage current model focusing on defect-related conduction is purposed to explain the experimental temperature-dependent reverse I-V characteristics. The model suggests that linear defects like dislocations would form a continuous conductive band within the forbidden band on which leakage electrons could move. Electrons from the contact metal could overcome the locally reduced Schottky barrier and tunnel onto the defect band through thermionic-field emission. Good agreement between the simulated I-V curves and the experimental results is obtained.2) Lateral AlGaN/GaN-based Schottky barrier diodes (SBDs) with Ni/Au Schottky contacts were fabricated. The impact of post-gate-annealing (PGA) process on the properties of the SBDs was investigated. I-V characterization shows that the PGA process could reduce the density of electrically active states at the Schottky metal/AlGaN interface, leading to lower reverse leakage current. C-V characterization shows that the PGA process could reduce the density of surface states and the AlGaN/GaN interface states, as well as the density of polarization charge within the AlGaN barrier layer. Analysis of capacitance frequency dispersion in depletion state indicates that Fermi level pinned by the AlGaN/GaN interface states can be unpinned by PGA.3) The impact of field plates (FPs) on the transient response characteristics of AlGaN/GaN HEMT devices is studied using numerical simulation method. FP can suppress the maginitude of lag phenomena in both cases of gate-lag and drain lag, but has no influence on the lapsed time. The change of ionization probability of surface states and the negative charges on FP induced by the potential difference between the FP and the semiconductor could modulate the carrier density within the device channel, which could reduce the lag effect.