Study On Electrical Transport And Breakdown Characteristics Of GaN-based Schottky Barrier Diodes

In recent years, the performance of power devices based on conventional semiconductors such as Si and GaAs is limited by their intrinsic material parameters. Wide band gap semiconductor materials, especiallyⅢ-nitrides (GaN, AlN, InN) and their compounds, have been a subject of intensive investigation during the last decades, which have exhibited promising potentials in UV-to-visible optoelectronic device applications as well as high frequency, high temperature and high power devices due to their outstanding properties such as wide bandgap energy, high electron saturation velocity, large breakdown electrical field and high thermal conductivity. However, heteroepitaxial GaN-based devices commonly suffer from high dislocation density within the GaN epi-layers, which seriously limits the device performance. In this work, we studied the leakage transport mechanism of homoepitaxial GaN Schottky barrier diodes and the breakdown mechanism of AlGaN/GaN-based planar Schottky barrier diodes.Firstly, we investigated temperature-dependent electrical characteristics of Schottky barrier diodes fabricated on homoepitaxial GaN grown by metal organic chemical vapor deposition on bulk GaN substrate. The dislocation density of the GaN homoepitaxial layer is estimated to be～4×106 cm-2 by cathodoluminescence mapping technique,which agrees with the x-ray diffraction measurements. The diode fabricated with a vertical geometry exhibits a low reverse leakage of～10-10 A at-20 V, a low turn-on voltage of～0.9 V, and a low on-resistance of～1.5 mΩ·cm2 at room temperature (RT). Within the temperature range of 300 K and 410 K, the Schottky barrier height and ideality factor are observed to increase and decrease respectively with increasing temperature due to barrier inhomogeneity. Large deviations of reverse leakage from the thermionic emission (TE) mechanism were observed. A modified thermionic-field emission (TFE) model was proposed to fit the temperature and field dependent reverse leakage in which the dislocations are believed to be the primary channels of the reverse leakage. Based on our purposed model, the simulated reverse leakage curves agree well with the experimental curves.Then, AlGaN/GaN-based planar Schottky barrier diodes with various spacings between ohmic and Schottky contacts are fabricated without any edge termination. The reverse leakage current of the devices quickly saturates at low reverse bias when the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is fully depleted. The corresponding breakdown voltage is found to follow a linear dependence on contact spacing and exceeds 1100 V at a contact spacing of 20μm, yielding a high VBR2/RON value of>280 MW·cm-2. The observations are tentatively explained by a "natural super-junction" theory.