Events

[Thesis Defense] : “Gallium Oxide based High Voltage Diode: Design, Fabrication, Characterization and Modelling.”

Thesis Title: Gallium Oxide based High Voltage Diode: Design, Fabrication, Characterization and Modelling
Name of the student: Mr. Jyotiranjan Sahoo
Degree Registered: Ph.D. Engineering
Advisor : Prof. Digbijoy N Nath
Date : 14th November 2024 (Thursday) 3:30 pm
Venue :: Hybrid ::Seminar Hall/

Abstract

High-efficiency power electronic devices intend to play a significant role in curbing global warming and enhancing energy conservation. Owing to its superior material properties, Gallium oxide (Ga2O3) has emerged as a new competitive candidate that promises to deliver performance beyond the capabilities of the current SiC or GaN technologies. In the absence of p-type doping in Ga2O3, the Schottky Barrier Diode (SBD) may be the most important Ga2O3-based rectifier. For higher breakdown voltage (BV) or lower reverse leakage current, innovative designs should be introduced to lower the surface electric field below the material’s critical electric field.

β-Ga2O3 based lateral Schottky barrier diodes were fabricated using Ni/Au as Schottky contact and Ti/Au as Ohmic contact. The Sn-doped β-Ga2O3 sample was grown by the optical float-zone (OFZ) technique. The effect of trenches, used for mesa isolation, on breakdown voltage was investigated. The device showed near-ideal characteristics in terms of built-in potential. The parasitic series and shunt resistances were extracted, and their dependency on temperature was established. Modeling of temperature-dependent reverse leakage current was demonstrated using the thermionic emission (TE) model, Poole-Frenkel (PF) emission model, and Fowler-Nordheim (FN) tunneling mechanism. The combined model showed excellent agreement with experimental data over a wide range of bias and temperature. The maximum electric field of 2.3 MV/cm was achieved.

Vertical Schottky barrier diodes based on industry-standard β-Ga2O3 samples were fabricated. The dopant concentration in the 7 μm-thick Si+Cl-doped epitaxial layer and the 641 μm-thick Sn-doped (001) substrate of the 2 inch β-Ga2O3 epitaxial wafer was specified to be 2.7E16 per cm3 and 4.8E18 per cm3, respectively. The effect of mesa etch before and after the Schottky contact deposition on the reverse-bias characteristics of the diode was studied. The built-in potential of 0.9 V and breakdown voltage of 362 V were reported for these diodes.

A computer-aided design technology tool, TCAD-SILVACO, was utilised to understand and simulate the device behaviour of β−Ga2O3-based lateral and vertical diodes, which incorporated the reduced surface field (RESURF) effect caused by Trench MOS Barrier Schottky (TMBS) structure, bipolar diode design utilizing p-type NiOX and field-plate structure. An TMBS diode is the basic building structure of a high-voltage vertical diode. The RESURF effect caused by TMBS structure and bipolar (heterojunction) diode design utilizing NiOX as a p-type epi-layer could further improve the surface electric field profile, thus increasing the breakdown voltage characteristic of the diode. Nickel Oxide (NiOX), with a bandgap of 3.7 eV, has been one of the most studied wide-gap oxide materials because of its p-type behavior owing to its nonstoichiometric nature. The carrier concentration in the sputtered and annealed NiOX film was mapped to its deposition recipe. Various etch-recipes of NiOX were optimised. The TCAD designs were then implemented through fabrication.

The p-type NiOX as a field plate decreased the surface trap states in an already mesa-etched vertical diodes based on industry-standard β−Ga2O3, thus reducing reverse leakage current. The breakdown voltage increased to 500 V in these structures. Heterojunction vertical Schottky barrier diodes based on industry-standard β−Ga2O3 sample using NiOX as a p-type material were fabricated. For these bipolar vertical diodes, the BV of 1317 V, ON-resistance of 7-8 mΩ−cm2 and Baliga’s figure of merit (BFOM) of 1.16 GW/cm2 were reported. Large area multi-finger vertical diodes were designed in various dimensions of finger width, pitch, and total contact area. In these devices, the breakdown voltage was reported to be 1173 V with an absolute forward current of 100 mA at 4.5-6 V and 1 A at 17-18 V. For mesa-free large-area diodes, an absolute ON-current of 1-1.5 A was reported at a relatively low forward voltage of 5-8 V. The reverse recovery time (trr) was estimated to be below 100 ns for these diodes.

Finally, the implantation of Ga2O3 with ions of Sn and Si was simulated using SRIM. The SRIM data was analysed systematically to deduce the selection of ion energies and the corresponding dose to design a desired dopant profile. These analyses are essential in designing transistors.

In summary, this thesis focuses on the steady design and implementation of high-voltage diodes. It encompasses a comprehensive study of the device’s behaviour through established models and TCAD simulation. This work aims to explore new prospects for achieving the capabilities of Ga2O3-based electronic devices.