Schottky power diodes designed for improved breakdown characteristics
Creators: name: Luong, Stanley
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400899
subject: Electrical engineering not elsewhere classified
classificationCode: 400908
subject: Microelectronics
classificationCode: 401603
subject: Compound semiconductors

description: Silicon carbide (SiC) is a semiconductor material sold as substrates (like silicon is) for making semiconductor devices. It has advantages (compared to other semiconductors like silicon) regarding making devices that operate at high temperature, high electric fields and high current density.<br><br>Overall, the semiconductor industry continues to expand and SiC products are a growing part of this. In 2016, global semiconductor sales reached nearly US$340 billion (the highest ever) according to The Semiconductor Industry Association (SIA). Market growth is driven by the ever-increasing amount of semiconductor technology in devices the world depends on for working (as reported by SIA). Power electronic components such as semiconductor SiC power diodes used in cars for example are among the numerous areas where improvements in performance are continually sought. ‘Increasing electrification in vehicles generally – and in hybrid and electric vehicles specifically is energizing the market for power semiconductors in vehicles’ (IHS Markit report). The HIS report shows that the total market for power semiconductors (including discrete SiC power diodes) will increase from US$5.5 billion in 2016 to more than US$8.5 billion in 2022. An increasing trend towards electric cars in the coming years is expected to drive the demand for electronic components made from suitable semiconductor materials, including SiC. The advantage of SiC semiconductor chips is that they have high-reliability in harsh environments like the environment of the drive train of vehicles which includes the engine and connected components to deliver power to the wheels of vehicles. Moreover, the car industry is just one area where improvements in power semiconductor devices are sought. Anywhere where there is control, or high transmission voltage and current and voltage conversion, will benefit from improvement in diode performance. Two important aspects of Schottky diode performance are how much current it can deliver when in the forward bias mode and how much voltage it can withstand when in current blocking mode. Too much current (forward bias) or too much voltage (typically a reverse bias consideration) across the diode will cause it to break down.<br><br>Considering the value of the power semiconductor device market, the industry push for performance, and the possibilities that improvements in SiC materials bring to semiconductor research, SiC Schottky diodes (also called Schottky Barrier Diodes, SBD) were investigated to determine the influence of several factors that affect device performance. Minimising the loss of energy and maximising the possible delivered electric current and also blocking voltage capability by improving SiC Schottky diode electrical performance is an important area of semiconductor research and of value to industry. Breakdown in the forward and reverse bias modes will be the focus of this research but the other aspects will also be reported on too. For example, high forward current is desired but if it comes at the expense of high forward voltage then there will be high power loss in the diode which should ideally act as a switch with no power loss. Similarly, a high reverse bias is desired but if leakage current (reverse bias current) is high then again there is power loss.<br><br>This study uses finite element modelling and experimental investigation of different metals for forming improved Schottky contacts. Contact geometry and electrode edge isolation techniques are investigated to optimise designs. Schottky contact geometry is optimised in order to minimise the incidents of maximum current density within the diode structure, where breakdown occurs. Surface preparation and surface treatment prior to Schottky formation and in particular the surface treatments used to give a carbon-rich SiC surface, which in this research has been found to reduce the turn-on voltage of SiC Schottky diodes, is also investigated. Optimised geometry and electrode edge isolation improvements are demonstrated using silicon substrates and this improvement can be applied to any metal-to-semiconductor combination. A diode requires an Ohmic contact and this is also studied here with the approach of using selective etching to prepare the SiC surface. SiC diodes were fabricated and used for electrical testing to determine the electrical characteristics. Moreover, the effects of the quality of the SiC itself on the breakdown voltage was investigated (the major qualifier for crystal quality is the value of the density of the defect known as a micropipe and this value is called MPD (for micropipe density) and given in SiC wafer specifications from suppliers.