An Analysis of Forward Conduction Characteristics of Ultra High Voltage 4H-SiC N-IGBTs

Abstract

Ultra High Voltage (UHV) 4H-SiC N-IGBTs, with drift layer thicknesses ranging from 140 μm to 240 μm, were fabricated and characterized. A blocking voltage of 25 kV, and a forward voltage drop (VF) of 12.8 V were measured from a 9 mm x 9 mm device with a 240 μm drift layer. A positive temperature coefficient of VF was observed, which is desirable for paralleling, but unusual for a bipolar device. The cause of this behavior was investigated using a test structure that allowed separate observations of electron and hole currents in the 4H-SiC IGBT structure. It was revealed that the hole current increases with temperature, due to increases in charge injection and carrier lifetimes at elevated temperatures, while the electron current decreases with temperature due to a unipolar resistance component in its path, most likely due to JFET resistance, formed by depletion regions extending into the lightly doped drift region. The concept of Carrier Storage Layer (CSL) was implemented in UHV 4H-SiC N-IGBTs to suppress this effect, resulting in a negative temperature coefficient of VF. A 15 kV 4H-SiC N-IGBT with a 1x1016cm-3 doped CSL showed a VF reduction of 3 V at a collector current of 20 A, at a junction temperature of 150°C, compared to a 15 kV SiC N-IGBT without a CSL at the same collector current value.