A Mechanism of Space Charge Formation in Silicone Gel and its Effect to Instability of High Voltage Power Devices

When the IGBT and diode devices are kept in turn-off state, the collector-emitter leakage current <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathbf{I}_{\text{ce}})$</tex> is very small initially, however it could increase suddenly after a certain time and result in avalanche breakdown failure depending on the environment such as temperature, humidity, and process conditions. In this paper, the mechanism of this <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{I}_{\text{ce}}$</tex> instability of high voltage power devices is investigated in relation to space charge in silicone gel encapsulation material covering the chips. It is shown that the instability depends on the height of emitter bonding wire which run through above the surface of the edge termination area of the chip. The lower the height, the sooner the initiation of the sudden increase of the Ice occurred, indicating the electric field in silicone gel below the emitter wires is a major factor of the instability. Space charge in silicone gel sheet was measured by PEA (Pulsed Electro Acoustic) method. The large density of negative charge was observed in the silicone gel sheet with aluminum electrodes while the charge density of the sample with Au electrodes is an order of magnitude lower. The electrode material dependence indicates the origin of the charge is electrons injected from Al since the work function of Al is smaller than that of Au. Furthermore, the conduction current of the silicone gel sheet with Al electrodes was far larger than that with Au electrodes. These results strongly support that the origin of the space charge is electrons injected from the surface of aluminum wires.