Current gain of 4H–SiC high-voltage BJTs at reduced temperatures

The dependences of the common-emitter current gain β on the collector current IC have been measured at reduced temperatures in 3 kV 4H–SiC epitaxial-emitter npn bipolar junction transistors with implanted p+ base contact and base–emitter distances Lbe ranging from 3 to 15 µm. The collector–emitter voltage was fixed (at 100 V) to provide the active operation mode at any collector current in a wide range from 80 mA to 10 A (current densities of 3.5–445 A cm−2). The maximum current gain steadily grows with Lbe from 8 (Lbe = 3 µm) to 21 (Lbe = 15 µm) at room temperature. For all values of Lbe, β steadily increases as the temperature becomes lower; however, the smaller Lbe is, the weaker the effect produced by temperature lowering. As, for example, the temperature is lowered to −42 °C, β increases from 8 to 9 for structures with Lbe = 3 µm and from 21 to 31 for structures with Lbe = 15 µm. The phenomena observed have been accounted for by recombination at the p+–p base boundary.