Non ideal current in SiGe/Si HBT

The non-ideal current in the I/sub b/ and I/sub c/ of the SiGe/Si HBT were studied. At low temperature it showed tunnel characteristics. From C-V and double crystal X-ray diffraction measurements, it was confirmed that this phenomena was related to the crystal quality in the base, emitter and their interface, but not related to the SiO-Si interface. The non-ideal current in I/sub c/ cannot be explained by the conventional drift-diffusion theory and is believed to be relevant to electrons tunneling from emitter or its depleted region to collector by means of an intermediate trap state in the base. However DLTS measurements shown that the phenomena have nothing to do with the deep levels in the emitter and collector.