Anomalous drain leakage current in Si1-xGex-channel pMOSFETs was analyzed by investigating the surface generation current in a gate-controlled p(+)-n junction, and determining how the leakage current is influenced by Si1-xGex thickness, Ge fraction, and temperature. It was concluded that the leakage current is related to interface traps generated at Si/SiO2 interface, and the current is due to trap-assisted band-to-band tunneling. The interface traps are generated in significantly numbers in MOSFETs with Si1-xGex layers thicker than the critical thickness of misfit-dislocation generation. The leakage current sets constraints for Si1-xGex thickness and/or Ge fraction in Si/Si1-xGex heterostructure MOSFETs. Moreover, instability of the drain current can be observed presumably due to a field effect derived from holes trapped and/or de-trapped in shallow states near Si/Si1-xGex interface.