Tunneling field effect transistors (TFETs) are a device technology that has the potential to achieve sub-60-mV/decade subthreshold slope and therefore could potentially overcome the power / performance bottleneck facing the CMOS industry today. It would be highly desirable to fabricate such devices using Si-based materials due to the proven capability of Si for VLSI circuits. However the large, indirect band gap of Si is not conducive to achieving large band-to-band tunneling currents. This paper presents experimental results on Si/SiGe heterojunction tunneling transistors (HETTs), where it is demonstrated that increasing the heterojunction band offset leads to enhanced device performance. An analysis is also provided addressing whether or not Si-based materials are suitable for TFETs or if material systems with direct band gaps and that allow more aggressive heterostructure engineering will be required to achieve the performance levels necessary for practical applications.