A minimum PVCR of about 3 is needed for typical circuit applications. Low current density Si/SiGe RITDs are suitable for low-power memory applications, and high current density tunnel diodes are needed for high-speed digital/mixed-signal applications. Si/SiGe RITDs have been engineered to have room temperature PVCRs up to 4.0. The same structure was duplicated by another research group using a different MBE system, and PVCRs of up to 6.0 have been obtained. In terms of peak current density, peak current densities ranging from as low as 20 mA/cm2 and as high as 218 kA/cm2, spanning seven orders of magnitude, have been achieved. A resistive cut-off frequency of 20.2 GHz has been realized on photolithography defined SiGe RITD followed by wet etching for further reducing the diode size, which should be able to improve when even smaller RITDs are fabricated using techniques such as electron beam lithography.