Circular Geometry Transistors Fabricated on Germanium-on-Alumina Bonded Substrates

Germanium (Ge) has been bonded to fine grain alumina (Al2O3) by means of a polished polycrystalline silicon interface layer. After room temperature bonding and subsequent bond strength annealing at 150 oC for 24 hrs, the germanium layer was ground and polished to 100µm. Low temperature Tungsten (W) gate circular geometry devices, W/L = 9, fabricated on this layer exhibited effective mobility values of 150 cm2/Vs. This is much lower than results obtained on equivalent devices on bulk Ge. Improvement in the germanium polish process resulted in transistors with an effective mobility of 415 cm2/Vs, comparable to that of the bulk Ge devices. Low temperature measurement, down to temperatures of 173 K, showed an improvement in device performance resulting in both an increase in effective mobility to 591 cm2/Vs and a decrease in sub threshold slope from 180 mV/dec to 60 mV/dec indicating a reduction in leakage current.