On the collapse of drain I-V characteristics in modulation-doped FET´s at cryogenic temperatures

The collapse of the drain current-voltage characteristics of modulation-doped field-effect transistors (MODFET´s) at cryogenic temperatures, previously thought to be unavoidable, has been investigated. The results indicate that the mechanism responsible for the collapse is dependent on both the device fabrication steps and the parameters of crystal growth. Bulk AlxGa1 - xAsFET´s fabricated in our laboratory exhibited little or no collapse in the I-V characteristics at 77 K in the dark, demonstrating that the mechanism responsible for this pheonomenon is not related to problems associated with contacting AlxGa1 - xAs. MODFET´s with proper fabrication and growth procedures showed no collapse. In those devices exhibiting no collapse, the source resistance exhibited a substantial decrease upon cooling. At 300 K source resistances slightly over 1.0 Ω . mm with a transconductance of 170 mS/mm were obtained. Upon cooling, the source resistance decreased to 0.5 Ω . mm with a transconductance of 280 mS/mm. These results demonstrate that MODFET´s will exhibit enhanced performance at 77 K without exposure to light. Specific contact resistivities measured at room temperature ranged from 2 × 10^-7to 2 × 10^-6Ω cm²depending on the structural parameters.