Instabilities in modulation doped field-effect transistors (MODFETs) at 77 K

The DC characteristics of AlxGa1¿xAs/GaAs MODFETs at 77 K have been investigated. At 77 K the MODFETs typically showed a threshold voltage shift of ¿0.2 V with respect to room temperature. In devices with x = 0.24 large drain biases sometimes resulted in a semipersistent distortion of the drain I/V characteristics. Illuminating the devices removed both the distortion and threshold voltage shift. By using a mole fraction of x = 0.33 and optimising the growth temperature the maximum threshold shift could be reduced to about 0.05 V.