The dependence of 77 K electron velocity-field characteristics on low-field mobility in AlGaAs-GaAs modulation-doped structures

We have used the geometrical magnetoresistance method to measure the electron velocities and mobilities as functions of electric field in AlGaAs-GaAs modulation-doped structures at 77 K. These structures had a variety of AlGaAs mole fractions and undoped setback layers resulting in different low-field mobilities in the different structures. We find that higher low-field mobility does not lead to a higher high-field velocity, and in fact, that at electric fields found in operating MODFET's, the mobilities and velocities were about the same in various AIGaAs-GaAs MODFET's. The results indicate that high low-field mobility is not an important parameter for MODFET design and does not even significantly reduce the parasitic source resistance in an operating FET. Furthermore, because of the strong electric field dependence of the mobility, low-field source resistance measurements are not adequate for determining the source resistance in an operating FET.