Subpicosecond characterization of ballistic parallel transport in GaAs quantum wells

For the first time, we observe directly ballistic acceleration of electrons by an applied electric field in semiconductors on a time scale of 0.15 picosecond, by femtosecond optical techniques. While hot-electron transistors are based on ballistic drift in a field-free region, the understanding of ballistic acceleration in parallel transport in the presence of an external electric field is important for the understanding of many short channel devices. Also, for high carrier densities ( approximately 10/sup 17//cm/sup 3/) which are typical for many devices, we demonstrate that THz electromagnetic radiation due to accelerating electrons can be a very important loss mechanism on picosecond time scale. Therefore, for high-speed devices which require strong acceleration of electrons, the transport must be considered self-consistently with the resulting radiation and space-charge field in the devices.<>