The frequencyhyphen;dependence of the carrier drift velocity in superlattice miniband conduction in response to a small signal ac electric field is investigated for laterally unconfined and onehyphen;dimensionally confined (2D) superlattices biased in the negative differential mobility regime. Numerical calculations show that in both systems there exists a mobilityhyphen;transition frequency of order of 100 GHz, above which the differential mobility becomes positive. This frequency in a 2D superlattice is about three times that in an unconfined system, owing to enhanced carrier scattering. In a domainhyphen;driven microwave oscillator, the mobilityhyphen;transition frequency is expected to be relatively small compared to the oscillator frequency. Nonetheless, the higher mobilityhyphen;transition frequency in the 2D superlattice would render it more advantageous for use in negative differential mobility microwave oscillators. copy;1994 American Institute of Physics.
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