Frequency limitations of negative differential mobility in vertical conduction in superlattices

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Abstract
The frequency‐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 one‐dimensionally confined (2D) superlattices biased in the negative differential mobility regime. Numerical calculations show that in both systems there exists a mobility‐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 domain‐driven microwave oscillator, the mobility‐transition frequency is expected to be relatively small compared to the oscillator frequency. Nonetheless, the higher mobility‐transition frequency in the 2D superlattice would render it more advantageous for use in negative differential mobility microwave oscillators.