Response and transit times in quantum-well structures

The response of a biased double-barrier quantum well to a small ac voltage with characteristic time ${\mathrm{\tau }}_{\mathrm{resp}}$ and the transit or dwell time ${\mathrm{\tau }}_{\mathrm{trans}}$ are calculated using nonequilibrium Green’s-function techniques. The tunneling process is shown to be predominantly sequential for an ${\mathrm{In}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/${\mathrm{In}}_{\mathit{y}}$ ${\mathrm{Al}}_{1\mathrm{-}\mathit{y}}$As structure due to only well alloy scattering treated in the coherent-potential approximation. The magnitudes of ${\mathrm{\tau }}_{\mathrm{resp}}$ and ${\mathrm{\tau }}_{\mathrm{trans}}$ are in close agreement and about the same for either sequential or coherent tunneling.