Analysis of Transit Time Effects due to Spacer Layers in Quantum Well Oscillators
Quantum well oscillators [1–9] are the subject of much research activity, as potential candidates for millimeter-wave sources. Resonant tunneling through quantum wells is inherently a very fast transport mechanism, and hence very high cut-off frequencies (in the order of hundreds of GHz) have been suggested. The most common heterostructure used in quantum well oscillators has been the AIxGaI-x As/ GaAs system. However, the output power obtained from pure quantum well oscillators has been very low due to low dc bias voltages and area and impedance limitations associated with the capacitance of very narrow junctions. In order to improve room-temperature operation of quantum wells undoped spacer layers on either side of the quantum well have been used to prevent dopant migration from the heavily doped contact layers into the quantum well region. The thicknesses of these nominally undoped spacer layers vary greatly in thickness from 50Å to 3000Å [8–10]. Previously no transit time effects due to these spacer layers have been considered in the analysis of the rf performance of quantum wells when used as an oscillator.
KeywordsMigration GaAs Lime Kesan
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