Concentration of electrons in selectively doped GaAlAs/GaAs heterojunction and its dependence on spacer-layer thickness and gate electric field

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Abstract
The concentration Ns of two‐dimensional electrons in N‐GaAlAs/GaAs systems is studied; Ns shows a systematic decrease when the thickness Wsp of an undoped GaAlAs ‘‘spacer layer’’ is increased. Such a dependence is shown to be well explained by the theory in which the size quantization is taken into account. Furthermore, Ns is studied as a function of gate voltage in field‐effect transistor (FET) structures and found to saturate once Ns reaches its limiting value. This phenomenon is strongly dependent on Wsp and is well ascribed to the formation of nondepleted region in the GaAlAs layers. Implications for FET designs are discussed.