Modulation doped GaAs/Al(x)Ga(1-x)As layered structures with applications to field effect transistors

Traditionally, there have been two approaches to achieving high switchings speeds in semiconductor devices. One is to improve processing and lithographic techniques thereby reducing device dimensions and reducing parasitic losses. The other is to fabricate devices from material with improved transport properties such as GaAs or InP as opposed to Si. With the advent of epitaxial techniques which can grow thin reproducible heterolayer structures on substrates large enough for LSI and VLSI application, devices which employ conduction band engineering to improve device performance are becoming strong contenders in the race for faster devices. Examples of devices which have been improved by conduction band engineering are heterojunction lasers, heterojunction bipolar transistors junction field effect transistors and metal semiconductor field effect transistor (MESFETS). In the aforementioned structures conduction and valence band discontinuities at the heterojunction are used to confine carriers to the active region of the device and/or reduce parasitic currents. A modulation doped heterostructure (MDH), on the other hand, is employed to actually improve or enhance the transport properties of the majority carriers in the channel of a MESFET. In a modulation doped (Al, Ga)As/GaAs heterostructure only the larger bandgap (Al, Ga)As is doped (n-type).