InAlGaP devices

A transistor comprising

• a source and a drain,
• a heterostructure including a narrow bandgap channel layer coupling said source to said drain and in which a quantum well is formed to confine electrons, and first and second wider bandgap donor layers disposed on opposite sides of said channel layer for supplying said electrons to said quantum well, and
• a gate contact structure for applying voltage to said heterostructure to control the flow of electrons in said channel layer between said source and said drain, characterized in that
• said channel layer comprises InGaAs and each of said donor layers comprises In.sub.0.5-q (Al.sub.x Ga.sub.1-x).sub.0.5+q P in which the mole fraction of Al is about 0.2<x<0.3, said channel layer and each of said donor layers forming a pseudomorphic heterojunction at the interface between them, and
• said heterostructure further includes a first pair of spacer layers disposed between said first donor layer and said channel layer and a second pair of spacer layers disposed between said second donor layer and said channel layer, each pair comprising an i-type As-containing, P-absent Group III-V compound inner spacer layer adjacent said channel layer and an i-type P-containing, As-absent Group III-V compound outer spacer layer adjacent one of said donor layers, said inner spacer layer having a bandgap which is intermediate that of said channel layer and said outer spacer layer.