Heterojunction field effect transistor device and process of fabrication thereof

A heterojunction field effect transistor fabricated on a semi-insulating substrate, comprising:

• (a) a first layer overlying said semi-insulating substrate and formed of a first high-purity compound semiconductor material, said first layer having a lower portion, an upper portion and a first shoulder portion interconnecting the lower and upper portions;
• (b) a second layer topographically overlying said first layer so that a step configuration of said first shoulder portion is transferred to said second layer to form lower and upper portions formed of a second compound semiconductor material and a second shoulder portion interconnecting the lower and upper portions and formed of a fourth compound semiconductor material, said second compound semiconductor material being smaller in electron affinity than said first high-purity compound semiconductor material, said fourth compound semiconductor material being smaller in electron affinity than said first high-purity compound semiconductor material but larger in electron affinity than said second compound semiconductor material, an active channel layer taking place in said upper portion of said first layer along a heterojunction formed between said first layer and said second layer;
• (c) a source contact region formed on the lower portion of said second layer so as to be in contact with said second shoulder portion, said source contact region being formed of a third compound semiconductor material;
• (d) a drain region formed on the upper portion of said second layer so as to be electrically coupled to said active channel layer, said drain region being spaced from said source region;
• and
• (e) a gate electrode formed on the upper portion of said second layer, said gate electrode being biased to control said active channel layer, wherein said fourth compound semiconductor material has a lower edge of a conduction band higher in energy level than that of said first high-purity compound semiconductor material but lower in energy level than that of said second compound semiconductor material.