Analysis of charge control in stepped channel InP HFETs

An analysis by self-consistent calculation of the intrinsic charge control in stepped channel InP HFET (heterostructure field-effect transistors) is presented. It is shown that the notch depth, its width, and its aspect ratio with the total channel width significantly influence the charge control characteristics. It is shown from charge control considerations that the aspect ratio, notch width to total channel width, of stepped In/sub x/Ga/sub 1-x/As channel HFETS should be > approximately 0.3-0.4 if the full benefits to transport of including an In-rich layer in the channel are to be realized. Otherwise, any putative improvements in carrier transport will be offset by the majority of the 2DEG (two-dimensional electron gas) residing in the outer In/sub 0.53/Ga/sub 0.47/As channel region. The ideal layer structure should maximize the energy separation of the first and second subbands, with the ground state subband strongly confined to the In-rich notch. To achieve this the In/sub 0.53/Ga/sub 0.47/As layer should be as thin as epitaxial growth considerations allow.<>