A new analytical model for InAlAs/InGaAs heterojunction, InP based high electron mobility transistor (HEMT) incorporating the depletion effect in the InAlAs region is developed and extended to predict sheet carrier density, the effect of parallel conduction through variation of depletion width, potential at the interface of depletions and DeltaE(c) - E-f with gate voltage. The sheet carrier density obtained shows excellent agreement with the available results and gives a new solving procedure for HEMT from MESFET analysis. (C) 2002 Elsevier Science Ltd. All rights reserved.