Calculating drain delay in high electron mobility transistors

•Derived drain delay (τD) expression in FETs with non-uniform velocity.•For same depletion length (xm) & velocity profile (v[x]), τD> collector delay.•For same xm & v[x], τD for FETs with field plates >τD for FETs without field plates.•For constant velocity (vsat), τD=xm/(αvsat).•Derived α in terms of weighting function and in terms of image charges.