Mobility profiles in WnX‐LDD (lightly doped drain region) and WNX‐BPLDD (buriedp‐type buffer lightly doped drain region) GaAs metal–semiconductor field‐effect transistor (MESFET) with variousp‐buffer layers are discussed. The mobility profiles are evaluated using frequency dependent admittance studies. A slightly dopedp‐buffer layer seems to have no influence on the carrier concentration while there is a significant modification of the mobility profile at lower gate voltages. The channel drift mobility in BPLDD‐MESFETs with heavily implantedp‐buffer layers is degraded over the whole gate voltage swing. Mg‐p‐type implants have no effect on the carrier profile near its peak while the tail region is shifted to higher gate voltages. However, to observe a significant sharpening of the channel electron concentration profile a certain amount of Mg implantation dose seems to be necessary.