to realize a depletion - mode modulation - doped field - effect transistor with high gate - length to depletion - depth ratio that is capable of providing high power gain at millimeter - wave frequencies , an ohmic gate or a heterojunction gate is used on the n - algaas / gaas layered structure , replacing the prior art shottky - barrier metal gate . the depletion - mode operation is desirable for analog signal amplifying circuits as opposed to the enhancement - mode device commonly used for switching or digital circuits . in the case of schottky - barrier gate , high aspect ratio structures naturally operate in the enhancement mode , hence the need for the change in the gate electrode structure .