Traveling-wave amplification of microwave energy having frequencies up to 500 GHz is provided by a semiconductor (GaAs, GaP, InAs, InP) which has a thin epitaxial layer capable of exhibiting the property of negative differential conductivity. A thin resistive layer is applied on the skin of the epitaxial layer to smooth out the dc electric field profile and allow use of longer layers with a selective control of band pass and gain properties of the amplifier.