A travelling wave amplifier of signals in the millimeter wavelength range consists of a monolithic solid state waveguide structure wherein space harmonics of the input electromagnetic energy wave (signals) are generated due to periodic corrugations of the guide's top surface. The waveguide structure includes a current conductive layer supportive of a stream of electrons with an electron velocity v.sub.e, the stream of electrons being located where the amplitude of the spatial first harmonic is a maximum. The corrugation periodicity L is selected so that the equality v.sub.e = K (.omega./2.pi.) L is satisified. In the equality, .omega. is the angular frequency of the input wave and K is a factor which is not less than and on the order of one.