substantially with the 100 direction of the silicon monocrystal it has already been mentioned that a low electron density is advantageous this also gives distribution of the dissipated heat over an adequately large volume the cooling means 16 may consist of copper plates which are thermally coupled to the semiconductor crystal 12 or to the waveguide portion 14 containing said crystal or it may form the waveguide portion the crystal 12 may also be employed free i e without surrounding waveguide portion or cavity resonator in this case the holding is by dielectric components and the cooling expediently by a nonabsorbent liquid such as alcohol the coupling in and out of the microwave radiation can be carried out in this case by horn radiators or the like the cooling of the crystal to for example 70 k and below reduces the microwave absorption and at the same time increases the thermal conductivity if a phase mismatch manifests itself in a troublesome manner phase matching can be achieved by forming the semiconductor crystal itself as waveguide portion 22 as shown in simplified manner in fig2 and by choosing the boundary conditions so that the waveguide dispersion compensates the material dispersion formation of the semiconductor body as hollow waveguide also makes it possible to control the mode composition in such a manner that a substantially flat intensity profile results by means of an external magnetic field b as indicated schematically in fig2 a resonant increase of the nonlinearity and a phase matching can be achieved the magnetic field forces the free electrones between the surges on circular orbits and in the microwave range the cyclotron resonance is a few tenths of a tesla a few kilogauss the then also.