The fabrication of experimental solid state heterojunction triodes is described using high resistivity cadmium sulphide films evaporated in vacuo on to low resistivity single-crystal silicon. Interfacial transmissivities for electrons through the heterojunction gate as high as 90% are obtained, with excellent saturation of drain characteristics. It is expected that useful incremental operation up to microwave frequencies should ultimately be possible with large signal switching times less than 0.1 nsec.