 In 1956, Philco had developed an "improved" higher-speed version of its original surface-barrier transistor, which was used in military applications and was called the surface-barrier diffused-base transistor (SBDT).Philco had used surface diffusion of a gaseous form of phosphorus atom particles, to penetrate the surface of the intrinsic semiconductor base material.The Philco SBDT transistor was capable of operating in the UHF range.