Diffusion masking and all-diffused germanium NPN mesa transistors

A simple process for the fabrication of all-diffused germanium NPN mesa transistors is described. The starting material is an N-type wafer with a P-type diffused skin. A key feature of the process is the use of a film of silicon monoxide evaporated through a mask to coat selected regions of the germanium wafer. The silicon monoxide serves as a diffusion mask to restrict the subsequent diffusion of N-type emitter doping elements to uncoated regions of the wafer. Ohmic contacts to the emitter and base regions are made by vacuum evaporation through appropriate masks. The process is applicable to a wide range of NPN transistors and permits excellent control of device geometry and electrical characteristics. The process is discussed in connection with the development of a high-speed switching transistor. Essential characteristics of this device are: emitter breakdown voltage--5 volts, base resistance--150 ohms, common base alpha frequency cut-off > 500 megacycles, common emitter current gains--100.