An NPN germanium double-diffused mesa transistor

A double-diffused NPN germanium mesa transistor has been designed for ultra-high Speed switching applications. This device is intended for switching circuit use in the 1 to 100 mA range, affording the advantages of complementary circuitry in conjuction with such standard PNP devices as the 2N705 and 2N828. Switching times of the device described here are, however, lower by a factor of from two to three than in the PNP devices. The geometry of the active device approaches the smallest compatible with fabrication techniques. The fabrication of the device involves the use of tunnel PN junction ohmic contacts, the selective masking of diffusions, and the formation of ohmic contacts by photo-resist processes. Some effects of surface passivation on this NPN germanium device are also discussed. By virtue of its small geometry and the higher minority carrier mobility inherent in the NPN structure, frequency response is much improved over the standard PNP mesa transistors. Current gain bandwidth products as high as 1500 MCPS are commonly observed.