Problems in the design and fabrication of uni-junction transistors by planar technology

Numerous unijunction transistor (UJT) devices, fabricated by conventional planar technology from nominal 100 ohm-cm n-type silicon, yielded measured results quite irreconcilable with those predicted by a simple model. Three major considerations are identified. (1) Thermal oxidation produced a substantial positive surface potential; the resultant surface conductance is an important contributor to the current flow and invalidates any simple 'resistance' model. (2) Typically, diffusions designed to produce light-heavy (L-H) 'junctions' are considered prerequisite to the formation of satisfactory ohmic contacts to lightly doped material. (3) Planar-diffused n-type L-H 'junctions,' if sufficiently abrupt, exhibited avalanche breakdown when biased as an exit contact; breakdown was never observed to occur when biased as an entrance contact. A p-n junction guard ring surrounding an n-type L-H 'junction' was conceived of as a composite base one contact.