Limiting layer concentration for thin-film or narrow base devices

摘要

Charge control theory has been applied to the determination of relationships between surface charge, carrier density and space-charge widening phenomena at PN junctions. This has brought to light a very useful relationship. It appears that a variety of thin-layer semi-conductor and even tunneling devices are limited by the onset of avalanche breakdown at the edge of the layer when the total fixed charge within the space-charge depletion region is of the order of 1012carriers/cm2. The detailed results are applied to a number of materials and devices. Limits for the design of field effect and avalanche devices of minimum extrinsic layer thickness, including varactor diodes and intrinsic barrier transistors, have been derived. One conclusion is that any silicon NPN transistor which does not punch through before avalanching occurs should have an intrinsic base sheet resistance less than 6000 ohms per square. Conversely, a field effect depletion type transistor must have a P channel exceeding this resistance to avoid avalanche limitations. Metal-base transistors also should have more than 1012effective carriers per square centimeter provided by the base layer in order to operate over an adequate voltage range. Other applications will be presented, relating to limits of doping or trap densities of the layers in a variety of thick or thin-film solid state devices.