Saturation Current in Alloy Junctions

According to theory, the current which flows across a P-n junction when it is biased in the reverse direction should be nearly independent of the applied voltage. This current is called the saturation current, I8. In actual practice, the current often increases with voltage (due to leakage paths across the junction at the surface) and eventualy large currents flow as the breakdown voltage of the junction is approached. However, for "good" junctions well below their breakdown voltage, the reverse current follows theory so that the magnitude of I8 is of considerable practical interest. A previous equation for saturation current has been given which applies where the effect of free surfaces may be ignored and where there are quite thick (greater than a diffusion length) layers of material on either side of the junction. For alloy junctions made on thin pieces of semiconductor, these conditions are violated. In this paper a new equation is developed which applies specifically to diodes made by alloying circular junctions on thin wafers. Over its range of applicability, it gives accurate values of I8. It is shown that most of the saturation current comes from thermal generation at the free surfaces of the base wafer. The equation for I8 consists of a geometrical term and a coefficient which depends on the physical constants of the material. The principal dependences are these: I8 increases linearly with base wafer resistivity and exponentially with temperature. It also increases, but more slowly, with wafer thickness and surface recombination velocity.