The distribution of the electrical potential around the edge of the field plate in a silicon planar diode has been solved by means of a computer. The breakdown voltage related to this region has been derived as a function of impurity doping in silicon and of oxide thickness. If the oxide layer is not thick enough, avalanche breakdown voltage is often considerably lower than the value allowed by the material. The effect of Q ss has been found to be negligible. Breakdown voltage vs. gate voltage measurement on gated diodes and avalanche light emission support an edge breakdown mechanism. Breakdown voltage on field plated diodes matches rather well with the calculated values. It normally drifts towards higher values and during this walkout a hot carrier current is collected by the field plate. The field plate current is fairly proportional to avalanche current and increases with bulk doping. A new planar diode structure suitable for a BV up to 900 V has been designed. The discharge voltage in air between field plate and equipotential ring (EQR) is shown to be correlated with the actual field at the edge.