A new noncapacitive method of determining net surface charge densities using floating field ring devices

A new method has been developed for determining net surface charge densities in semiconductor surfaces that are passivated with materials such as glasses or silicone rubber for which it is difficult or impossible to make standard measurements. The method is based on measurements of the potential of floating field rings around reverse-biased planar diodes and theoretical calculations of this potential that include the effects of the surface-charge density. A feature of the technique is that the surface charge is determined under conditions of deep depletion and after all processing is completed. This is important if the surface-charge density is to be used to help predict field ring behavior or avalanche breakdown voltage. The experimental technique used is first to measure the floating field ring potential as a function of the applied reverse bias between the anode and cathode. The value of surface-charge density is obtained by comparing the measured field ring dependence with theory. Although the specific dependence of the field ring voltage on the surface charge is unique to the device under test, shifts in voltage by 70 V for a surface charge density of 10¹¹cm^-2are typical of 1000-V devices.