Geometric Control of Surface Leakage Current and Noise in Lithium Drifted Silicon Radiation Detectors

Lithium drifted silicon radiation detectors exposed to normal ambient have been shown to have a surface n-type layer which extends from the "n" side over to the " p" side of the device. In this paper a continuation of the study of surface problems is presented, and a successful method for the control of leakage current and noise due to surface breakdown is shown. Noise measurements carried out on Li-drifted silicon devices show clearly the existence of two mechanisms for surface noise generation. One has the characteristics of shot noise, while the second one is much noisier than shot noise. Surface potential measurements correlated with light probe scanning give indication as to the origin of the currents which cause the two types of noise. The measurements also suggest the desirability of increasing the resistivity of the surface layer, and show that this can be attained by increasing the magnitude of the internal fields normal to such surface. One possible way of increasing the magnitude of the fields normal to the exposed junction surface consists in shaping the Si crystal so that, after the Li drifting process has been completed, regions of high fields exist within the compensated region. A comparison of cylindrical, planar, and "inverted T" types of p-i-n devices shows the superiority of the third type of device in that it is possible to apply high bias (between 150 and 200 V/mm at room temperature) before the onset of the high noise mechanism.