X-ray response of silicon surface-barrier diodes at 8 and 17.5 keV: Evidence that the x-ray sensitive depth is not generally the depletion depth

The absolute x-ray response of 18 EG&G Ortec partially depleted silicon surface-barrier diodes (SBDs) has been measured at 8 and 17.5 keV. In addition we have examined the x-ray response of four Tennelec and two United Detector Technology partially depleted SBDs. The variation in response to 8 keV x rays, for which the optical depth is about 5, is comparatively slight (≲12%). The variation in response to 17.5-keV x rays, for which the optical depth is only ∼0.7, is comparatively large (∼100%). These variations are mainly attributable to differences in the SBD physical thicknesses, and thus to differences in optical depth. At both 8 and 17.5 keV the diodes respond linearly to large variations in incident flux (over three orders of magnitude). This linearity, and the diode x-ray response, in general, is insensitive to large changes in the detector bias voltage; thus the depletion depth, proportional to the square root of the bias voltage, does not play a critical role in determining the x-ray sensitive depth. It is important to emphasize that this finding is contrary to the commonly held belief that the x-ray sensitive depth is equivalent to the depletion depth. In addition, this result has a direct bearing on both SBD and p-i-n detectors intended for fully depleted operation, but used in an underbiased mode such that they are actually partially depleted. We conclude that SBDs have attractive features for quantitatively measuring x radiation from high-intensity sources for which hν≳10 keV.