Linearity of P–N junction photodiodes under pulsed irradiation

Abstract

The dependence of the sensitivity on the radiation pulse length for a P–N junction photodiode has been investigated over an extended range of pulse lengths, from 170 ns to 1.2 ms. The power incident on the diode surface was varied between 1.6 and 118 mW. A novel method was used to generate the light pulses with variable length, while keeping the temporal pulse shape and the intensity constant. The method consists of using a rotating mirror in combination with a DC light source, in our case at 633 and 532 nm. In this way, the pulse shape only depends on the geometry of the setup, with the pulse length solely determined by the rotation frequency of the mirror. No further calibration is needed for determination of the pulse intensity and shape. Accuracies obtained are better than 2%, mainly determined by instabilities in the setup. The sensitivity of an IRD AXUV-100 photodiode was studied, both with and without a reverse bias voltage applied. At unbiased conditions and irradiation levels well below the saturation intensity, the photodiode had a constant, frequency-independent sensitivity over the full range of pulse lengths investigated. The sensitivity decreased at irradiation levels approaching saturation, with the largest decrease at longer pulse lengths. This decrease was fully accounted for by electron–hole recombination at a typical time scale of 15±5 μs. Under biased conditions and pulse lengths below 5 μs, the diode sensitivity remained constant for all incident power levels investigated. At longer pulse lengths a decrease in sensitivity was observed, both at low and high power. This second type of decrease is attributed to the frequency response of the bias electronics. An extrapolation of the results to other wavelength ranges is given, including the extreme ultraviolet (EUV) and soft X-ray ranges.