International Radiation Detectors, Inc.

Linearity

Figure 1 shows the linearity of the AXUV/SXUV/UVG photodiode and a widely used p-on-n photodiode of equivalent area when exposed to increasing levels of 430 nm radiation. All the diodes were tested without any reverse bias. The standard ac-dc method was used to measure linearity. The p-on-n photodiode showed a noticeable decrease in responsivity at photocurrents greater than 500 µA while the IRD photodiode showed only 0.02% decrease in responsivity at a photocurrent of 3 mA. Application of a reverse bias will extend the linear range of the UVG photodiodes when measuring UV radiation. It is believed that the difference in the series resistance of these diodes can explain the large difference in linear range [1].

Fig. 1: Linearity of IRD photodiode and a widely used P-on-N photodiode when
tested at 430 nm with no reverse bias.

Response linearity to pulse radiation has been studied recently [2,3]. The higher the reverse bias on the detector, the higher the upper limit on the linear range. Because of their lower responsivity, SXUV diodes have higher upper limit of linear range compared with the AXUV and UVG diodes for a given reverse bias. Because of their high responsivity, IRD diodes can not be used for measurement of pulse energies in excess of 100 μJ even with a front attenuating layer or a directly deposited filter. For measurement of UV/EUV pulses with high energy, an attenuating metal mesh can be used in front of the diodes. Care needs to be taken to conduct the heat generated in the metal mesh which typically is in the vacuum system.

References:

1] R. Korde et. al. “Present Status of Radiometric Quality Silicon Photodiodes”,
Metrologia, Vol. 40, S145-S149 (2003).

2] R. Stuik and F. Bijkerk “Linearity of P-N junction photodiodes under pulsed irradiation”, Nuclear Instruments and Methods in Physics Research A 489, 370-378 (2002).

3] R.E. Vest and Steven Grantham “Response of a Silicon Photodiode to Pulsed Radiation”, Applied Optics, Vol. 42, No. 25, 5054-5063 (2003).