Reverse bias and PIN diode sensitivity. Building a solid state radiation detector

Contents

1 Introduction

2 Motivation

3 Small Radiation Detector set-up

4 Laboratory testing
4.1 Finding the optimal reverse bias voltage
4.1.1 Introduction
4.1.2 Experiment
4.1.3 Results and data analysis
4.1.4 Interpretation and conclusion
4.2 Temperature dependence of parcel sensitivity
4.2.1 Introduction
4.2.2 Experimental set-up
4.2.3 Result and conclusion

5 Conclusion

Appendix A: Basic Specifications of the Small Radiation De- tector

Appendix B: Technical details

Abstract

A prototype of a solid state radiation detector based on a PIN diode has been developed to the point where it was ready for a test flight on a weather balloon. This device is intended to be used to monitor secondary cosmic radiation and may have the potential to be produced commer- cially since space weather is an issue of growing importance concerning the economy and technology of modern civilisation. Some of the most crucial experimental work in preparation for the test flight is discussed in this report. One important question was what reverse bias should be applied to the PIN diode. As a result, it was found that the detector’s sensitivity increases with increasing reverse bias until it reaches a satura- tion value at ∼ 25V. Another concern was that the prototype may not be robust against temperature differences. This concern could be ruled out to a certain extent.

1 Introduction

In an ongoing project in the Department of Physics at the University of Ox- ford there’s an effort to build a solid state detector for atmospheric ionisation measurements, in particular the measurement of secondary muons generated by cosmic rays.

During a 7 week project, several tasks related to the development and laboratory testing of the device were completed, such that in the end the detector was ready for a test flight on a weather balloon. The most important tasks will be discussed in this report.

2 Motivation

Galactic cosmic radiation (GCR) has effects in the atmosphere that are still not understood very well.

Also, space weather is dictated by GCR and it is very important for the safety of spacecrafts, especially satellites. It has been estimated that during the period 1994-1999 alone, over 500 000 000 $ in insurance claims were disbursed due to on-orbit failures of satellites related to space weather.¹

It should come as no surprise that space weather is of concern both scientifically and commercially, especially in this day and age where there are more satellites than ever before orbiting Earth and there is even more money at stake than in the aforementioned period.

Still, there’s a discrepancy between models and measurements that needs to be understood better. Further research in this field could be facilitated with the Small Radiation Detector because of its unique qualities compared to other devices used in ionisation measurements. Typical measurements can be done with different kinds of technology: Ionisation chambers, Geiger counters or solid state detectors.

Ionisation chambers contain an isolated electrode in a chamber filled with a fixed amount of gas. The ionisation rate can then be calculated from the rate of decay of voltage on said electrode. Cosmic rays were first discovered in 1908 by Bergwitz using this technology. He didn’t publish his results, though, because he suspected an error in his measurements. However, he told his colleague Victor F. Hess about it and in 1912 the latter could prove in a more rigorous experiment the existence of what is now called secondary cosmic radiation.² With Geiger counters, the idea is to have a voltage of a several hundred volts across a low pressure noble gas in a sealed pipe. When any ionising particle enters the pipe, the gas gets ionised in a cascade effect which results in a drop in voltage that can be measured and thus the amount of such events can be counted. While ionisation chambers give us some idea of the energy distribu- tion of ionising particles from looking at the length of the track that such a particle leaves in the chamber, Geiger counters don’t have that feature. Yet, Geiger counters are widely used for air ionisation measurements because the technology is simple and effective compared to ionisation chambers. However,

Geiger counters have further disadvantages, including fragility and the need for high voltage, which can be difficult or expensive to provide on a test fligth. In a PIN diode setup under reverse bias, energetic particles can ionise solids just like they ionise gas in the two measurement methods discussed above. However, PIN diodes enable energy discrimination and can be run at very low voltages compared to Geiger counters (∼ 10V instead of ∼ 500V). This is why a solid state detector can be a good alternative for these kinds of measurements.

3 Small Radiation Detector set-up

Figure 1 shows the basic set-up of a PIN photodetector. Basically, it is a PIN diode with a voltage applied such that the negative terminal is connected to the p-doped material and the positive terminal is connected to the n-doped material. The result of this so-called reverse bias is that holes (positive charge carriers) in the p-type region and electrons (negative charge carriers) in the n-type region get pulled away from the depletion layer. This means that the depletion layer widens and in a static situation there is practically no current flowing, apart from a small residual, ”dark” current IS . Furthermore, the I-layer between the p- and the n-layer is an intrinsic semiconductor, which also increases the size of the depletion area by a multitude. This is important in a photodetector because only electron-hole pairs created by incident photons in (or near) the depletion area will get sweeped out of the region by the reverse bias which then creates a current. In conclusion, a large depletion region is needed for maximum efficiency of the photodetector. As one can see from figure 1, only photons (or other kinds of ionising radiation) of sufficient energy, i.e. energy higher than the band gap in the intrinsic semiconductor, will create electron-hole pairs in the active area of the PIN diode.

The PIN diode used as a detector in this project is the PS100-7-CER-2. Because we are mainly interested in high energy radiation, namely muons, the sensor circuit needs to be shielded from lower energy radiation, like visible light, which would result in unnecessary noise. This is done by wrapping the sensor circuit in copper foil, as shown in figure 2.

Figure 3 shows the complete set-up of the Small Radiation Detector. The parcel in figure 2 is installed on a PCB (printed circuit board) with different PICs. Its main functions are to amplify the signal coming from the photodiode and to convert these analogue pulses into digital ones.

The whole detector runs on a supply voltage of 16V, has dimensions of approxi- mately 10cm×5cm×1cm and weighs about 30g. The data output is RS232/TTL to USB. It gives data in the ASCII format about the total number of counts, the time since start in seconds and milliseconds, as well as the pulse height.

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Figure 1: Schematic of a PIN photodiode with reverse bias, credit to³

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Figure 2: Sensor circuit, including the PIN diode, wrapped in copper foil

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Figure 3: Complete detector: Parcel on PCB with PIC microcontrollers

4 Laboratory testing

4.1 Finding the optimal reverse bias voltage

4.1.1 Introduction

The sensitivity of the PIN photodiode used in the Small Radiation Detector project, PS100-7-CER-2, was tested by measuring signal rates for different reverse bias voltages while exposing the photodiode to a radioactive source of a known, constant amount of radiation.

The active region in the PIN photodiode is identical to the depletion layer within the intrinsic semiconductor (the I-part of the PIN-photodiode). The size of the depletion layer increases when the reverse bias voltage is increased, up to a point where the entire intrinsic semiconductor is depleted (saturation). In this experiment, the goal was to find out how the depletion layer size is scaling with reverse bias, and in particular to find the saturation point for the reverse bias to allow for maximum sensitivity. It is important to know this value because applying reverse bias voltages higher than this would be unnecessary and a waste of energy.

4.1.2 Experiment

In order to learn more about the effect of different reverse biases on the sensitiv- ity of the PIN photodiode, the parcel containing it was separated from the rest of the Small Radiation Detector. Cables were attached such that the reverse bias could be changed and regulated by a DC power supply. The measurements at different reverse bias voltages were done using an amplifier (ORTEC 440A), adjusted for a gain of 32 and a pulse shaping time of 0.25µs

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