Phys 586 Laboratory

Lab 12

Goal: In this lab you will make measurements basic V-I measurementswith a MOSFET and (optionally) measure x-ray spectra with a silicon PINphotodiode.

Reading: Knoll p408-410, p774-775, and additional reading and materialsposted with the lab.

Lab:

1. Look over the data sheets for the p-channel MOSFETs we are using.What is the gate-source threshold voltage? Under what conditions isthis specified?

2. Also note the characteristic ID versus VDS curves and the ID versus VGS

curve on the second page. You can see the meaning of the gate-sourcethreshold voltage from the second curve.

3. Figure 1 shows a simplified circuit that is typically used to measure thechange in threshold voltage as function of dose. This circuit is used inmany dosimeter applications. Note the drain and gate are connectedtogether so VGS = VDS here.

4. Figure 2 shows the actual implementation of the the circuit in Figure 1.A resistor pot controls the magnitude of the constant current source.

5. Using the test setup, measure the threshold voltage as a function ofthe drain-source current for currents of 0 to 1000 µA as defined in theworksheet. You will have a set of VI curves for the pre-irradiated MOS-FETs and a set for the irradiated MOSFETS. For a given MOSFET,plot the pre-irradiated and irradiated VI curves on the same plot.

6. There are several (arbitrary) ways to define the threshold voltage. Oneis to choose the voltage value at 800 µA. Another is to draw a tangentto the higher values of IDS and call the point where the line crossesIDS = 0 the threshold voltage.

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7. Whichever method you choose, make a plot of ∆Vth (after-before) as afunction of irradiation dose. You can also assume the line should passthrough the point (0,0). The irradiation dose in grays is the last twonumbers of the MOSFET serial number. Note these MOSFETS are notdesigned as RADFET’s so this is just an experiment. Hopefully youwill see an effect. RADFET’s designed for this purpose usually havea thicker SiO2 region and different fabrication processes compared tostandard MOSFETs.

8. Comparing data with your classmates, which MOSFET performed bestas a RADFET?

9. Look over the data sheet for the XR-100CR x-ray detector. What isthe specific silicon detector used? What is the bias voltage? Why isthe detector cooled?

10. Collect data with the Maestro MCA for Cu, Rb, and Mo. Identify andcomment on any peaks found in the spectra for each.

11. Make a plot of the lnR versus lnE where R is the energy resolutiondefined by FWHM/peak.

In your lab writeup, please include:

1. Briefly explain how a MOSFET works. Note this information is in-cluded in the silicon 2 lecture posted on D2L.

2. Briefly explain why the threshold voltage changes in a MOSFET whenit is subjected to radiation.

3. The VI curves for each MOSFET. Plot the VI curve for the pre-irradiated and irradiated MOSFET on the same plot. You will have atotal of five plots with two curves per plot.

4. The ∆Vth versus irradiation dose curve. Note these MOSFETS are notdesigned as RADFET’s so this is just an experiment. Hopefully youwill see an effect.

5. As with proportional counters, one typically quotes energy resolutionsat 5.9 keV, the energy of an 55Fe source. What is the energy resolutionof the XR-100CR at 5.9 keV? How does this compare to the data sheetspec?

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IntroductionMOSFET dosimeters are now used as clinical dosimeters for radiotherapy radiation beams.1 The main advantages of MOSFET devices are:

1. The MOSFET dosimeter is direct reading with a very thin active area (less than 2 m).

2. The physical size of the MOSFET when packaged is less than 4 mm2.

3. The post radiation signal is permanently stored and is dose rate independent.2

This paper reviews the basic MOSFET structure and how it is used as a dosimeter.

The MOSFET DosimeterThe basic MOSFET structure is depicted in Figure 1. The type shown is a P channel enhancement MOSFET which is built on a negatively doped (n-type) silicon. When a sufficiently large negative voltage is applied to the polysilicon gate a significant number of minority carriers (holes) will be attracted to the oxide/silicon surface from both the bulk silicon substrate and the source and drain regions.

Once a sufficient concentration of holes have accumulated there, a conduction channel is formed, allowing current to flow between the source and drain (Ids). The voltage necessary to initiate current flow is known as the device threshold voltage (VTH ).

When a MOSFET device is irradiated, three things happen within the silicon dioxide layer (sensitive region): a build-up of trapped charge in the oxide; the increase in the number of interface traps; and the increase in the number of bulk oxide traps.

Introduction to the MOSFET Dosimeter

Technical Note: 4

� | Technical Note 4: Introduction to the MOSFET Dosimeter | TN#101248.04

X-RAY SOURCE

FIGURE 1. Schematic cross section of a P-channel MOSFET

Si02 passivation

Si02

AI AIpolysilicon gate

Si02gate oxide [sensitive regions]

P+ P+

drainsourceSi Substrate [525 m]

Au [1 m] + Ni [3 m]

Kapton [.25mm]

Electron-hole pairs are generated within the silicon dioxide by the incident radiation. Electrons, whose mobility in SiO

2 at room temperature is about 4 orders of magnitude greater than holes, quickly move out of the gate

electrode while holes move in a stochastic fashion towards the Si/SiO2 interface where they become trapped in

long term sites, causing a negative threshold voltage shift ( VTH), which can persist for years.

The difference in voltage shift before and after exposure can be measured, and is proportional to dose.

At energies below 50 keV photoelectric absorption is the dominant process while between 60 keV and 90 keV both photoelectric and compton scattering become important. For both of these mechanisms the energy deposited to the oxide (sensitive region) of the MOSFET is delivered mostly by secondary electrons.

The photoelectric cross-section depends strongly on the target material, approximately Z4. The result of this is that interface dose enhancement occurs because the photoelectric cross section is different in the Si than in the SiO2. More electrons will be produced in the Si, the higher cross section material, so more will cross from Si into SiO2 than will cross the interface in the other direction. Dose enhancement and the increasing dominance of photoelectric absorption, as the incident energy decreases from 150 keV to 10 keV, results in an increase in device sensitivity. This energy dependence is typical of most dosimeters.

The range of photon energies used in fluoroscopy is narrow and the observed MOSFET energy dependence is 10%. This is small compared to other errors inherent in skin dose measurements, making it possible to use the dosimeter in the fluorographic energy range 40kVp to 140kVp.

Oncology procedures consist of irradiating an immobilized patient with a high energy photon (1 to 20 MeV)or electron beam( 6 to 20 MeV). In these energy ranges the observed energy dependence does not exceed 5% and the MOSFETs respond identically to electrons and photons.3,4

ConclusionsThe physics of MOSFET dosimeters is now well understood so that these devices can be characterized and used for many applications in radiation dosimetry.

Small size and radio-transparency make the device an excellent choice for dosimetry in fluorography.

Low energy dependence, high sensitivity and immediate read out make the MOSFET a good replacement for TLD in radiation therapy dosimetry.

� | Technical Note 4: Introduction to the MOSFET Dosimeter | TN#101248.04

References1. Soubra, M., Cygler, J. and Mackay, G.F., “Evaluation of a Dual Metal Oxide-Silicon Semiconductor Field Effect Transistor Detector as a Radiation

Dosimeter”, Med. Phys. 21 (4) April 1994.

2. Thomson I., Reece M.H., “Semiconductor MOSFET Dosimetry”, Proceedings of Health Physics Society Annual Meeting (1988).

3. Oldham T.R., and McGarrity J.M., “Comparison of 60Co Response and 10 keV X-Ray Response in MOS capacitors,” IEEE Trans. Nucl. Sci NS-30 4377 (1983).

4. Tallon R.W., Ackerman M.R, W.T. Kemp W.T., Owen O.H., and Saunders D.P., “A Comparison Of Ioinizing Radiation Damage in MOSFET’s from 60Co Gamma Rays, 0.5 To 22 Mev Protons and 1 to 7 MeV Electronics”, IEEE Trans. Nucl. Sci. NS-32 4393 (1985)

5. Ma, T.P., Dressendorfer, P.V., “Ionizing Radiation Effects in MOS Devices and Circuits”, 1st edition, (Wiley New York, 1989).