CA1204885A

CA1204885A - Dosimeter

Info

Publication number: CA1204885A
Application number: CA000491034A
Authority: CA
Inventors: Ian Thomson
Original assignee: Thomson and Nielsen Electronics Ltd
Current assignee: Thomson and Nielsen Electronics Ltd
Priority date: 1985-09-18
Filing date: 1985-09-18
Publication date: 1986-05-20
Also published as: US4678916A

Abstract

ABSTRACT

The invention is a radiation dosimeter and method of operation of its elements comprising a pair of silicon insulated gate field effect transistors integrated into the same substrate, each having a gate, a source and a drain. The dosimeter includes apparatus for measuring a first differential threshold voltage between the transistors, apparatus for forward biasing the gate of the first transistor and inhibiting operation of the second, while allowing both to be subjected to ionizing radiation, and apparatus for measuring a second differential threshold voltage between the transistors following the irradiation. The first differential threshold voltage can be subtracted from the second to obtain a measure of the radiation dosage. Threshold drift and offset is thereby substantially eliminated.

Description

~LZ~8~3S

01 This invention relates to a dosimeter for 02 measuring ionizing radiation and particularly to a 03 dosimeter using an insulated gate field eEfect 04 transistor (IGFET) as a sensor, having substantially 05 improved accuracy.
06 IGFET sensors have previously been used in 07 ionizing radiation dosimeters. It had been found that 08 radiation shifted the threshold voltage of the IGFET;
09 in order to determine the amounk of radiation dose received, the amount of shift of the threshold voltage 11 was measured.
12 The theory behind the use of IGFETs as 13 sensors in dosimeters, and an operational dosimeter 14 have been described in the papers "RADIATION DOSIMETRY
WITH MOS SENSORS" by I. Thomson, R.E. Thomas and 16 L.P. Berndt, "RADIATION PROTECTIO~ DOSIMETRY", 17 Volume 6, No. 1 - pp 121 - 124, December 1983, and 18 "NEW DIRECT READING SOLID STATE DOSIMETER" by 19 I. Thomson given at the 28th Annual Meeting of the Health Physics Society, June 1983, Baltimore, 21 Maryland, U.S.A.
22 In this specification, the term IGFET
23 refers to a field effect transistor fabricated on a 24 silicon substrate and having an oxide insulator between the gate electrode and ~he silicon substrate.
26 The gate electrode can be either metal or 27 polycrystalline silicon dioxide.
28 In use of such IGFETs, a number of 29 problems have been recognized. The threshold voltage has been found to vary with temperature, i.e.
31 approximately -2mV/C. With no temperature 32 compensation, a 1C. shift in temperature has been 33 found to cause an approximately error of 1 cGy. In 34 military applications, dosimeters are typically required to cover the temperature range -20C. to 36 +50C., a 70 difference, which would give rise to a 37 70 cGy error. Due to the large error, use of the ~2~

01 system described earlier in which the threshold 02 voltage shift is measured to define received dosage 03 has been found to be unfeasible in practice where wide 04 temperature ranges are expected to be encountered.
05 Such devices also have an inheren-t zero 06 offset which varies from device to device and from 07 wafer to wafer. For example, the threshold volt~ge at 08 zero dose has been found to average approxima-tely 5 09 volts. However even if the threshold voltage was controlled to 10%, -this would be insufEicient to give 11 all devices the same threshold voltage. In volume, 12 10% threshold voltage is considered to be as good as 13 is practically possible for volume production.
14 However this represents approximately 500 millivolts variation, or approximately 250 cGy error. This error 16 has been found to be too great a spread for sensors in 17 the 10 cGy to 1,000 cGy range. The previous solution 18 to this problem was to offset each device individually 19 with a resistor or to measure, record and calculate the zero offset using a computing device.
21 A third problem associated with such prior 22 art devices is zero drift in the threshold voltage, in 23 addition to the temperature effect. This phenomenon 24 has been found to be reproducible and occurs on switching on a P-channel IGFET. The phenomenon is 26 believed to be due to slow surface states in the 27 silicon/silicon dioxide interface. This drift has 28 been measured as typically 10-20 mV maximum, and is a 29 drift in the same vol-tage direction (i.e. the threshold voltage increases by 10-20 mV).
31 The present invention is a radiation 32 dosimeter which uses an IGFET sensor, which 33 substantially overcomes the aforenoted problems to an 34 extent that a substantially improved and prac-tical dosimeter results. Both indirect and direct reading 36 dosime~ers can be fabricated, useful by the military, 37 for example, both as personal and tactical 01 dosimeters. Such dosimeters operate in the range of 02 10-1,000 cGy (gamma + neutron) in the temperature 03 range noted earlier.
04 The present invention solves -the problems 05 of drit, whichever of the cause no-ted above, by 06 measuring the differential threshold between two IGFET
07 sensors exposed to the same radiation, in which one is 08 biased into its conducting region, and the other is 09 biased either off or to a conducting level less than the first. Since slow surface states and temperature 11 affect both IGFETs in the same manner, the drif-t will 12 proceed substantially the same, and any differential 13 resulting from the drift will be zero or minimized.
14 However since one of the IGFETs is biased either off or to a conducting level less than the first, the 16 threshold voltage of the first transistor varies with 17 the dosage to a greater degree than that of the second 18 IGFET. Thus the measured differential threshold 19 vol-tage between the two transistors will be a measure of the gamma radiation dose.
21 It should be noted that the reading of the 22 radiation dose is effected with the gates of each of 23 the transis~ors connected to their drains, and with 24 equal currents passing through their drain-source circuits. Thus because the differential threshold 26 voltage is measured, the effect of zero offset is also 27 eliminated.
28 In order to ensure as closely as possible 29 that both transistors would have the same threshold change with temperature, the same drift, and the same 31 offset, it is desired that both transistors should be 32 fabricated in the same silicon subs-trate, i.e. in the 33 same die. Preferably the gate oxide should be greater 34 than or equal to 0.5 microns, and preferably is P-channel. It is also preferred -that a polysilicon 36 gate should be used. However it should be noted tha-t 37 metal gate devices could be used, as well as ~-channel 01 Eield effect transistors.
02 In spite of the above precautions, it may 03 be found that -there remains a -threshold difference 04 between the two transistors. Thus prior to 05 irradiation, it is preferred that the differential 06 threshold between the two IGFETs should be measured.
07 After irradiation, and measurement of the differential 08 threshold, -the first differential threshold can be 09 subtracted from the second differential threshold, to obtain an accurate dosage m~asurement.
11 As a further alternative, a resistor can 12 be connected in series with the source-drain circuits 13 of each of the transistors, which can be varied (e.g.
14 by laser trimming) to precisely equalize the threshold voltages, i.e. to cause the differential threshold 16 voltage to be zero.
17 According to a further embodiment of the 18 invention, each transistor is replaced by a cascade of 19 source-drain connected transistors, in order to increase the sensitivity.
21 In another embodiment, four IGFETs are 22 connected as a bridge, which doubles the sensitivity 23 of the two transistor embodiments, allows the use of 24 only one current source as opposed to two during measurement of the dose, and allows use of the same 26 gate bias during irradiation as in the dual IGFEI' 27 embodiment.
28 During irradiation, in one embodiment, the 29 gate of the second transistor of the dual IGFET pair is connected to zero volts (e.g. to its own drain).
31 In another embodiment, however, i-t is given a voltage 32 bias, which is in the same polarity direction but 33 lower value as the first IGFET. This embodiment has 34 been found to minimize sensitivity difference between the two IGFETs.
36 Once the first IGFET has received 37 sufficient radiation to saturate it, the second IGFET

~2~885 01 can merely replace it, and the saturated IGFET be used 02 as the second IGFET. This can be accomplished by the 03 use of switches or by a simple connector which is 0~ reversed in position, and which may be permanently 05 fixed to the substrate or base carrying the IGFETs.
06 This allows the dosimeter to have twice the dosage 07 range of a dosimeter which uses a single IGFET.
08 Thus by the use of at least two IGFETs in 09 the sensor, not only i3 -the dosage range doubled, but the problems of voltage drift with temperature or due 11 to slow surface states and zero offset are 12 substantially reduced or eliminated.
13 In basic form, the invention is a 14 radiation dosimeter and method of operation of its elements comprising a pair of silicon insulated gate 16 field effect transistors integrated into the same 17 substrate, each having a gate, a source and a drain.
18 The dosimeter includes apparatus for measuring a first 19 differential -threshold voltage between the transistors, apparatus for forward biasing the gate of 21 the first transistor and inhibiting operation of the 22 second, while allowing both to be subjected to 23 ionizing radiation, and apparatus for measuring a 24 second differential threshold voltage between the transistors following the irradiation. The first 26 differential threshold voltage can be subtracted from 27 the second to obtain a measure of the radiation 28 dosage.
29 A better understanding of the invention will be obtained by reference to the detailed 31 description below, in connection with the following 32 drawings, in which:
33 Figures lA, lB and lC respectively 3~ illustrate schematic diagrams of a IGFET, an IGFET in a circuit ready for irradiation, and in a circuit for 36 m~asurement of its threshold, all according ~o the 37 prior art;

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~2~4885 01 Figure 2 is a basic circuit according to 02 the present invention in its form prepared for 03 irradiation;
04 Figure 3 is a circuit according to the 05 present invention prepared for measurement of dosage;
06 Figure 4 is a plan view of a physical 07 layout for implementing the present invention in a 08 locket or badge;
09 Figure 5 is section A-A of the apparatus illustrated in Figure 4;
11 Figure 6 is a schematic diagram of the 12 circuit shown in Figure 4 in its configuration 13 prepared for accepting irradiation;
14 Figure 7 is a schematic diagram of a circuit for measuring the radiation received by the 16 sensor of Figure 6;
17 Figure 8 is a schematic diagram of a 18 direct reading dosimeter according to the invention;
19 Figure 9 is a schematic diagram of a basic sensor for measuring both gamma and neutron radiation;
21 Figure 10 is a plan view of a physical 22 implemation of the circuit of Figure 9;
23 Figure 11 is a schematic diagram of 24 another embodiment of the sensor according to the present invention;
26 Figure 12 is a schematic diagram of 27 another embodiment of the sensor according to the 28 present invention;
29 Figure 13 is a graph of sensitivity against gate voltage of the IGFETs of Figure 12;
31 Figure 14 is a schematic diagram of a 32 sensor according to a further embodiment of the 33 invention; and 34 Figure 15 is a schematic diagram of the circuit of Figure 14 in a configuration in which 36 dosage is to be read.
37 Turning now to Figure lA, an IGFET is s 01 shown of the kind used in the present invention. The 02 IGFET is preferably an MOS field effect transis-tor 03 using either a metal gate or silicon gate, having 0~ source, drain and gate terminals, and is preferably of 05 P-channel -type. The ga-te oxide is preferably 0.5 06 microns or greater in order -to operate suitably. The 07 body or substrate of the device is connected -to the 08 source.
09 In Figure l(B), battery 2 is shown connected -to the IGFET 1 as in the prior art, with its 11 positive pole connected to the gate and its negative 12 pole connected to both the source and drain 13 terminals. The device is now connected in a circui-t 14 whereby i-t can be exposed to radiation. The threshold of operation moves in the negative direction for bo-th 16 P-channel or ~-channel devices.
17 In Figure l(C), the same IGFET 1 which was 18 shown in Figure l(B) is connected with its gate 19 connected to its drain, and a current source ISD
connected to the source as in -the prior art. The 21 threshold voltage VT is measured across the source 22 and drain. The -threshold which is measured provides a 23 determination of the dose received by the IGFET.
24 As noted earlier, the prior art dosimeter described above suffers from variation in threshold 26 voltage with temperature, drift of threshold and zero 27 offset.
28 Figure 2 illustrates the basic embodiment 29 of the present invention. Two IGFETs 3 and 4 have their sources connected together at S and their drains 31 Dl and D2 connected together, with a battery 5 or 32 other device to provide bias potential connected 33 between the gate of IGE'ET 3 and its drain Dl and 34 source S, the gate of IGFET 4 connected to its drain D2~ Both IGFETs are of the same type, similar to the 36 IGFET 1 o~ Figure 1, but are ~abricated in the same 37 die in order that they should have the same :~L2(~38S

01 temperature variation charac-teristics, the same 02 substrate resistivity and the same slow surface s-tates 03 prior to irradiation. Preferably the gate o~ide of 04 each IGFET should be greater or equal to approximately 05 0.5 microns, i.e. is a thick oxide IGFET.
06 Both IGFETs shown in Figure 2 are 07 subjected to -the sarne ionizing radiation. It has been 08 Eound that more charge will accumulate under the gate 09 of biased IGFET 3 compared with that of IGFET 4, shifting its threshold.
11 Figure 3 illustrates the same IGFETs 3 and 12 4 in a circuit prepared in order that the dosage can 13 be read. The sources of the IGFETs are connected 14 together but the drains are not. The gate of IGFET 3 is connected to its own drain Dl and the gate of IGFET
16 4 is connected to its own drain D2. A source of D.C.
17 or A.C. current Il is connected to drain Dl and a 18 source of D.C. or A.C. current I2 is connected to 19 drain D2. Both sources of current pass through the common source, thereby causing current Il to pass 21 through IGFET 3 and current I2 to pass through IGFET
22 4. Both currents are the same value.
23 The voltage ~VT is measured across the 24 drains Dl and D2. This is the differential threshold voltage, and will be found to be a relatively accurate 26 representation of the dosage received, with 27 tempera-ture and surface state drift as well as zero 28 offset substantiall~ cancelled out.
29 In case there is a difference between the thresholds of both transistors 3 and 4 prior to 31 irradiation, the circuit of Figure 3 is set up prior 32 to irradiation, and the voltage ~VT measured as a 33 irst differential -threshold. The cir~uit oE Figure 2 34 is then set up and the IGFETs irradiated. In order to read the accumulated dose, the circuit of Figure 3 is 36 then again set up, and ~VT measured. The difference 37 between the second differential threshold voltage and 01 the first differential threshold vol-tages is an even 02 accurate represen-tation of the dosage which has been 03 received.
04 The sensitivity of the dual IGFETs can be 05 expressed as ~VT/D = Sl - S2 which equals 0 for the 06 case in which the gate bias equals zero, or about 07 2mV/cGy for the case in which the gate bias is equal 08 to 3 volts, or about 5mV/cGy for the case in which the 09 gate voltage is greater than 10 volts.
The temperature sensitivity of the dual 11 IGFET sensor has been found to be smaller than 12 0.1mV/C. Over the military temperature range 13 described earlier avT = 7mV = 1-3cGy, depending on 14 bias, which is a substantial improvement over the 70cGy of the single IGFET sensor of the prior art.
16 Experiments with irradiated type TN502 17 IGFET devices in a dual mode as described above 18 indicate that at least for low radiation dosage, the 19 effect of -threshold volta~e increases due to surface states is reduced to smaller or equal to lmV, which 21 appears to be adequate ~or most applica-tions in the 22 10 - 1,000 cGy range.
23 To fabricate a remote reading type of 24 dosimeter, ~he two IGFETs can be connected in a dual in line package 5 shown in plan in Figure 4 within a 26 housing. Also located within the housing is a our 27 switch minidip switch 6, and several wristwatch 28 batteries 7. The housing can be a badge or locket 8 29 worn on the lapel on a person.
The designations of the terminals in the 31 dual in line package 5 are shown, in which the gates 32 and drains of the two IGFETs Gl, D2, Dl and G2 are 33 opposite each other in the two inner terminals of 34 opposite sides of an eight terminal dual in line 3$ package, the source being at either one or the o~her 36 end. This allows the minidip package to be removed, 37 turned 180 and plugged back into i~s socket, which 38 _ 9 _ 01 facilitates eEfective interchanging of the two IGFETS.
02 Such a package is shown in Fiyure 5 in 03 which the package 5 is plugged into a socke-t 9 which 04 is retained within a locket base 10. The bottom oE
05 the socket 9 con-tains terminals 11 for connection to 06 an external circuit. The minidip switch 6 is also 07 retained on the base 10. Connections between the dual 08 in line package terminals and the minidip switch are 09 not shown, but can be fabricated by a person skilled in the art understanding this invention upon a readiny 11 of the description related to Figure 6.
12 A protective cover 12 is screwed over the 13 base, which cover is preferably plastic and may 14 include a photon filter for stopping x-rays. The housing 10 is preferably made out of the same material 16 as cover 12 and may also include a photon filter for 17 x-rays.
18 The circuit of Figure 6 illustrates the 19 internal connection within the base 10. IGFETs 3 and 4 have their sources connected together, the common 21 source being connected through SW4 to the gate of 22 IGFET 4. The drain D2 of IGFET 4 is connected to its 23 own gate through switch SW3. Battery 5 has its 24 positive terminal connected through switch SWl to the gate Gl of IGFET 3, and its negative terminal 26 connected to the source (or the common drain, if the 27 appropriate type of IGFET is used). The two drain 28 terminals Dl and D2 are connected together through 29 switch SW2.
Each of the switches correspond to a 31 switch of minidip switch 6. While both IGFETs of the 32 sensor are being irradiated, all of the switches are 33 closed. However during reading of the dosage, all of 34 the switches are opened. The circuit terminals are accessed through terminals 11 (Figure 5) and the 36 circuit of Figure 7 is set up. The sensor IGFETs 3 37 and 4 with the switches open contacted at terminals 11 ~Z~

01 are as shown within the dashed box 13.
02 To read the dosage the common source is 03 grounded, the gate of each IGFET is connected to its 04 own drain, and equal sources of current Il and I2 are 05 applied, preferably through variable resistors 14 and 06 15 to the drains Dl and D2. Resistors 14 and 15 07 having been pxeviously been ad~usted to provide a null 08 reading of ~VT. The differential voltage between 09 the drains D1 and D2 (which could be read on either side of the resistors), is shown ~VT, and is read on 11 a meter 16. A suitable range meter is 200mV full 12 scale to 2 volts full scale.
13 The circuit of Figure 7 can of course be 14 set up with a permanent socket so that when the dosimeter locket as shown in Figure 6 is brought in 16 for reading, the minidip switches are turned off and 17 the terminals 11 at the bottom of the locket are 18 plugged into the corresponding socket in -the reading 19 circuit. The manner of operation is preferably as follows:
21 Prior to irradiation, the resistors 14 and 22 15 are set to 0, the locket is plugged into the 23 reading circuit so that the differential threshold 24 voltage of the IGFETs are measured. This voltage will be found to be nearly 0. The bias voltage on 26 transistor Gl should then be switched into the circuit 27 as in Figure 6, and switches SWl-SW4 closed. The 28 meter will read the battery voltage, which can be a 29 fundamental check that the dosimeter may operate.
The dosimeter sensor is unplugged and the 31 IGFETs are irradiated at a distance from the reader.
32 In order to read the dosimeter, it is again plugged 33 into the reading circuit and the switches SWl-SW4 34 opened. The differential threshold voltage is again read, the dosage received during irradiation will be 36 the latter differential threshold voltage minus the 37 originally read differential threshold voltage.

~2~L1385 01 If required, the resis-tors 14 and 15 can 02 now be rezeroed. The dosimeter sensor can now be 03 continued to be irradiated and read as beEore.
04 When the IGFET 3 has been found to run out 05 of range, i.e. to saturate, the dual in line package 06 is removed from its socket, turned 180, plugged back 07 into -the same socket, and the second IGFET 4 now 08 becomes the IGFET which has i-ts threshold voltage 09 increased by radiation. Thus the range of the dosimeter is effectively doubled.
11 Figure 8 illustrates a direct reaaing 12 dosimeter. In this case IGFETs 3 and 4 are connected 13 to similar current sources 17 and 18, which pass 14 similar currents through the source-drain circuits of IG~ETs 3 and 4. The sources of the IGFETs are 16 connected together as before. The gate of IGFET 3 is 17 connected through single pole double throw switch 19, 18 which has one terminal connected to the positive pole 19 of battery 5, and its other terminal connected drain Dl. The negative pole of battery 5 is also connected 21 to ground.
22 The gate of IGFET 4 is connected to a 23 switch 20 which has one pole connected to a posi-tive 24 battery voltage + and its other pole to ground.
The drains of the IGFETs 3 and 4 are 26 connected to the input of analog to digital converter 27 21, which has its outputs connected to the inputs of a 28 digital readout 22. Of cours~ the analog to digital 29 converter 21 can be deleted and readout 22 can be an analog meter.
31 In order to read the differential 32 threshold at the input of analog to digital converter 33 21, switches 19 and 20 are connected as shown in 34 Figure 8. The gate of IGFET 4 is connected through switch 20 to its own drain Dl and the gate of IGFET 3 36 is connected to its drain ~2. Readout 22 will provide 37 a constant accumulated dosage reading.

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01 When IGFET 3 has been used up, i.e.
02 saturated switch 20 is connected to the positive 03 source +, and switch 19 is switched to Dl. IGFET 4 04 thus becomes the sensing device.
05 The IGFETs thus far d~scribed provide a ~6 gamma dosage indication. In order to also facilita-te 07 reading of neutron dose, a PIN diode 23, having anode 08 A and cathode K is preferably used. A variable 09 resistor 24 is connected in series with i-t to provide a predetermined voltage for a given current through 11 the PIN diode. For exarnple if a voltage is applied 12 across the series circuit of the PIN diode 23 and 13 resistor 24 to provide a current of e.g. 25mA, 14 resistor 24 can be adjusted to provide a voltage between terminal A and terminal T of e.g. 2 volts.
16 Physically located to receive the same 17 radiation as the PI~ diode are IGFETs 3 and 4 as 18 described earlier, with their sources connected 19 together and drains connected through resistors 25 and 26. The resistors can be trimmed to provide a 21 differential threshold voltage across terminals 27 of 22 0 volts.
23 The resistors 24, 25 and 26 can be thick 24 film transistors on an alumina substrate, or can be thick film resistors which are laser trimmed, for 26 example, to 0.1% tolerance and have temperature 27 coefficients of less than 100 parts per million.
28 Figure 10 illustrates in plan view a 29 hybrid substrate on which the components described with reference to Figure 9 are mounted. Integrated 31 circuit chip 28 in which IGFETs 3 and ~ are integrated 32 is bonded to an alumina substrate 29. PIN diode 30 is 33 bonded to a conductive layer 31 deposited on the 3~ substrate. Terminals Dl, S, D2, A, 27, 27, and T
corresponding to similarly labelled terminals of 36 Figure 9, are located on substrate 29. I'hick film 37 resistors 24, 25 and 26 corresponding to similarly ~LZ~

01 labelled resistors in Figure 9 are silk screened 02 printed or otherwise deposited on the substrate, over 03 evaporated on plated conductors 32, which are 04 connected -to the aforenoted terminals. The anode of 05 the PIN diode and the gates, drains and common source 06 of the IGFET chip are wire bonded to the conductors 32 07 to form the circuits shown in Figure 9.
08 The above hybrid circuit provides a 09 combination gamma ray neutron sensor suitable for both indirect or direct reading systems, which is also 11 suitable for mass manufacture.
12 In another embodiment of the invention 13 shown in Figure 11, two groups of IGFETs 3A, 3B... 3N
14 and 4A, 4B4N have their IGFETs connected in cascade within each group, the sources and drains of 16 the cascaded transistors in each group being connected 17 together. The drains or sources of the unconnected 18 ends of each of the cascade of IGFETs are connected 19 together as shown in Figure 11, the drains being shown connected to ground. The differential voltage ~VT
21 is measured between the two sources.
22 It will be seen that if all the gates of 23 each of the transistors 3A, 3B3N are made posi-tive 24 (biased conducting) during irradiation, and all of the gates of transistors 4~, 4B...4N are biased to zero, 26 non-conducting, during irradiation, the threshold 27 voltages of all of the transistors 3A, 3B...3N
28 increase. Since the threshold voltages of the 29 transistors 4A, 4B4N are smaller, the sum of the changes of the threshold vol~age differentials can be 31 measured. This has been found to increase the 32 sensitivity of the sensor without the requirement of 33 high irradiation bias.
34 It will be noted that since one side o~
the dual IGFET sensor is at a positive bias, and the 36 other i5 at ~ero bias, there will be a differential 37 sensitivity to radiation, which is high enough to be ~L2~ S

01 of practical use. The sensitivity of a single or dual 02 sensor i6 a function of bias, and the relationship of 03 bias to sensitivity is not linear. For low bias 04 voltages, such as used in a low voltage battery locket 05 type instrument or low voltaye direct reading 06 instrument, the variation in gate voltage gives rise 07 to variations in sensitivity. If the gate of one 08 IGFET is at say, ~3V, and during irradiation the 09 second IGFET gate is at O volts, the sensitivity of the first sensor is close to being linear with voltage 11 of e.g. 0.5mV/cGy/volt. However if the gate voltage 12 varies from battery to battery or with temperature, 13 then the sensitivity of the dual pair will also vary.
14 For a 10% change in a gate voltage +3V it has been found that there will be approximately a .15mV/cGy 16 change in sensitivity. Thus for a 10% change in bias, 17 there will be an approximate 10% change in 18 sensitivity.
19 If the sensor cannot be operated at saturation, e.g. at greater than 15 volts, then this 21 problem is exhibited. According to an embodiment of 22 the present invention, one method of solving this 23 problem is to bias both sensors with a positive bias.
24 One FET must have a high enough bias to ensure sensitivi-ty, the other low enough to give a reasonable 26 difference but not too low so that it reaches zero.
27 Both sensors must be biased with the same vol-tage 28 source so that as this varies the working points for 29 each IGFETs move together.
Figure 12 illustrates a circuit to fulfill 31 the require~ents described above. IGFET 3 has its 32 gate connected to the positive pole of a battery 5.
33 The same pole of the battery is connected through a 34 voltage divider comprised of resistors 33 and 34 connected across it. The gate of IGFET 4 is connected 36 to the junction between the resistors. This circuit 37 of course is set up prepared for lrradiation, and thus ~2~8S

01 the sources are connected together and the drains are 02 connected together.
03 Figure 13 is a curve illus-trating the 04 sensitivity as ordinate versus gate bias VG~ as 05 abscissa. The curve is shown as line 35.
06 For gate voltage VGl+, transistor 3 07 exhibi-ts sensitivity Sl. At the same time the gate 08 bias VG2+ of IGFET 4 exhibits sensitivit~ S2. The 09 differential sensitivity SDIFF is the differential sensitivi-ty.
11 As the battery voltage c~anges, it will be 12 found that the differential sensitivity will be 13 substantially immune to fairly large variations in 14 battery voltage, and it will be substantially constant from locket to locket or instrument to instrument for 16 sensors made from the same process. This occurs 17 because the slope of the sensitivity/bias curve has 18 been found to be more constant than the potential 19 variat;ons in battery voltage. This circuit has been found to be a substantial improvement over an attempt 21 to stabilize by clamping one sensor to zero voltage 22 during irradiation.
23 Figure 14 illustrates a bridge form of 24 sensor. This circuit is a limited form of the cascade circuit shown in Figure 11, in which four 26 transistors are used, two in each casecade series.
27 In this configuration, prepared ~or 28 irradiation, the drains DB of transistors 3B and 4B
29 are connected together to a common drain lead D, the sources SB of transistors 3B and 4B are respectively 31 connected to drains DA and DB of transistor 3A and 4A
32 respectively, and the sources SA o~ transistors 3A and 33 4A are connected to a common source lead S.
34 ~he gates of transistors 3A and 4B are connected to the same positive bias VGl and the 36 gates of transistors 4A and 3B are connected to the 37 same bias VG2+ (the substrates have been omitted in ~488~

01 this diagram for the purpose of clarity, but are all 02 connected together).
03 Prior to irradiation, all of the IGFETs 04 will have nominally the same threshold voltage, so 05 that driving the bridge with one cons-tant current 06 source ISD will result in a differential threshold 07 voltage ~VT close to zero, as shown in Figure 15.
08 To read the irradiation dosage, the gates of IGFETs 09 are connected to their respective drains. The common source S is driven from a constant current source 11 ISD which is also connected to the common drain 12 terminal D. The points DA, SB and DB, SB are the 13 points from which the differential threshold voltage 14 is measured. The common substrate should be connected to the most positive potential point.
16 Thus during irradiation, bias VGl and 17 VG2 is applied to the circuit as shown in Figure 18 14. During reading of the dosage, the value ~VT is 19 measured as shown in the circuit in Figure 15. Prior to irradiation the value ~VT measured by the circuit 21 shown in Figure 15 will result in a differential 22 threshold voltage close -to zero.
23 During irradiation, in the circuit shown 24 in Figure 14, all gates are positive with respect to the substrate, which should be at ground potential.
26 If VGl~ is larger than VG2+, the threshold 27 voltages of transistors 3A and 4B will increase by a 28 greater amount than the threshold voltages of 29 transistors 4A and 3B, as described earlier. The threshold voltage shift AVT due to irradiation will 31 be found to be twice that of the simpler dual IGFET
32 sensor with the same (different) gate biases during 33 irradiation described earlier with respect to Figure 34 12.
The benefit of the bridge form of the 36 circuit is that twice the sensitivity of a two IGFET
37 type sensor described earlier in the specification is ~l2~ 3i 3S

01 obtained, only one current source is required for 02 reading as opposed to two current sources required in 03 the earlier embodiments and the same gate bias is used 04 during irradiation as that of the dual pair described 05 earlier.
06 It may thus be seen that the various 07 embodiments of the present invention provide a 08 substantially rnore sensitive, stable and accurate 09 dosimeter sensor and dosimeter circuit than prior art MOS sensors and dosimeter circuits.
11 A person understanding this invention may 12 now conceive o~ varia-tions and other embodiments, all 13 usiny the principles described herein. All are 14 considered to be within the sphere and scope of the invention as defined in the claims appended hereto.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A radiation dosimeter comprising a pair of silicon insulated gate field effect transistors integrated into the same substrate, each having a gate, a source, and a drain, means for measuring a first differential threshold between the transistors, means for forward biasing the gate of the first transistor and inhibiting operation of the second, while allowing both to be subjected to ionizing radiation, means for measuring a second differential threshold voltage between the transistors following said irradiation, whereby the first differential threshold voltage can be subtracted from the second to obtain a measure of the radiation dosage.

2. A radiation dosimeter comprising first and second insulated gate field effect transistors integrated in the same substrate, each having a source, drain and gate, the sources being connected together, means for connecting a first voltage source positively poled toward the gate of the first transistor between its source and gate, and means connecting a second voltage source positively poled toward the gate of the second transistor between its source and gate, the second voltage source having less potential difference than that of the first voltage source, and means connecting the drains of said transistors together.

3. A dosimeter as defined in claim including means for disconnecting the drains of said transistors from each other, means for disconnecting the voltage sources from the gates, means for connecting the gates to the drains of the corresponding transistors, means for applying similar currents to each source-drain circuit of each transistor, and means for reading potential difference between the drains to obtain either an accumulated radiation dose indication following irradiation or an initial differential threshold measured prior to irradiation.

4. A dosimeter as defined in claim 2 or 3 in which the potential difference of the second voltage source is zero.

5. A dosimeter as defined in claim 2 or 3 in which the potential difference of the second voltage source is a fraction of the potential difference of the first voltage source.

6. A dosimeter as defined in claim 2 or 3 in which the transistors have polysilicon gates separated from the substrate by a gate insulating layer having thickness equal to or greater than about 0.5 micron, and have a common silicon substrate.

7. A dosimeter as defined in claim 2 or 3 in which the transistors have polysilicon gates separated from the substrate by a gate insulating layer having thickness equal to or greater than about 0.5 micron, are of P-channel types, and have a common silicon substrate.

8. A radiation dosimeter comprising a first drain to source, cascade interconnection of a first plurality of insulated gate field effect transistors (IGFETs), and a second drain to source cascade interconnection of a second plurality of insulated gate field effect transistors (IGFETs) similar in number to the first, all integrated into the same substrate, means for positive biasing each of the gates of the first plurality of transistors and for biasing each of the gates of the second plurality of transistors either to bias levels less than those of the first plurality of transistors, and means connecting drains of transistors at the ends of the respective cascade interconnections together.

9. A radiation dosimeter as defined in claim 8 including means connected otherwise sources of transistors at the other ends of the respective cascade interconnections together, means for applying a current through the source - drain circuits of the transistors, means for connecting each gate of each transistor to its own drain, and means for measuring a differential threshold of conduction between the cascade connections between corresponding drains in the cascade connections to obtain either a measure of initial or received radiation dosage.

10. A radiation dosimeter as defined in claim 9 in which the number of transistors in each cascade interconnection is two.

11. A radiation dosimeter as defined in claim 8 in which the transistors are of P-channel type, the number of transistors in each cascade connection is two, means connecting sources of transistors at the other ends of the cascade interconnections of respective transistors together, means for applying a positive bias to first and second cascaded transistors respectively in the first and second cascaded connections and a smaller positive bias to second and first cascaded transistors respectively in the first and second cascaded connections, during irradiation of all transistors together with ionizing radiation.

12. A radiation dosimeter as defined in claim 3 including resistors connected in series with each drain, the values thereof being predetermined to render any differential threshold voltage, which may be measured between the two transistors, to zero, prior to irradiation of the transistors.

13. A method of measuring ionizing radiation dosage comprising:
(a) measuring a first differential threshold voltage between a pair of similar insulated gate field effect transistors integrated into the same substrate, (b) positively biasing the gate of the first transistor and inhibiting operation of the second, while allowing both to be subjected to ionizing radiation, (c) measuring a second differential threshold voltage between the transistors following said irradiation, and (d) subtracting the first from the second differential threshold voltage to obtain a measure of the radiation dosage.

14. A method as defined in claim 8 in which said substrate is silicon and in which each transistor has gate oxide > 0.5 micron in thickness.

15. A method as defined in claim 13 or 1 in which the gates of the transistors are formed of polysilicon.

16. A method of measuring radiation dosage comprising:
(a) connecting the sources of two insulated gate field effect transistors (IGFETs), integrated in the same substrate, together, (b) connecting the gates of each transistor to its corresponding drain, (c) applying equal currents between the drains and sources of each transistor, (d) measuring a first voltage between the drains of the two transistors, (e) applying a first biasing voltage between the gate and source and drain of the first transistor, poled to positive bias the gate, (f) applying a second biasing voltage which is between zero and a level significantly less than the first position voltage between the gate and source, drain of the second transistor, poled to zero bias or positive bias the gate, (g) connecting the drains of the transistors together, (h) allowing both transistors to be subjected to ionizing radiation, (i) removing the application of first and second biasing voltage and disconnecting the drains from each other, (j) repeating steps (b), (c), and (d) but this time in step (d) measuring a second voltage between the drains of the transistor, and (k) subtracting the first voltage of step (d) from the second voltage of step (d) to obtain a measure of the radiation dosage recorded by the first transistor.

17. A method as defined in claim 13, 14 or 16 in which the gates of the transistors are formed of polysilicon, the gates have gate oxides of about 0.5 micron thickness and the transistors are of P-channel type integrated in a silicon substrate.