Method of monitoring radiation using a floating gate field effect transistor dosimeter, and dosimeter for use therein

• US 6,172,368 B1
• Filed: 08/24/1998
• Issued: 01/09/2001
• Est. Priority Date: 09/12/1997
• Status: Expired due to Term

• Current Assignee: Best Theratronics Incorporated, Carleton University, Thomson & Nielsen Electronics Limited
• Original Assignee: Carleton University, Thomson & Nielsen Electronics Limited
• Inventors: Tarr, Nicholas Garry, Thomson, Ian
• Primary Examiner(s): Hannaher, Constantine
• Assistant Examiner(s): Gabor, Otilia
• Application Number:
  US09/138,394
• Time in Patent Office: 869 Days
• Field of Search: 250/370.07, 250/370.14, 250/337
• US Class Current: 250/370.07
• CPC Class Codes: G01T 1/026 Semiconductor dose-rate meters

Abstract

An insulated gate field effect transistor dosimeter has a source and drain defining a channel region, a floating gate having a first portion extending over the channel region, and a second, larger portion extending away from said region, a control gate having at least a portion thereof overlapping a first part of the floating gate, and a charging gate overlapping a second part of the floating gate. The area of the second part of the floating gate is much smaller than the area of the first part, and the charging gate is separated from the channel region by the control gate. The dosimeter is charged, before irradiation, by connecting the source, drain and control gate to a common ground and applying a potential difference between the charging gate and the common ground. The charge is supplied to the floating gate by a path which does not require a significant electric stress to be created in the region of the gate oxide and the channel. The dosimeter may comprise two such transistors fabricated on a common substrate, conveniently with a common source. The pair of transistors may be charged by maintaining the sources, drains and control gates within the normal maximum operating voltage relative to each other, and applying different potential differences between the two charging gates, respectively, and the substrate. Following irradiation, the absorbed radiation does is determined by measuring the difference between the threshold voltages of the two transistors. Preferably, the transistors have charges of opposite polarities. This differential arrangement reduces the effects of temperature variations and enhances sensitivity.

Claims

1. A method of monitoring ionizing radiation using an insulated gate field effect transistor dosimeter having a source and a drain formed in a substrate, a floating gate separated from the substrate by an insulating layer, a control gate overlapping a first part of the floating gate and insulated therefrom, and a charging gate overlapping a second part of the floating gate and insulated therefrom, the second part being remote from a channel between the source and drain, the method comprising the steps of:

(i) maintaining potential differences between the substrate, source, drain and control gate lower than a maximum normal operating voltage of the device;

(ii) pre-charging the floating gate by establishing a potential difference between the charging gate and the control gate, monitoring a parameter dependent upon a threshold voltage of the transistor and increasing the potential difference to transfer charge between the charging gate and the floating gate through the insulating layer material between the floating gate and the charging gate until a predetermined threshold voltage is established without involving excessive electric field stress in the region of the channel;

(iii) with the substrate, source, drain, control gate and charging gate maintained at a common electrical potential, exposing the dosimeter to the ionizing radiation;

(iv) following such irradiation, measuring a parameter affected by change in the charge applied to the floating gate and determining the amount of such ionizing radiation absorbed by the transistor in dependence upon the difference between floating gate charge before and after irradiation.

2. A method as claimed in claim 1, wherein the parameter measured to determine the amount of radiation is the same parameter that was measured during charging of the floating gate.

3. A method as claimed in claim 1, wherein the pre-charging of the floating gate includes the step of measuring drain current corresponding to the predetermined threshold voltage and the step of determining the amount of absorbed radiation includes the steps of applying a fixed drain-source bias, measuring the drain current, and adjusting the control gate bias until the drain current is the same as that corresponding to the predetermined threshold voltage, the control gate bias to achieve this being equivalent to the threshold voltage of the transistor.

4. A method of monitoring ionizing radiation using a dosimeter comprising a matched pair of insulated gate field effect transistors sharing a common substrate, each having a source and a drain formed in the substrate, a floating gate separated from the substrate by an insulating layer, a control gate overlapping a first part of the floating gate and insulated therefrom, and a charging gate overlapping a second part of the floating gate and insulated from both the floating gate and the control gate, the second part being remote from a channel between the source and drain, the method comprising the steps of:

(i) maintaining potential differences between the substrate, source, drain and control gate of each transistor lower than a maximum normal operating voltage of the transistor;

(ii) establishing a potential difference between the charging gate of one of the transistors and its control gate, monitoring a parameter dependent upon a threshold voltage of the one transistor and increasing the potential difference to transfer charge between its charging gate and its floating gate through the insulating layer material between the floating gate and the charging gate until a predetermined threshold voltage is established without involving excessive electric field stress in the region of the channel of that transistor;

(iii) establishing a potential difference between the charging gate of the other of the transistors and its control gate, monitoring a parameter dependent upon a threshold voltage of said other of the transistors and increasing the potential difference to transfer charge between its charging gate and its floating gate through the insulating layer material between the floating gate and charging gate until a predetermined threshold voltage is established without involving excessive electric field stress;

(iv) with the substrate, sources, drains, control gates and charging gates of the transistors connected in common, exposing the device to the ionizing radiation; and

(v) following such irradiation, determining the amount of such ionizing radiation absorbed by the transistors by measuring a parameter dependent upon the difference between the threshold voltages of the pair of transistors.

5. A method as claimed in claim 4, wherein the floating gates of the two transistors are charged to equal and opposite levels.

6. A dosimeter comprising a matched pair of insulated gate field effect transistors sharing a common substrate, each of said transistors having a source and a drain formed in the substrate, a floating gate separated from the substrate by an insulating layer, a control gate overlapping a first part of the floating gate and insulated therefrom, and a charging gate overlapping a second part of the floating gate and insulated from both the floating gate and the control gate, the charging gate being remote from a channel between the source and drain.

7. A dosimeter as claimed in claim- 6, wherein the control gate has an area considerably larger than the area of the charging gate.