Patent Publication Number: US-6665161-B1

Title: Semiconductor circuit having increased susceptibility to ionizing radiation

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of U.S. Provisional Application No. 60/138,720, filed Jun. 11, 1999, which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor processing and integrated circuits. More particularly, the present invention relates to semiconductor circuits that are susceptible to ionizing radiation. 
     BACKGROUND OF THE INVENTION 
     Older semiconductor processing technologies produced integrated circuits that were highly susceptible to damage from ionizing radiation. Such ionizing radiation is emitted from a multitude of galactic sources (e.g., the Sun, etc.) and exists above the ionosphere, and is also emitted when nuclear weapons are detonated. The important consequence of such radiation susceptibility is that these integrated circuits were not well suited for use in satellites or in military applications. They could, therefore, be freely sold and exported without the fear that they would be used militarily against the United States or its allies. 
     In contrast, some state-of-the-art semiconductor processing technologies currently produce integrated circuits that are highly tolerant to damage from ionizing radiation. Such tolerance results, among any other reasons, from a decrease in semiconductor-feature size (e.g., interconnect line width, etc.) in integrated circuits. In particular, the “gate oxide” or “gate insulator” in field effect transistors (FETs) has thinned to the point where, by virtue of such thinness, it is inherently tolerant to ionizing radiation. 
     The relatively high radiation tolerance of state-of-the-art circuits is no benefit to most users and for most applications. This characteristic does, however, allow such circuits to be used in aerospace and military applications. In fact, such circuits may be so radiation tolerant that Department of Defense export restrictions (ITAR) are implicated so that such circuits can not be freely sold and exported. To the extent that a commercial CMOS fabricator is restricted from freely exporting its chips, it suffers financially. 
     Thus, a need exists for semiconductor circuits that exhibit increased susceptibility to ionizing radiation, but that retain the advantages that accrue from contemporary processing methods. 
     SUMMARY OF THE INVENTION 
     Some embodiments of the present invention provide semiconductor circuits that have increased susceptibility to ionizing radiation without the costs and disadvantages of techniques in the prior art. In particular, the illustrative embodiment of the present invention is an integrated circuit that possesses the benefits of contemporary processing technologies (e.g., small feature size, etc.) yet is advantageously irreparably damaged by ionizing radiation. Thus, some integrated circuits made in accordance with the present teachings can pass Department of Defense export restrictions and can be freely sold and exported. 
     In accordance with an illustrative embodiment of the present invention, a radiation-susceptible integrated circuit comprises a radiation sensor, a differential amplifier and a circuit disabler that are operable, collectively, to disable an IC when it is exposed to a sufficient amount of ionizing radiation. The radiation sensor is operable to generate a first signal and a second signal, and the differential amplifier is operable to receive the first and second signals and to generate a third signal having a value that is a function of the first and second signals. The circuit disabler, which is electrically connected to the differential amplifier, is operable to receive the third signal from the differential amplifier and to disable the integrated circuit, or not, responsive to the value of the third signal. 
     In the illustrative embodiment, the radiation sensor comprises two devices that have a different tolerance to ionizing radiation. In one embodiment, such devices are transistors that have different structures such that they exhibit a different susceptibility to ionizing radiation. When exposed to a total dose of ionizing radiation that exceeds the radiation tolerance of one of the transistors but not the other, only the more radiation-susceptible transistor will exhibit an increase in leakage current. 
     The differential amplifier receives, as its input, two signals from the radiation sensor. In the illustrative embodiment, those two signals are the output (i.e., the leakage current) from the two devices (e.g., transistors). The differential amplifier is operable to generate an output signal having a value that is indicative of a difference or offset that exists between the input signals. Such an offset results when the IC is exposed to an amount of ionizing radiation sufficient to cause only one of the devices in the radiation sensor to exhibit an increase in leakage current. In one embodiment, the differential amplifier generates a “high”-voltage signal when an offset (increased leakage current from one device) is observed, and a “low”-voltage signal in the absence of an offset. 
     The output signal from the differential amplifier is received by a circuit disabler. The circuit disabler is activated, or not, as a function of the value of the output signal. For example, a high-voltage output signal from the differential amplifier activates the circuit disabler, while a low-voltage output signal does not. When activated, the circuit disabler is operable to cause the integrated circuit to stop functioning, and preferably causes irreparable damage thereto. 
     The circuit disabler can be implemented in a variety of ways. For example, in a first embodiment, the circuit disabler comprises a switching FET that shorts power to ground in the IC when activated. In a second embodiment, the circuit disabler comprises a switching FET that shorts signal to ground in the IC when activated. In a third embodiment, the circuit disabler comprises circuitry for disabling the IC&#39;s clock driver, and in a fourth embodiment, the circuit disabler comprises circuitry suitable for activating the chip-reset function. 
     A further illustrative embodiment of the present invention is a method that comprises comparing a first signal indicative of a first current leakage with a second signal indicative of a second current leakage; and generating a third signal having a value that is a function of the difference between the first and second signals. In particular, the third signal has a first value when the difference between the first and second signals meets or exceeds a threshold difference, and has a second value when the difference between the first and second signal is less than the threshold difference. The first and second current leakages are indicative of an amount of radiation exposure. In a further embodiment of the method, an integrated circuit is disabled when the third signal has the first value and is not disabled when the third signal has the second value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a portion of an integrated circuit in accordance with the illustrated embodiment of the present invention. 
     FIG. 2 depicts a first embodiment of a circuit disabler in accordance with the present teachings. 
     FIG. 3 depicts a second embodiment of a circuit disabler in accordance with the present teachings. 
     FIG. 4 depicts a third embodiment of a circuit disabler in accordance with the present teachings. 
     FIG. 5 depicts a fourth embodiment of a circuit disabler in accordance with the present teachings. 
     FIG. 6 depicts a further embodiment of an integrated circuit in accordance with the present teachings wherein a latch circuit is electrically connected between a differential amplifier and a circuit disabler. 
     FIGS. 7A-7C depict a n-channel transistor in the prior art. 
     FIG. 8 depicts an illustrative embodiment of a multi-edge device that is relatively susceptible to ionizing radiation. 
     FIG. 9 depicts an illustrative embodiment of an enclosed device that is relatively tolerant to ionizing radiation. 
    
    
     DETAILED DESCRIPTION 
     For the purposes of this Specification, the terms “circuit” and “device” are used interchangeably, unless other noted. 
     The terms “radiation susceptible,” “radiation intolerant” and “radiation tolerant,” are, of course, relative terms. In the context of the illustrated embodiment of the present invention, such relative designations suffice, since it is the difference in radiation susceptibility, rather than absolute tolerance levels, that are important. The following quantitative descriptions of such terms are offered, however, to provide context for these designations. A radiation tolerant device or circuit exhibits a TID tolerance in the range of about 50 to about 200 Krad (Si). A radiation-susceptible or radiation-intolerant device or circuit has a maximum TID tolerance (i.e., the point at which a substantial leakage current is exhibited) of about 50 Krads (Si), and more typically as low as about 1 to 3 Krad (Si). Again, these quantitative descriptions are offered by way of illustration, not limitation. In the context of the present invention, it is the relative, rather than the absolute radiation susceptibility that is of primary importance. 
     The designation “total” in the phrase “total ionizing dose” signifies that the radiation exposure may occur over a period of time; in other words, TID is a cumulative dose. Furthermore, as the amount of radiation absorbed by different materials for a given exposure varies, radiation tolerance must be referenced to a particular material, such as silicon (Si). 
     FIG. 1 depicts an embodiment of a radiation-susceptible integrated circuit in accordance with the illustrated embodiment of the present invention. It will be clear to those skilled in the art that integrated circuits typically include many other devices/elements than are depicted in FIG.  1 . Such other devices are not shown so that attention can be focused on those elements that are germane to an understanding of the present invention. 
     As depicted in FIG. 1, illustrative integrated circuit  100  comprises radiation sensor  110 , high-gain differential amplifier (e.g., comparator)  140  and circuit disabler  150 , which are interrelated as shown. A description of the structure and implementation of various illustrative embodiments of radiation sensor  110  and circuit disabler  150  is deferred until an overview of the inventive circuit is presented. 
     When a signal that is capable of enabling or activating circuit disabler  150  is received thereby, the circuit disabler disables, preferably permanently, integrated circuit  100 . In the illustrative embodiment, the signal that controls whether or not circuit disabler  150  is activated is output signal DAO from differential amplifier  140 . In some embodiments, the output DAO of differential amplifier  140  is fed to a second amplification stage (not shown) before feeding it to circuit disabler  150  as may be required to provide appropriate voltages. 
     Differential amplifier  140  receives, as input, two output signals S 1  and S 2  from radiation sensor  110 . Output signal DAO is a function of the difference between signals S 1  and S 2 . 
     For example, in one embodiment, when there is substantially no difference or offset between signals S 1  and S 2 , differential amplifier  140  generates a signal having a first value (e.g., a “low” voltage). When there is an offset between signals S 1  and S 2 , differential amplifier  140  generates a signal having a second value (e.g., a “high” voltage). Continuing with the example, when signal DAO has the second value (i.e., when there is an offset between signals S 1  and S 2 ), circuit disabler  150  is enabled and operation of integrated circuit  100  is thereby disrupted. When signal DAO has the first value (i.e., when there is no offset between signals S 1  and S 2 ), circuit disabler  150  is not enabled and has no effect on the “normal” functioning of integrated circuit  100 . 
     Hereinafter, the term “goes low,” as applied to the output DAO of differential amplifier  140 , means that as a result of there being essentially no difference between signals S 1  and S 2 , output signal DAO generated by differential amplifier  140  is a low-voltage signal. Also, the term “goes high,” as applied to the output DAO of differential amplifier  140 , means that as a result of a difference or offset between signals S 1  and S 2 , output signal DAO generated by differential amplifier  140  is a relatively higher-voltage signal. The low-voltage signal is not capable of enabling or activating circuit disabler  150 , while the higher-voltage signal does enable the circuit disabler. 
     As explained further below, an offset between signals S 1  and S 2  occurs when radiation sensor  110  is exposed to ionizing radiation. To implement such functionality, in one embodiment, radiation sensor  110  comprises first device  120  and second device  130 , wherein the second device is more tolerant to ionizing radiation than the first device. First device  120  generates signal S 1  and second device  130  generates signal S 2 . 
     By way of explanation, when first device  120  receives a cumulative amount of ionizing radiation (e.g., TID) that exceeds its radiation tolerance, an increase in leakage current occurs. As the leakage current of first device  120  increases, a value of signal S 1  increases. Since the radiation tolerance of second device  130  has not yet been exceeded, no such increase in leakage current, or in a value of signal S 2 , occurs. A difference or offset therefore arises between signal S 1  and signal S 2 . Again, it is this difference that ultimately causes circuit disabler  150  to disable integrated circuit  100 . Description of the underlying physical phenomena that results in radiation-induced leakage current is described later in this Specification in conjunction with FIGS. 7A-7C. 
     The first device  120  and second device  130  should be fabricated such their “pre-radiation” current leakage are about equal. Thus, until one of the devices exhibits increased current leakage, the output of differential amplifier  140  goes low, and circuit disabler  150  is inactive. 
     Circuit Disabler 
     Those skilled in the art will understand that there are many ways to disable an integrated circuit. By way of illustration, not limitation, four approaches for implementing circuit disabler  150  are described below. An illustrative arrangement for realizing each of the approaches is depicted in FIGS. 2-5. 
     In a first approach, circuit disabler  150  disables an integrated circuit by coupling power to ground. In an illustrative arrangement for coupling to power to ground that is depicted in FIG. 2, circuit disabler  150  comprises a switch, realized in the illustrated embodiment as field effect transistor (FET)  260 . The gate of switching FET  260  receives signal DAO from differential amplifier  140  (FIG.  1 ). The drain of FET  260  is electrically connected to power through electrical conductor  202  and the source of FET  260  is electrically connected to ground through electrical conductor  204 . Electrical conductors  202  and  204  can be metal bus lines or specific transistor diffusion nodes, as are well known in the art. It will be clear to those skilled in the art how to make and use electrical conductors  202  and  204  in conjunction with a switch such as FET  260 . 
     When the output of differential amplifier  140  goes low, signal DAO will not turn on switching FET  260  (there is no current flow through the transistor). On the other hand, when the output of differential amplifier  140  goes high, signal DAO will turn on switching FET  260 . When switching FET  260  turns on, the switching FET couples power to ground thereby disabling devices in the integrated circuit. The circuit disabler is advantageously designed so that when switching FET  260  turns on, the current flow therethrough is sufficient to “fuse” the bond wire (open), ensuring that current continues to flow. Further disclosure concerning an arrangement wherein switching FETs short power to ground on exposure to ionizing radiation is provided in applicant&#39;s co-pending patent application filed on even date herewith as Ser. No. 09/590805 and entitled “Increasing the Susceptibility of Integrated Circuits to Ionizing Radiation”, incorporated by reference herein. 
     In a second approach, circuit disabling means  150  disables an integrated circuit by coupling information-bearing (“I-B”) signals to ground. In an illustrative arrangement for coupling I-B signals to ground that is depicted in FIG. 3, circuit disabler  150  comprises a switch, realized in the illustrated embodiment as field effect transistor (FET)  360 . The drain of FET  360  is electrically connected to I-B signal(s) through electrical conductor  302  and the source of FET  360  is electrically connected to ground through electrical conductor  204 . 
     As in the previous embodiment, when the output of differential amplifier  140  goes low, signal DAO will not turn on switching FET  360 . When the output of differential amplifier  140  goes high, however, signal DAO will turn on switching FET  360 , causing signal(s) in conductor  302  to short to ground. The present embodiment of circuit disabler  150 , like the previous embodiment, is advantageously designed so that when switching FET  360  turns on, the current flow therethrough is sufficient to fuse the bond wire ensuring that current continues to flow. 
     It should be understood that, in most embodiments, the two previously described embodiments of circuit disabler  150  comprise a plurality of switching FETs (rather than one FET as illustrated) that are ganged in parallel. Moreover, in some embodiments, the switch is realized any one of a number of devices other than a FET, including, without limitation, a bipolar transistor, a silicon-controlled rectifier (SCR) and a thyristor or triac. 
     In a third approach, circuit disabler  150  disables an integrated circuit by disabling the clock driver circuit. In an illustrative arrangement for disabling the clock driver circuit that is depicted in FIG. 4, circuit disabler  150  comprises AND gate  460  and three conductors  442 ,  482  and  492 , interrelated as shown. Conductor  482  electrically connects master clock  480  to one input of AND gate  460 . Conductor  442  electrically connects the output of differential amplifier  140 , which functions as a control signal, to a second input of AND gate  460 . Conductor  492  electrically connects the output of AND gate  460  to clock driver  490 . 
     If circuit disabler  150  is to be enabled when the output of differential amplifier  140  goes high (as in the previous examples), then output signal DAO must be inverted in inverter  470  before being fed to AND gate  460 . As an alternative, the polarity of the inputs to differential amplifier  140  can be inverted to the same effect. 
     When the output of differential amplifier  140  is low, AND gate  460  passes clock signal CS to the output (since DAO is inverted) and to clock driver  490  via conductor  492 . As such, the operation of the integrated circuit is not disrupted. When the output of differential amplifier  140  goes high, AND gate  460  does not pass clock signal CS to clock driver  492 , thereby disabling the integrated circuit. 
     In a fourth approach, circuit disabling means  150  disables an integrated circuit by placing the circuit in a “perpetual” reset mode via the circuit&#39;s internal reset circuitry. In an illustrative arrangement for asserting reset that is depicted in FIG. 4, circuit disabler  150  comprises OR gate  560  and three conductors  542 ,  572  and  582 , interrelated as shown. Conductor  572  electrically connects external reset  570  to one input of OR gate  560 . Conductor  542  electrically connects the output of differential amplifier  140 , which functions as a control signal, to a second input of OR gate  560 . Conductor  582  electrically connects the output of OR gate  560  to internal reset  580 . 
     When the output of differential amplifier  140  is low, and in the absence of external reset signal ERS, OR gate  560  will not assert reset (i.e., does not pass a signal via conductor  582  to internal reset  580 ). The “normal” operation of the integrated circuit is therefore not disrupted. When the output of differential amplifier  140  goes high, OR gate  560  asserts reset (i.e., sends a signal to internal reset via conductor  582 ), thereby disabling the integrated circuit. 
     As previously described, radiation sensor  110  comprises two devices, one of which devices has a greater tolerance to ionizing radiation than the other. As a consequence, on exposure to ionizing radiation, the less-radiation-tolerant device will typically show an increase in current leakage. It is possible, however, that with continuing exposure to ionizing radiation, the more-radiation-tolerant device will also exhibit an increase in current leakage. It is expected that the less radiation-tolerant device will always show a greater current leakage than the more radiation-tolerant device. There is, however, a possibility that with both devices exhibiting substantial current leakage, the offset between signals  51  and S 2  might decrease and the output from differential amplifier  140  may go low. 
     Such a happenstance will not affect the embodiments utilizing the switching FET, since the current flow fuses the bond wire (open) so that circuit disabler remains enabled regardless of upstream indications. On the other hand, as to the third and fourth approach described above, the circuit disabler may not remain enabled in such a situation. In particular, with reference to FIGS. 4 and 5, a disabled clock driver may be re-enabled, and the reset may be de-asserted, such that the integrated circuit may begin to function again. 
     To prevent such an occurrence, some embodiments of the present invention further comprise a “latch” circuit, such as is depicted conceptually in FIG.  6 . In the illustrative embodiment depicted in FIG. 6, latch circuit  660  is fed output signal DAO from differential amplifier  140  and delivers an output signal LO to circuit disabling means  150 . When latch circuit  660  detects a “failure,” (i.e., when DAO indicates an offset between signals S 1  and S 2 ), output LO locks. Once locked, signal LO does not change, regardless of changes in DAO. The latch is advantageously designed to avoid prematurely latching during start-up, when false signals may be generated. The latch may suitably be designed for synchronous or asynchronous operation. The design and implementation of such latch circuits are within the capabilities of those skilled in the art. 
     It will be understood that in other embodiments, other arrangements for implementing the circuit disabler as will occur to those skilled in the art in view of the present teachings may suitably be used. In addition to using other arrangements for implementing the four “approaches” described above, additional approaches may suitably be used. For example, in addition to shorting power to ground, shorting signal to ground, disabling the clock driver and resetting the circuit, a further way to disable the circuit involves disabling the output buffer of the integrated circuit. 
     Radiation Sensor 
     Before describing an embodiment of radiation sensor  110 , it will be instructive to consider the mechanism by which exposure to ionizing radiation affects the electrical characteristics of CMOS and NMOS transistors and other semiconductor devices. This mechanism is described in conjunction with FIGS. 7A-7C, which depict an n-channel transistor  702  in the prior art. 
     FIG. 7A depicts, via plan view, n-type drain region  704 , n-type source region  706  and gate electrode  708  (typically polysilicon). Gate electrode  708  lies over a channel region  710  between drain region  704  and source region  706 . Drain region  704  and source region  706  are bounded by field oxide (e.g., silicon dioxide)  712  at regions remote from gate electrode  708 . 
     FIG. 7B is a cross-section of transistor  702  along line  1 - 1  (shown in FIG. 7A) as viewed in the direction indicated. FIG. 7B shows the “operating” transistor in p-type substrate  714 . Gate electrode  708  overlies gate dielectric (e.g., silicon dioxide)  718 . 
     A parasitic transistor having field oxide  712  as a gate dielectric is present in transistor  702  along the side walls of field oxide  712  at regions  720  at which gate electrode  708  extends over the edge of field oxide  712 . (See FIG. 7C, which depicts a cross-section of transistor  702  along line  2 — 2  in FIG. 7A as viewed in the direction indicated; see also FIG. 7A.) This parasitic transistor will normally be in a non-conductive state since field oxide  712  is significantly thicker than gate dielectric  718 . As such, the parasitic transistor will be off when the operating transistor is off. When the operating transistor is turned on, effectively all of the source-to-drain conduction will flow in the channel under gate dielectric  718 , such that the parasitic transistor is non-conducting. Moreover, p-type regions  716  are relatively more heavily-doped than substrate  714  (e.g., silicon) to increase the threshold voltage of parasitic transistors, thereby improving the electrical isolation between neighboring transistors (not shown). 
     On exposure to a sufficient amount of ionizing radiation, however, the threshold voltage of the operating and parasitic transistor may shift by amounts such that the parasitic transistor will conduct when the operating transistor normally conducts. Moreover, a sufficient amount of ionizing radiation may cause the parasitic transistor to become an enhancement-mode device (i.e., conduct with zero potential difference between gate electrode  708  and source region  706 ). The reason for this shift in threshold voltage on exposure to ionizing radiation is explained below. 
     Threshold voltage is given theoretically by the equation: 
     
       
           V   T =φ−(σ/ε) d-F   [1] 
       
     
     where: φ is the work function of the gate region; 
     σ is the total charge at the dielectric(insulator)-semiconductor interface; 
     ε is the dielectric constant of the insulator; 
     d is the insulator thickness; 
     F is a term that can be considered a constant. 
     If, somehow, positive interface charge, σ, is added, then threshold voltage, V T , decreases (because a larger number is being subtracted from φ). A decrease in threshold voltage makes a n-channel device (wherein conduction carriers are electrons) easier to “turn on.” It can be seen from expression [1] that with sufficient positive interface charge, σ, an n-channel device can turn on at a threshold voltage, V T , equal to zero. In fact, the addition of positive interface charge can occur on exposure to ionizing radiation. 
     When exposed to ionizing radiation, electron-hole pairs are formed in the gate dielectric and the field oxide. Some of the holes become trapped in the gate dielectric and field oxide as various gate-induced fields sweep out the electrons as part of normal circuit operation. Since holes behave like positive charge, this phenomenon is referred to as positive-charge trapping. The trapped “positive” charges migrate toward the silicon-silicon dioxide interface, thereby adding positive interface charge, σ, and, as a consequence, decreasing the threshold voltage, V T . 
     A dielectric having a relatively greater thickness will trap more holes than a dielectric having a relatively lesser thickness. For example, field oxide  712  will trap more positive charge than gate dielectric  718  since it is significantly thicker (e.g., about 1000 angstroms versus about 70 angstroms). More positive interface charge will therefore be added at the field oxide ( 712 )/semiconductor interface than at the gate dielectric ( 718 )/semiconductor interface. 
     Since the field oxide “traps” more “positive” charge than the gate dielectric, and since the threshold voltage decreases as positive interface charge increases, the threshold voltage of the parasitic transistor at regions  720  will therefore shift downwardly more than will the threshold voltage of the operating transistor (i.e., the transistor having gate dielectric  718 ). If the dose of radiation is sufficiently great, the parasitic transistor will conduct at regions  720  (i.e., under the edge of field oxide  712 ) when the operating transistor normally conducts. 
     In accordance with the present teachings, aspects of the physical phenomena that underlie radiation susceptibility as described above are utilized to form radiation sensor  110 . As previously described, in the illustrative embodiments, radiation sensor  110  comprises first device  120  and second device  130 , wherein first device  120  has a greater susceptibility to ionizing radiation than second device  130 . Based on this difference in radiation susceptibility, device  120  will begin to exhibit an increase in leakage current before device  130 . Radiation is therefore “sensed” by the generation of an offset or difference between the output signals of the two devices. 
     FIGS. 8 and 9 depict illustrative embodiments of first device  120  and second device  130 . FIG. 8 depicts device  120  that is relatively radiation intolerant. Illustrative device  120  depicted in FIG. 8 may be considered to be a plurality of transistors. Region  822  comprises a thick (e.g., about 1000 or more angstroms) layer of field oxide. The vertically-oriented rectangular shapes designated by the numeral “ 823 ” represent thin (e.g., about 70 angstroms) oxide regions. Regions  824  overlying thin oxide regions  823  function as a transistor gate. Regions  826  and regions  828  sandwiching gate  824  function as drain and source regions. Drain  826  is biased to power (e.g., Vdd), and gate  824  and source  828  to ground. 
     Recalling the earlier description of the underlying mechanism for radiation susceptibility, it will be appreciated that the large number of “edges” or “interfaces”  820  wherein gate  824  extends over field oxide region  822  renders device  120  very radiation intolerant. In particular, parasitic transistors are present at each of the many interfaces  820 . And exposure to a sufficient level of ionizing radiation will “turn on” those parasitic transistors. Thus, exposure to ionizing radiation will result in a large leakage current from device  120 . 
     On the other hand, illustrative device  130  depicted in FIG. 8 provides a very radiation tolerant configuration. Device  130  comprises drain  936 , gate  934 , source  938  and field oxide region  932 . Drain  936  is biased to power, and gate  934  and source  938  to ground. 
     Device  130  has an “enclosed” configuration wherein gate  934  does not extend over field oxide region  932  at any point. As a consequence, parasitic transistors are not present, and, by virtue of their absence, device  130  is extremely resistant to ionizing radiation. 
     It should be understood that in other embodiments, other configurations that provide radiation tolerant and radiation intolerant devices are suitably used to implement radiation sensor  110 . See, for example, applicant&#39;s previously-referenced patent application Ser. No. 09/590,805 entitled “Increasing the Susceptibility of Integrated Circuits to Ionizing Radiation”, as well as co-pending patent applications Ser. No. 09/590,806 and entitled “Semiconductor Device and Circuit Having Low Tolerance To Ionizing Radiation” and Ser. No. 09/590,809 and entitled “Apparatus and Method for Manufacturing a Semiconductor Circuit”, both of which were filed on even date herewith and both of which are incorporated by reference herein. 
     It is to be understood that the above-described embodiments are merely illustrative of the invention and that many variations may be devised by those skilled in the art without departing from the scope of the invention and from the principles disclosed herein. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.