A p-MOSFET total dose dosimeter where the gate voltage is proportional to the incident radiation dose. It is configured in an n-WELL of a p-BODY substrate. It is operated in the saturation region which is ensured by connecting the gate to the drain. The n-well is connected to zero bias. Current flow from source to drain, rather than from peripheral leakage, is ensured by configuring the device as an edgeless MOSFET where the source completely surrounds the drain. The drain junction is the only junction not connected to zero bias. The MOSFET is connected as part of the feedback loop of an operational amplifier. The operational amplifier holds the drain current fixed at a level which minimizes temperature dependence and also fixes the drain voltage. The sensitivity to radiation is made maximum by operating the MOSFET in the OFF state during radiation soak

Blaes, Brent R.

Buehler, Martin G.

NASA Technical Reports Server

111ll1111111 Ill IIIII 11111 lllll1 11111111ll1111111 Il1 
US005332903A 
United States Patent [19] [113 Patent Number: 5,332,903 
Buehler et al. r45i Date of Patent: Jul. 26. 1994 
[54] P-MOSFET TOTAL DOSE DOSIMETER 
[75] Inventors: Martin G. Buehler, LaCanada; Brent 
R. Blaes, San Dimas, both of Calif. 
1731 Assignee: California Institute of Technology, 
Pasadena, Calif. 
[21] Appl. NO.: 983,380 
[22] Filed: Nov. 30, 1992 
Related U.S. Application Data 
[63] Continuation-in-part of Ser. No. 956,252, Oct. 5, 1992, 
which is a continuation-in-part of Ser. No. 672,705, 
Mar. 19, 1991. 
[51] Int. ( 3 . 5  ........................................... HOlL 31/119 
[52] U.S. c1. .......................... 250/370.14; 250/370.15; 
257/428 
[58] Field of Search ...................... 250/370.07, 370.14, 
250/370.15; 257/428, 429, 290, 291, 292 
[561 References Cited 
US. PATENT DOCUMENTS 
3 , 5 7 0 ~  12 3/1971 Barry et al. ......................... 257/428 
4,788,581 11/1988 Knoll et al. .................... 250/370.07 
4,808,834 2/1989 Kimata ................................ 250,578 
4,996,576 2/1991 Lynch et al. ........................ 257/428 
OTHER PUBLICATIONS 
Thomson et al. “Radiation Dosimetry with MOS Sen- 
sors”, Rad. Protect. Dosimetry, vol. 6, No. 1-4 pp. 
Soubra et al, “MOSFET Dosimeters as Rad. Det.”, 
Conference: Amer. Asso. of Phys. in Med. (1990) St. 
Louis, Mo. 
Primary Examiner-Paul M. Dzierzynski 
Assistant Examiner-Richard Hanig 
Attorney, Agent, or Firm-Leonard Tachner 
121-124 (1984). 
P I  ABSTRACT 
A p-MOSFET total dose dosimeter where the gate 
voltage is proportional to the incident radiation dose. It 
is configured in an n-WELL of a p-BODY substrate. It 
is operated in the saturation region which is ensured by 
connecting the gate to the drain. The n-well is con- 
nected to zero bias. Current flow from source to drain, 
rather than from peripheral leakage, is ensured by con- 
figuring the device as an edgeless MOSFET where the 
source completely surrounds the drain. The drain junc- 
tion is the only junction not connected to zero bias. The 
MOSFET is connected as part of the feedback loop of 
an operational amplifier. The operational amplifier 
holds the drain current fixed at a level which minimizes 
temperature dependence and also fixes the drain volt- 
age. The sensitivity to radiation is made maximum by 
operating the MOSFET in the OFF state during radia- 
tion soak. 
12 Claims, 2 Drawing Sheets 
. $3 
ID=Vl /R  
https://ntrs.nasa.gov/search.jsp?R=20080012414 2019-08-30T04:03:48+00:00Z
U.S. Patent July 26, 1994 Sheet 1 of 2 5,332,903 
V I  
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t - -  s3 R 
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ID=Vl /R 
FIG. 1 
U.S. Patent 
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1 
July 26, 1994 Sheet 2 of 2 5,332,903 
. FIG. 2 
1 9 
5.332,903 
P-MOSFET TOTAL DOSE DOSIMETER 
ORIGIN O F  INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract, and is sub- 
ject to the provisions of Public Law 96-517 (35 USC 
202) in which the Contractor has elected to retain title. 
APPLICATIONS 
This application is a continuation-in-part of applica- 
tion Ser. No. 07/956,252 filed Oct. 5, 1992 which is a 
continuation-in-part of application Ser. No. 07/672,705 
filed Mar. 19, 1991. 
TECHNICAL FIELD 
The present invention relates to MOSFET dosimetry 
and more specifically to an improved dosimeter provid- 
ing increased accuracy in reading total radiation expo- 
sure by eliminating temperature effects. 
BACKGROUNDART 
The space radiation effects of concern to modem 
microcircuits are Single-Event Upsets (SEUs) and 
Total Ionizing Dose (TD). In the SEU effect, cosmic 
rays and high energy protons, that undergo electronic 
reactions with the integrated circuits, deposit sufficient 
charge in memory cells and latches to flip bits and cor- 
rupt data. Such events do not induce any physical dam- 
age and thus are nondestructive. On the other hand, in 
the TD effect, gate oxides are charged which is a per- 
manent change. This shifts transistor threshold voltages 
and reduces the channel mobility. A change in these 
parameters degrades the performance of CMOS inte- 
grated circuits (ICs) by changing the propagation delay. 
Radiation sensitive field effect transistors have appli- 
cation as integrating dosimeters providing a measure- 
ment of the amount of dose absorbed from various radi- 
ation sources. Field effect transistor (FET) dosimeters 
are advantageous as compared to more conventional 
dosimeters because FET dosimeters are small and they 
provide continuous readout. They operate on the prin- 
ciple that ionizing radiation causes a shift in threshold 
voltage due to the accumulation of trapped charge in 
the gate oxide and at the oxide interface. Unfortunately, 
FET dosimeters also suffer from a number of shortcom- 
ings. In particular, their sensitivity to radiation is low 
and their accuracy is diminished by changes in thresh- 
old voltage caused by temperature variations. Thus it 
would be highly desireable to have an FET dosimeter 
which is relatively insensitive to variations in tempera- 
ture. 
SUMMARY O F  THE INVENTION 
The present invention comprises a p-MOSFET total 
dose dosimeter where the gate voltage is proportional 
to the incident radiation dose. It is confieured in an 
CROSS-REFERENCE TO RELATED 
5 
10 
15 
20 
25 
30 
35 
L 
current fixed at a level which minimizes temperature 
dependence and also fixes the drain voltage. The sensi- 
tivity to radiation is made maximum by operating the 
MOSFET in the OFF state during radiation soak. 
OBJECTS O F  THE INVENTION 
It is therefore a principal object of the present inven- 
tion to provide a p-MOSFET total dose dosimeter with 
increased radiation sensitivity and decreased tempera- 
ture sensitivity. 
It is another object of the invention to provide an 
improved dosimeter yielding more accurate readings of 
total radiation exposure by eliminating temperature 
effects. 
It is another object of the invention to provide more 
accurate dosimeter measurement by employing a p- 
MOSFET dosimeter in which the MOSFET is edgeless 
to eliminate leakage current and in which an operational 
amplifier virtual ground fixes the drain current at a 
magnitude selected to substantially eliminate tempera- 
tiire sensitivity. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The aforementioned objects and advantages of the 
present invention, as well as additional objects and ad- 
vantages thereof, will be more fully understood herein- 
after as a result of a detailed description of a preferred 
embodiment of the invention when taken in conjunction 
with the following drawings in which: 
FIG. 1 is a schematic diagram of the dosimeter of the 
present invention; and 
FIG. 2 is a graph of the temperature dependence of 
the dosimeter of the invention showing the drain cur- 
rent point at Ghich the dosimeter operation is tempera- 
ture independent. 
DETAILED DESCRIPTION O F  A PREFERRED 
EMBODIMENT 
Dosimetry via MOSFET threshold voltage shifts is 
influenced by at least two second order effects. These 
must be compensated out by design or included in the 
data analysis in order to obtain accurate dose measure- 
ments. The second order effects are (a) the radiation 
sensitivity of the transconductance factor, KP, via 
45 radiation-induced mobility, p, degradation and (b) the 
temperature sensitivity of both of these effects. In addi- 
tion the total operation of the device must be considered 
in order to obtain accurate dose measurements. 
The p-MOSFET is operated in the saturation region 
50 which is ensured by connecting the gate to the drain as 
seen in FIG. 1. For this case the drain current is given 
by: 
KP W 
2L I D = - - ( V G -  55 
the above square-law relationship is written as a VG- 
expression: 
n-WELL of a p-BODY substrate. It is opGated in the 
saturation region which is ensured by connecting the 60 
gate to the drain. The n-well is connected to zero bias. 
Current flow from source to drain, rather than from 
peripheral leakage, is ensured by configuring the device 
surrounds the drain. The drain junction is the only junc- 65 
tion not connected to zero bias. The MOSFET is con- 
nected as part of the feedback loop of an operational 
amplifier. The operational amplifier holds the drain 
VG= m+d[2ID.L/(Kp.W)]  
The radiation sensitivity is: 
as an edgeless MOSFET where the source completely S=dVG/dVT= 1 
The temperature dependence is determined by ex- 
panding V I  and KP in a Taylor Series: 
5,332,903 
3 
M- n o -  V A T -  To) age path. Thus the device is designed as an edgeless 
MOSFET where the source completely surrounds the 
drain. In addition, all junctions are held at zero bias 
except for the drain junction. The operational amplifier 
5 holds the source at zero bias providing a drain current 
ID=Vl/R. The drain voltage is fixed by the amplifier at 
VG which, for the case of the p-MOSFET, is negative. 
Having thus disclosed an exemplary embodiment of 
the invention, what is claimed is: 
1. A total dose dosimeter comprising: 
a p-MOSFET configured in a zero-biased n-WELL; 
an Operational amplifier having a feedback loop; 
said p-MOSFET being connected into said feedback 
loop, the gate and drain of said p-MOSFET being 
connected to the output of said operational ampli- 
tier and the source of said p-MOSFET being con- 
nected to an input of said operational amplifier; 
the gate voltage of said p-MOSFET being propor- 
tional to total radiation dose incident upon said 
2. The dosimeter recited in claim 1 wherein said p- 
MOSFET is operated in its saturation region. 
3. The dosimeter recited in claim 1 wherein said 
source physically surrounds said drain to prevent pe- 
ripheral current leakage between said source and said 
where T is the absolute temperature, Tois room temper- 
ature, and VTT= 1 dVT/dT 1. The temperature dePen- 
dence of KP is: 
KP= KPdT/To)-" 
and the Taylor Series expansion is: 
KP-KPO-KPfiT- To) 
where KPT= IdKP/dTI =n.KPa/To and n is between 
1.5 and 2.0. 
Substituting the above two equations into the VG- 
expression yields: 
VG =VGo+ { -VT*+~[rD.L.KP~/(2.KPo3 W)])(T- 
- To) 
where VGo=VTo+g[21D.L/(KPo.W 3. The above 
for small values of x. Thus VG will be independent of T 
for: 
15 
derivation used the approximation 1 4 1  -XI= 1 +x/2 2o p-MOSFET. 
ID=2KPJT$ (W/L)  [(MT/M~)/(KPT/KPJ~ 
where ID= 19.2 pA for a MOSFET length of 1.6 pm 25 drain. 
4 
and width of 3.2 pm and for the parameters given in 
FIG. 2 where the temperature dependence is shown 
explicitly. 
The total dose, D, is evaluated is a similar manner. 
The Taylor series expansions for VT and KP are: 
M= Mo+ VTDD 
where the VT damage coefficient is VTD=dVT/dD 
and 
KP= KPO- KPDD 
where the KP damage coefficient is KPD=dKP/dD. 
Substituting the above VT and KP expressions into the 
VG-expression leads to: 
D=(VG- VGo)/( MD+KPDVTT/KPT) 
where again the approximation of l d ( 1  -x) was used. 
The above dose expression requires the evaluation of 
VTDand KPDwhich is done using Co-60 irradiation. In 
this evaluation dose rate effects must be evaluated and 
then extrapolated to operating conditions. The tempera- 
ture coefficients, VTT and KPT, are determined from 
temperature measurements. 
The p-MOSFET dosimeter operating modes include: 
MEASure, OFF (soak/power down), CALibrate 1 and 
CALibrate 2. For maximum sensitivity to radiation, 
p-MOSFETs should be operated in the OFF state dur- 
ing the soak state. In the OFF state the electric field is 
directed from the gate toward to silicon. This means 
that during the radiation damage process when positive 
oxide charge is mobile, it will be distributed more 
toward the silicon which makes the MOSFET harder to 
turn on. This can be achieved in the implementation 
shown in FIG. 1 by applying zero bias to the gate. This 
requirement has the advantage that the soak and power 
down state will be identical and means the MOSFET 
will be operated in a consistent bias state providing a 
well-known biasing scenario. The key to this bias re- 
quirement 'Is that the n-well be connected to a zero 
potential and not be biased to VDD as is the normal cas 
in integrated circuit design. 
The design of the dosimeter requires current flow 
only from source to drain and not via a peripheral leak- 
4. The dosimeter recited in claim 1 wherein the drain 
current in said p-MOSFET is fmed by said operational 
amplifier at a magnitude where temperature variations 
have substantially no affect on gate voltage. 
5. The dosimeter recited in claim 1 further compris- 
ing a resistor having a resistance R and a voltage V also 
connected to said input of said operational amplifier, the 
drain current ID  of said p-MOSFET being fixed at 
ID=V/R by the virtual ground effect of said opera- 
6. The dosimeter recited in claim 5 wherein ID  is 
fixed, by selecting V and R, at a level which makes the 
gate voltage of said p-MOSFET substantially insensi- 
tive to temperature. 
7. The dosimeter recited in claim 1 further compris- 
ing means for turning said p-MOSFET to an OFF state 
during radiation exposure and to an ON state for mea- 
surement of accumulated total radiation dose. 
8. A p-MOSFET total dose dosimeter substantially 
a p-MOSFET in which the source is made to physi- 
cally surround the drain to prevent peripheral leak- 
age current between said source and said drain; and 
means to fm the drain current at a constant magnitude 
where gate voltage is independent of ambient tem- 
9. The dosimeter recited in claim 8 wherein said drain 
current fixing means comprises the virtual ground effect 
of an operational amplifier and said p-MOSFET is con- 
nected into the feedback loop of said operational ampli- 
10. The dosimeter recited in claim 9 wherein the gate 
and drain of said p-MOSFET are both connected to the 
output of said operational amplifier and the source of 
said p-MOSFET is connected to the input of said opera- 
11. The dosimeter recited in claim 9 wherein said 
drain current is fixed by a selected fixed voltage and 
resistance connected to the input of said operational 
amplifier. 
12. The dosimeter recited in claim 8 wherein said 
p-MOSFET is configured in a zero-biased n-well in a 
p-BODY substrate. 
30 
35 tional amplifier. 
40 
45 insensitive to temperature variations and comprising: 
50 perature. 
55 fier. 
60 tional amplifier. 
65 
* * * * *  


p-MOSFET total dose dosimeter

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