The same test setup is used for irradiating all the transistors. The transistors are irradiated with the following specifications.
Area of the Radiation beam : 10cm x 10cm
Depth under the skin : 5cm
Beam Energy : 6MV
Dose (CGy)
Rate of Dose (MU/min)
Monitor Units(MU)
100
361
117
1000
354
1170
3000
359
3513
5000
375
5855
Table 1
The Devices under test are TI-CD4007UBE and Fairchild 2N7000 (N-Channel Enhancement Mode Field Effect Transistor). These products have been selected because of their specifications such as minimum on-state resistance, rugged, reliable, and fast switching performance. For each radiation dosage two chips of each type are used to consider the effects of inter-device variations.
4. MOSFET Characterization
Fig. 6
The purpose of MOSFET characterization is to obtain the device operating curves accurately and estimate the transistor parameters. The characterization is performed on the transistors before and after radiation. The Test setup for this process is shown in the figure 6. Two DC generators are used for Vdd and Vgg. The gate current is measured by an ammeter Ig. The voltages Vgs (across gate and source) and Vds (across drain and source) are measured using two multimeters Vgs and Vds respectively. A resistance Rg of 1k ohm is in series with the gate circuit to protect the device from high currents. A resistance Rd is placed in series with the drain to source circuit to limit the saturation current. This resistance needs to be changed accordingly to obtain desired transistor operating curves. This value is changed from 30 Ohms to 1K Ohm.
Fig. 7
The voltage Vgg is set at three values 2V, 3V and 5V. The voltage Vdd is varied from 0V to 8V for each value of Vgg. The readings Vgs, Id , Vds and Vrd are noted down for each iteration. The setup for the characterization is shown in the figure 7.
The inter-device variation was found to be minimal in the devices. The error in readings between the devices was less than 1%. The curves obtained for different radiation dosages are presented in the figure 8.
Fig. 8
The curves shifted up with an increase in radiation dose. The radiation dose caused the threshold voltage of the NMOS to reduce. This phenomenon can be seen clearly from the curves. For a Given Vds the current obtained is increasing. The theory behind this operation is explained in the above section. Since the theory supports the test data PMOS transistor was also tested for the same. A NMOS Transistor was irradiated on irradiated on November 23, 2007. The Vds and Ids curves of this transistor was measured on November 23,2007, November 29,2007 and November 30,2007. The change over time was very negligible. The maximum change of current (Ids) for a particular Vds was below 7%. The results obtained are presented in figure 9.
Fig. 9
Fig. 9
5. Radiation Sensing Scheme
Fig. 10A CMOS inverter consists of a PMOS and a NMOS transistor arranged in a configuration as shown below. The schematic diagram of an inverter is shown in figure 10. The drain terminal of the PMOS and NMOS transistor are connected together.
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Source:
OpenStax, Design of an in-vivo radiation measurement scheme using a reliable wireless detector. OpenStax CNX. Jan 16, 2008 Download for free at http://cnx.org/content/col10508/1.1
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