Abstract ：The electrical characteristics of solid state devices such as BJT (Bipolar Junction Transistor) and MOSFET, etc, are altered by impinging photon radiation and temperature in the space environment. In this paper, the threshold voltage and breakdown voltage for the two kinds of MOSFET's (200 (V) and 100 (V) of VDSS) are tested to γ-irradiation and compared with the specifications under the pre and post irradiation. The experimental results shows the lowering of threshold voltage with increasing dose for the device exposed to γsource. 
Key words: Radiation effect Power MOSFET


I. Introduction

The types of radiation are generally divided into particle radiation and photon radiation. The particle radiation consists of the charged particles which have protons, electrons, α particles, ions, and neutral particles that are the neutrons. The particle radiation may also induce ionization so that excess carriers are generated within a semiconductor material and device. The photon radiation consists of g rays and/or x-rays. The units primarily used in radiation effects, which deal with ionization induced by g rays are rad. The rad is the amount of radiation which deposits 100 ergs of energy per gram of material. When dealing with particle radiation, the units are flux (number/cm2-sec) and fluence (number/cm2). 
A simple method for estimating the threshold voltage shift [1] due to low dose rate ionizing irradiation was recently proposed for power MOSFETs. Briefly, the method consists of estimating the low dose rate threshold voltage shift by summing the threshold shift due to the oxide charge trapping at the surface of the p-n junction immediately after irradiation and annealing 100 oC. 


II. Radiation Effects on Power MOSFET

MOS devices are among the most sensitive of all semiconductors to radiation, in particular ionizing radiation, showing much change even a relatively low dose. In fact, the effects of radiation on MOS devices must be at about the same level of total dose. The gate field oxide structures give the main influence on the changes in electrical characteristics [2, 3] affected by irradiation. A simple translation of the I-V characteristic towards more negative values of gate voltage is brought by charge trapping at the oxide-silicon interface. This is much serious for n-channel devices when the I-V curve is shifted past zero volts as the current increases sharply. This effect is often considered as a change in gate threshold voltage. The main effect is a distortion of the I-V curve due to a lowering of trans-conductance. 

III. Experiment Results

The two kinds of MOSFET's (200 [V] and 100 [V] of VDSS) are chosen as the tested parts. The threshold voltage (Vth) and breakdown voltage (BVDSS) are tested and compared with the specifications under the pre-irradiation and the irradiation of low dose rates of 4.97 and 9.55 rad/sec, and maximum total dose of 30 (Krad). The test procedure and method is carried out by Mil-Std-883 Method 1019. The g source using 60Co is used for the test of commercial IR (International Rectifier) MOSFET products. The sample size of testing is 6 pieces for each test.
Fig. 1 to 5 show threshold voltage and breakdown voltage characteristics of irradiated MOSFETs based on the quantity of dose and after annealing at 100oC for 168 hours. The total doses are 0, 5, 10, 15, 20, and 30 [Krad], respectively, and the dose rate is either 9.55 or 4.97 [rad/sec]. The annealing at 100oC and 168 hours is carried out right after irradiation. 
As shown in Fig. 1, the threshold voltage, Vth of MOSFET (200 [V]) is decreasing as the quantity of the total dose increases up to 30 [Krad] with dose rate of 9.55 [rad/sec], but the voltage was not recovered after annealing 100 oC and 168 hours, but was recovered to the level of 20 [Krad] irradiation with 4.97 [rad/sec] of 1.9 [V] in Fig. 2. 

Fig. 1. Threshold voltage (Vth) characteristics of a MOSFET(200 V).

The slope of the threshold voltage in Fig. 3 for the dose rate 4.97 [rad/sec] for MOSFET (100 [V]) is not steeper than that of MOSFET (200 [V]).
The breakdown voltage characteristics [4] are shown in Fig. 4 and 5, and the results are met within the specifications of minimum 200[V], and 100[V], respectively. The breakdown voltage is not changing due to the irradiation depend on the total dose and after the annealing is finished. It is expected that the field limit ring, for achieving breakdown voltage, of the sample device have an enough margin for the charge accumulation occurred when the device is exposed to the radiation source. 

Fig. 2. Threshold voltage characteristics of a MOSFET(200 V).

Fig. 3. Threshold voltage characteristics of a MOSFET(100 V). 

Fig. 4. Breakdown voltage (BVDSS) characteristics of a MOSFET(200 V).

Fig. 5 Breakdown voltage characteristics of a MOSFET(100 V). 

IV. Conclusion

The breakdown voltage does not change significantly with respect to the dose rate and total dose. The experiment shows that IR commercial products are robust in the breakdown voltage. However, it is suggested that the new structure of radiation hardening to prevent the shift of threshold voltage should be considered for the radiation hardened device. 

References：
[1] P. Khosropour, K.F. Galloway, D. Zupac, R.D. Schrimpf, and P. Calvel, “Application of Test Method 1019.4 to Non-hardened Power MOSFETs”, 1993 RADECS Record, pp. 303-305 and IEEE Trans. Nucl. Sci., vol. 41, pp. 555-560, 1994
[2] R.E. Sharp, “Radiation Effects on Power Semiconductors”, IEE, Savoy Place, London WC2R 0BL, UK 1994
[3] The Technical Reports for Radiation Effects, 1988, Neamen, Univ. of New Mexico, ALBQ, NM, U.S.A.
[4] S.Y.Lee, Y.H.Lho, “On the Design of Field Limiting Ring for Improving Corner Breakdown Voltage”, Proc. PEMC98, Prague, Czech, pp. 41-44, 1998.