Three gamma-cells were used in the present investigation, the first was a “GC-220” having a Co-60 source with dose rate of 80 mGy/sec. The second was “GC-4000 A” having a Co-60 source of a dose rate of 1.30 Gy/sec, and the third one was the “J-6500” irradiator which furnished with a Co-60 source plaque and used for high irradiation levels where its dose rate distribution was measured, calculated and reported. On the other hand, silicon-photovoltaic cells with 10×5 mm in dimensions have been irradiated at Centre Hospitlier, Lyon Sud, France, with different electron fluencies up to 1×1014 electron/cm2 at energies up to 12 MeV.
1.1. Varactor Devices
1.1.1. (I-V) characteristics
Varactor devices of the type BA102, as an example, showed higher radiation resistance than the LEDs 3,4. However, the forward-current voltage characteristics measured at different doses confirmed that the conduction mechanism in both involves an initial increase in the forward current which reaches a broad peak occurring after a specified dose above which the conduction current decreases (Fig. 1a). The initial increase in current is due to the contribution of a conduction current component via the radiation induced trapping centers, while the decrease is due to majority carrier removal by the radiation fluence. On the other hand, the reverse saturation current varies also with the dose as shown in Fig. (1b). The current increases in a linear function of the absorbed dose due to the increase in the leakage current component. The slope of the reverse current – dose dependence increases at higher reverse voltages as shown in Fig. (1a).
Fig. (1): Gamma-dose dependence of forward current (a) – reverse current (b) for BA102 type varactor.
1.1.2. (C-V) Characteristics
The (C-V) relationship for the varactor devices was studied in both the forward- and reverse-bias modes (A programmable automatic RCL meter). The applied forward –and reverse –bias voltages are shown to be from zero up to 0.60 volt and 10 volts, respectively. Fig.(2a) shows the radiation gamma dose dependence of the diffusion capacitance of the BA102 varactor. From which, it is clear that such devices are very sensitive to gamma-radiation whenever operates at forward bias voltage value of 0.60 volt.
If Co is the initial capacitance at zero dose, then it is clear from Fig.(2a) that the capacitance decreases as the dose increases. This behavior of Varactor – capacitors is confined by several tests for both the forward-and reverse-biased devices. The sensitivity of a reverse-biased Varactor is relatively much smaller than that of a forward-biased Varactor. Noting that, in practical applications a Varactor is used as a variable capacitor under reverse bias. An important characteristic is the range of capacitance variation with the bias voltage. If the circuit including a Varactor is located in a gamma radiation field, the range of capacitance will change with the change in absorbed dose. The capacitance will decrease at higher doses in the manner shown in Fig. (2a). However, the ratio between the maximum to minimum capacitance which can define the range of variation was found to increase from about 2.7 at zero dose to 4.5 at 500 kGy for the Varactor employed in this work (type BA102, Fig. 2b).
Fig. (2): Gamma-dose dependence of forward capacitance (a) and Cmax/Cmin variations
for reverse biased BA102 Varactor.
1.2. LED Devices
1.2.1. (I-V) Characteristics
Light emitting diodes (LEDs) devices are permanently damaged after exposure to gamma-radiation. This effect leads to the interruption of the LED device (I-V) characteristics (Fig.3), leading to pronounced changes the devices emitted light intensity level 5. In this concern, exposure to low gamma doses up to 50 Gy leads to an increase in the reverse saturation current, and consequently changes the forward (I-V) characteristics curves, where it shifts toward a lower voltage. For higher gamma doses, the forward voltage tends to increase (Fig. 3). The obtained results are mainly due to the formation of a very narrow intrinsic region in the neighborhood of the metallurgical PN-junction 6-8. Compensation of the chemical doping by radiation-produced deep traps will tend to produce such a region. At low gamma- doses, where the generation of current by recombination centers in the depletion region dominates the characteristics, the increase in the volume of the generation by formation of an intrinsic layer gives rise to a significant increase in the forward current, and consequent increase in the light emitted.
Fig. (3): Gamma-radiation effects on the forward (I-V) characteristics of LED
At higher doses, above 200 Gy the damage in carrier lifetime appears serious and hence, a significant voltage drop across this intrinsic region causes the observed shifts in the electrical characteristics (and light emitted) as shown in Fig. (4). The decrease in brightness, efficiency, and peak spectral intensity of the main emission line upon irradiation can be explained by a change in carrier lifetime, where the light intensity for a linearly graded LED is given by:
Where, C: constant , T: absolute temperature, V: drop voltage, k: Boltzmann’s constant, and q : electron charge,
A possible mechanism which could explain the change in lifetime in the diffusion region is that the radiation introduces defects which decrease the lifetime upon irradiation and which are removed on annealing, thus increasing the lifetime. The nature of the defect responsible for the change in lifetime and efficiency is not known, but several general statements can be made. The constancy of the recombination current with irradiation implies the defect is either not located in the space charge region and/or is a shallow level located throughout the junction. A deep level in the space-charge region could act as an efficient recombination center causing an increase in the recombination current. If it is assumed that the defects are introduced throughout the junction, the defect energy level is shallow. The shallow level is responsible for reducing the efficiency of light emission and for decreasing the minority carrier lifetime in the diffusion region of junction. Shallow defects introduced by irradiation have been observed.