The forward voltage drop is a function of high level lifetime.^[4] The high-level lifetime does not directly depend on the defect center position, but on trap concentration and capture coefficients.^[5] The high-level lifetime (τ _HL) formula is given by

where C_ni and C_pi are the capture coefficients for electrons and holes, respectively; τ _pi and τ _ni are the minority carrier lifetimes in heavily doped N-type and P-type silicon, respectively; and N_Ti is the defect density.

Nevertheless, recombination center parameters are different with the same lifetime control techniques, which are interfered by many factors such as semiconductor material, irradiation energy, annealing temperature and time. As indicated in a large number of papers, defect parameters induced by electron irradiation differ largely, which is mainly affected by irradiation energy.^{[6, 7]}

Figure 1 shows the majority carrier DLTS spectrum of the N-base of FRD after different lifetime control techniques. Seven levels E1– E7 are found in both types of the electron irradiated FRD and the CLT FRD. Based on the peaks in Fig. 1, through the Arrhenius system, all defect parameters including capture coefficients, energy position, and density are calculated as shown in Table 1. Comparing the peak values for the DLTS signal in Fig. 1(a), it is found that the vacancy oxygen (VO^{(− /0)}) defect, double divacancy (V2^{(= /− )}) defect, H-related (VOH^{(− /0)}) defect, and single divacancy (V2^{(− /0)}) defect are responsible for the electron irradiated peaks. The levels E1 and E4 have a dominant effect on the change of carrier lifetime. The peak E4 is the acceptor level of divacancy (V2^{(− /0)}), which determines the high-level lifetime. The levels E2 and E3 each have a low concentration and capture coefficient and are annealed at relatively low temperatures, which have a negligible influence on the recombination activity.^[8] According to Ref. [3], acceptor level (E_C-0.23 eV) of platinum is an effective recombination level at high injection, while it acts as a weak recombination center at low injection.

Fig. 1.

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	Fig. 1. DLTS measurements of the N-base of FRD (a) after individual irradiation effects by electrons irradiation with a dose of 90 kGy and (b) after combined effects by electron irradiation with a dose of 90 kGy and platinum diffusion performed at 890 ° C for 1 h.

Table 1.

Table 1.

Table 1. Survey of deep levels identified in the N-base of irradiated diodes. Dose Level (EC– ET)/eV Capture cross section/cm2 Concentration/cm− 3 Lifetime (300 K)/s Idebtity 90 kGy E1 0.128 1.23× 10− 15 1.33× 10+ 11 6.1× 10− 4 VO− /0 E2 0.214 7.22× 10− 17 1.07× 10+ 11 1.29× 10− 2 E3 0.347 3.46× 10− 16 4.21× 10+ 10 6.8× 10− 3 VOH(− /0) E4 0.476 6.44× 10− 14 1.45× 10+ 11 1× 10− 5 890 ° C + 90 kGy E5 0.140 1.02× 10− 15 1.84× 10+ 10 5.3× 10− 3 VO− /0 E6 0.376 1.44× 10− 14 1.18× 10+ 11 5.9× 10− 5 – E7 0.476 1.50× 10− 14 4.00× 10+ 11 1.6× 10− 5

Table 1. Survey of deep levels identified in the N-base of irradiated diodes.

Compared with DLTS signal peaks of individual electron irradiation, the levels E2 and E3 disappear in the DLTS signal of the CLT FRD. Meanwhile, platinum level (E_C-0.23 eV) is not found and a new level E6 which belongs to neither the electron irradiation levels nor the platinum levels appears. Obviously, the level E6 is not a simply superposition with electron levels and platinum levels, but must be caused by combined effects.

Lifetime-controlled samples show different temperature dependences of the forward voltage depending on the properties of induced defect.^[2] Figure 2 shows the forward voltage dependences of the electron irradiated FRD on temperature, while figure 3 shows the measurements for platinum diffused FRDs. It is convinced that the temperature has a small influence on the V_F of each of the electron irradiated samples. That is to say, at rated current, electron irradiated FRDs work at the positive or zero temperature coefficient, which is an advantage for paralleling applications. On the contrary, the V_F of platinum diffused sample decreases with increasing temperature. This implies that FRD shows a negative temperature coefficient, which would stimulate the continuing current rise with the increasing of the temperature.

Fig. 2.

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	Fig. 2. Temperature dependences of forward voltage drop for FRD after electron irradiation with different doses.

Fig. 3.

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	Fig. 3. Temperature dependences of forward voltage drop for FRD after platinum diffusion at different temperatures for 1 h.

Unlike the unirradiated FRDs, electron irradiation improves the temperature coefficients of the FRDs. The reason is that the concentration ratio between the E4 and E1 levels is very large (see Table 1). The level E4 is responsible for the carrier recombination in the lifetime at the high-level, that is deduced from the value of τ _HL which is given by Eq. (1). Therefore, the forward voltage is strongly influenced by the temperature dependence of the capture coefficient of level E4. For platinum diffused samples, the forward voltage is mainly affected by the acceptor level (E_C-0.23 eV), while the acceptor level has an inverse effect on the sample.

Figures 4 and 5 show the influences of the temperature on the forward voltage for the FRDs. Based on the results in Figs. 4 and 5, it is clear that with increasing temperature the forward voltage drop decreases. This is further supported by the change of the temperature coefficient as shown in Fig. 6. With the increase of electron irradiation dose, the Δ T/Δ T₀ of the CLT sample increases negatively. This means that the CLT FRD irradiated by high energy electron cannot improve the temperature characteristic but aggravates it.

Fig. 4.

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	Fig. 4. Temperature dependences of forward voltage for FRD after combined effects by platinum diffusion performed at 870 ° C for 1 h and electron irradiation with different doses.

Fig. 5.

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	Fig. 5. Temperature dependences of forward voltage for FRD after combined effects by platinum diffusion performed at 900 ° C for 1 h and electrons irradiation with different doses.

Fig. 6.

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	Fig. 6. Temperature coefficients of forward voltage for FRD after combined effects by platinum diffusion and electron irradiation.

In order to explain the intrinsic reasons of this phenomenon, we carry out the DLTS measurements for electron irradiated samples and the CLT samples. Based on analyses of the energy levels (see Fig. 1), it is demonstrated that there is an interaction between platinum and electron in the experimental process of combined lifetime, resulting in a change of recombination center data. It is worth pointing out that this kind of interaction produces a new level.

After DLTS measurements, the forward I– V characteristics on samples are shown in Fig. 7. It is clear that the voltage drop of the CLT FRD is higher than those of electron irradiated FRD and diffusion platinum FRD. As shown in Table 1, the capture rate and concentration of the level E5 are very small, so the level E5 gives little contribution to the high level lifetime of the CLT FRD. The E4 and E7 levels make similar contributions to the high level lifetime. According to the above analysis, the new level E6 must play such an important role in the high level lifetime that the CLT sample has a larger voltage. It is reasonable to make a conclusion that the level E6 is electrically active.

Fig. 7.

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	Fig. 7. Forward I– V curves for the different lifetime controlled FRDs.

The levels E6 and E7 make the main contributions to the forward voltage of the CLT FRD. Thus, the temperature dependences of the capture coefficient of the levels E6 and E7 determine the forward characteristics of the CLT FRDs. On the other hand, the levels E4 and E7 that are found to have the same position of the recombination center belong to the same defect. Therefore, the levels E7 and E4 have the same temperature dependence properties, which makes the FRD possess a positive temperature coefficient. However, the CLT FRD shows a negative temperature coefficient. It can be easily inferred that the temperature dependence of the level E6 is the most significant factor. Hence the level E6 dominates.