3.1. I-V characteristics

Room temperature sub threshold I-V characteristics of the n-channel ZVN4424A and the p-channel ZVP4424A irradiated to 0, 50, 100 and 200 kGy are shown in Fig. 1. It is observed that the threshold voltage of the devices, V_th, shifted as a function of radiation dose for all devices. Before irradiation, the threshold voltage was V_th = −1.5 V for the p-channel ZVP4424A and V_th = 1.7 V for the n-channel ZVN4424A; while after 50, 100 and 200 kGy irradiation, the threshold voltages decreased by 8±3V, 10±5V, 13±6V in ZVP4424A and by 2±1V, 4±1V, 7±1V in ZVN4424A,respectively.

Fig. 1Full-screen View

Sub threshold I-V characteristics of (a) ZVP4424A and (b) ZVN4424A before and after electron irradiation

The threshold voltage shifts in the p-channel is due to radiation induce positive charges in the interface traps^[8]. As more negative voltage is applied to the gate of a pmos device, the interface traps moves below the Fermi level and act as donors^[11]. When this negative gate-source voltage increases, the donor interface traps contribute more positive charges for channel conduction. Therefore the threshold voltage becomes more negative in the p-channel VDMOSFET.

As for the n-channel VDMOSFETs, the threshold voltage becomes negative as a result of radiation-induced positive charges dominated in the oxide traps^[12]. Charges in the traps forced the device to turn-on at negative bias when the electric field direction change from the silicon substrate to the insulator.

Figure 2 shows the threshold voltage shift (∆V_th) for the n-channel and p-channel devices irradiated to 50, 100 and 200 kGy. The square and circular bars indicate the n-channel and p-channel VDMOSFET threshold voltage shifts, respectively. Each data point represents the average ∆V_th while the error bars indicate the standard deviations. It can be seen that the threshold voltage shifts in the p-channel VDMOSFET is larger than that of the n-channel VDMOSFET^[13]. As Ristic et al. discussed in Ref. [14], this is due to the hot-carrier injection by negative electric field in the p-channel VDMOSFET. The hot-carrier injection affects the threshold voltage through the tunneling process of hot electrons from the gate/substrate into the oxide. Hot electrons accelerate by the negative electric field and produce more electron-hole pairs in the oxide. These cause a large decrease in threshold voltage of pMOS devices.

Fig. 2Full-screen View

∆V_th of ZVN4424A and ZVP4424A irradiated to 50, 100 and 200 kGy. Error bars indicate standard deviations of the threshold voltages.

Another important factor is the drain current degradation, as this affects the switching characteristics^[15]. Fig. 3 shows the drain current characteristics of p-channel and n-channel VDMOSFET before and after EB irradiation. At drain-source voltage of −3V, the drain current decreased by 0.047, 0.048 and 0.050 µA for 50,100 and 200 kGy irradiations, respectively. The 0.050 µA decrease at 200 kGy corresponds to 2 orders of magnitude decrease in the drain leakage current. This decrease is likely due to radiation-induced oxide damage^[16] also known as the formation of physically damaged region (PDR). When these defects localized in the interface of the p-channel VDMOSFET, it decreases the drain leakage current by formation of scattering centers that reduces the carrier mobility^[6]. Unlike the p-channel VDMOSFET, the drain leakage current of the n-channel is found to increase by 5 orders of magnitude for all dose levels. As Cester et al. pointed out, [17, 18] this may be due to accumulation of positive charges in the oxide layer of the n-channel VDMOSFET. The positive charges induce current through recombination process.

Fig. 3Full-screen View

Drain current characteristics of (a) ZVP4424A and (b) ZVN4424A before and after EB irradiation. The insert shows the increase in drain current before saturation of ZVN4424A.

The drain current degradation in the n-channel and p-channel VDMOSFET is a consequence of negative threshold voltage shifts which forces the device to turn on earlier in the n-channel and delay the turn on in the p-channel VDMOSFET. The next part illustrates these effects of degradation in the buck converter.

3.2. Irradiated VDMOSFETs impact on buck converter

The degradation of threshold voltage affects the I-V characteristics and hence the switching behaviors of the VDMOSFET in power converter applications^{[15, 19].} A buck converter circuit with 10V DC input and 5.5V output shows degradation in its switching characteristics through radiation-induced threshold voltage shifts in VDMOSFET. The circuit of the buck converter is shown in Fig. 4. The switching speed is increased by varying signal generator output frequencies from 1 kHz to 100 kHz while a clamped inductive load is connected at the VDMOSFET drain to reduce power dissipation^[20].

Fig. 4Full-screen View

The DC/DC buck converter circuit.

A buck converter circuit shown in Fig. 4 was simulated in pspice for the n-channel and p-channel switching transistors. The output voltage of the circuit was investigated for each model of the VDMOSFET before and after irradiation. The simulated of the resistors, capacitors and inductors were chosen to be implemented in the experimental setup. In the experimental analysis, the amount of output voltage degradation was measured by calculating the ratio of the output voltage. Fig. 5(a) shows a plot of the experimental output voltage ratio of buck converter consisting of p-channel ZVP4424A before and after being irradiated to 50, 100 and 200 kGy. One sees that the ratio of output voltages decreases with increasing doses. This is due to the consequence of the drain current decrease of the VDMOSFET after irradiation as shown in Fig. 3(a).

Fig. 5Full-screen View

Ratio of output voltage of (a) ZVP4424A p-channel and (b) ZVN4424A n-channel buck converter

The output voltage of the p-channel buck converter decreases with the drain leakage current of the VDMOSFET. This implies that there is degradation of the power conversion in the buck converter. In addition to this, the output voltage ratio V_{out_after}/V_{out_before} was investigated at frequencies of 1, 10 and 100 kHz, which were chosen based on its application in radiation harsh environments as shown in previous work ^[21]. The V_{out_after}/V_{out_before} ratio decreased with the initial output voltage before irradiation. Table 1 summarizes the p-channel buck converter output voltage at 50, 100 and 200 kGy with respect to switching frequencies..

From Table 1, the V_{out_after}/V_{out_before} ratio increases with the frequency for all dose levels. The output voltage ratio calculation shows that when the frequencies increases the output voltage comes closer to the desired value. It is believed that this is due to the increase in drain current through charge pumping for larger frequencies^[22]. When the current passes the inductive circuit, the output voltage increases by mutual inductance.

Figure 5(b) shows the V_{out_after}/V_{out_before} ratio of buck converter consisting of ZVN4424A n-channel before and after being irradiated to 50, 100 and 200 kGy. Unlike the results of the p-channel circuit, the V_{out_after}/V_{out_before} ratio of the n-channel irradiated buck converters increase with the irradiation dose. This is due to the VDMOSFET drain current increase after irradiation as shown in Fig. 3(b). The buck converter output voltage increases with the drain leakage current which reduces the efficiency of the buck converter applications.

Also, the output voltage and V_{out_after}/V_{out_before} of the n-channel VDMOSFET buck converter circuit was investigated at 1, 10 and 100 kHz. Table 1 also shows the summary of the n-channel buck converter output voltage response at 50,100 and 200 kGy with respect to the switching frequencies. The output V_{out_after}/V_{out_before} ratio increases with the frequency for all dose levels. It is found that when the frequency increases the output voltage are closer to the desired value. The power loss of the buck converter is calculated using the equation below:

P_loss = I²_out RDs_(on) V_out/V_in

As shown in Table 2, the power loss increases for the irradiated n-channel VDMOSFET, but reduces for the irradiated p-channel VDMOSFET. After weighing all the results it is clear that the degradation of the output voltage in p-channel is worse than the magnitude of the power loss than the n-channel. However for critical applications where power losses are important perhaps p-channel based buck converter circuit may be preferred.