4.1.2 Results

Fig. 4.3 shows the results of a breakdown simulation for the two-dimensional LDMOSFET and the three-dimensional SJ-LDMOSFET. With both structures a breakdown voltage of $120\,{\mathrm{V}}$ could be achieved. Since the SJ-LDMOSFET has been designed to have a fully depleted n-drift region for very low voltages the off-state current for low voltages is slightly higher than for the conventional LDMOSFET.

Figure 4.3: Off-state characteristics of a conventional LDMOSFET and a SJ-LDMOSFET.

The n-drift doping concentration for LDMOSFET has been fixed to $N_{\mathrm{D}}=1{\cdot}10^{16}\,{\mathrm{cm}}^{-3}$. Best results for the SJ-LDMOSFET could be achieved using an asymmetric doping for the n-columns ( $N_{\mathrm{D}}=9.9{\cdot}10^{16}\,{\mathrm{cm}}^{-3}$) and the p-columns ( $N_{\mathrm{A}}=7.0{\cdot}10^{16}\,{\mathrm{cm}}^{-3}$). At breakdown the lateral electric field strength at the surface of the device is shown in Fig. 4.4 for the middle of the p-column, at the p-n-column junction, and the middle of the n-column. The maximum values appear at the edges of the gate, the field plate, and the drain. At the p-n-column junction the maximum values are almost equal. In the middle of the n- and the p-column the maximum values appear at the gate and at the drain, respectively. Therefore, an excellent almost constant distribution of the electric field at the surface of the whole device could be achieved. To influence the electric field several approaches are feasible, e.g., graded n- (higher at the drain, lower at the gate) and p-doping (lower at the drain, higher at the gate) in the n- and p-columns, respectively. A different idea has been proposed in [141] using mutual tapered column widths.

Figure 4.4: Electric field strength at the surface of the SJ-LDMOSFET at $V_{{\textrm {DS}}} = 120{\hspace {.35ex}}{\textrm {V}}$.

For the SJ-LDMOSFET an on-resistance, $R_\mathrm{sp}$, which is $29$% lower compared to that of the LDMOSFET could be achieved (see Table 4.1).

Table 4.1: Doping concentrations and specific on-resistance for the LDMOSFET and the SJ-LDMOSFET.

Device Parameter	LDMOSFET	SJ-LDMOSFET
$N_{\mathrm{D}}$ [ ${\mathrm{cm}}^{-3}$]	$1.0{\cdot}10^{16}$	$9.9{\cdot}10^{16}$
$N_{\mathrm{A}}$ [ ${\mathrm{cm}}^{-3}$]	--	$7.0{\cdot}10^{16}$
$R_\mathrm{sp}$ [ ${\mathrm{m}}\Omega\,{\mathrm{cm}}^2$]	$2.88$	$2.03$

The on-state characteristics of both devices are shown in Fig. 4.5. The increased on-current of the SJ-LDMOSFET is due to the lower on-resistance of the device. Further improvement of the on-resistance can be achieved by doping engineering or by using trench gate structures [68,K6] at the expense of increased device capacitances. A different approach is shown in [142] for a structure using a folded gate to increase the channel area.

Figure 4.5: On-state characteristics of a conventional LDMOSFET and a SJ-LDMOSFET at $V_{{\textrm {GS}}} = 12{\hspace {.35ex}}{\textrm {V}}$.

In Fig. 4.6 the contour lines of the potential distribution at breakdown are shown. The contour lines are nearly equidistantly distributed by the RESURF principle. The optimum RESURF dose could be found which depends on the n-column, p-column, and p-body doping. One can clearly see the rolling curves at the surface. Moreover, the buried oxide supports the high voltage too and dense contour lines can be observed in this area.

Figure 4.6: Contour lines of the electrostatic potential $[\textrm {V}]$ of the LDMOSFET at $V_{{\textrm {DS}}} = 120{\hspace {.35ex}}{\textrm {V}}$.

The distribution of the electron current density for the SJ-LDMOSFET is shown in Fig. 4.7. The whole current flow happens in the n-column drift region and the p-column does not contribute to the current flow. Thus, the on-resistance of the device can be further improved using a larger n-column width than the p-column width ensuring complete charge compensation in the drift region.

Figure 4.7: Contour lines of the electron current density $[\textrm {A}/\textrm {cm}^2]$ of the SJ-LDMOSFET at $V_{{\textrm {DS}}} = 15{\hspace {.35ex}}{\textrm {V}}$ and $V_{{\textrm {GS}}} = 12{\hspace {.35ex}}{\textrm {V}}$.