4.4.3 Simulation Results

$R_\mathrm{sp}$ and the BV of high-voltage SOI-LDMOSFETs strongly depend on the doping and the length of the drift layer. The drift doping of conventional SOI-LDMOSFETs is restricted by the RESURF effect. To increase the BV the drift length must be increased and the doping decreased. This results in an increase in the on-resistance. With the SJ structure it is possible to increase the doping concentration of the drift layer drastically, and $R_\mathrm{sp}$ can be reduced effectively.

The on-resistance of SJ devices has a linear voltage dependence instead of the square-law dependence of standard power MOSFETs [75,159]. The BV of the SJ depends on the critical electric field $E_\mathrm{c}$ of the device and the length of the $n$- and $p$-columns. With the SJ concept the $n$-column charge $Q_\mathrm{n}$, the $p$-column charge $Q_\mathrm{p}$, and the charge $Q_\mathrm{db}$ of the $p$-body depletion region should be balanced in this structure. To reduce the column length of the SJ SOI-LDMOSFET a trench oxide is proposed in this study.

Figure 4.33 shows a comparison of the BV of conventional SOI-LDMOSFETs which have an $n$-drift length $L_\mathrm{d}$ $=$ 20.0$\mu$m and 13.0$\mu$m, respectively, and the SJ SOI-LDMOSFET with a trench oxide and $L_\mathrm{d}$ $=$ 13.0$\mu$m. As shown in the figure the BV of the conventional devices strongly depends on the drift length. The conventional SOI-LDMOSFET with $t_\mathrm{soi}$ $=$ 7.0$\mu$m and $t_\mathrm{ox}$ $=$ 2.0$\mu$m has a BV of 300V (the solid line) at $L_\mathrm{d}$ $=$ 20.0$\mu$m and $N_\mathrm{D}$ $=$ 2.3 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$. If the $n$-drift length is reduced to 13.0$\mu$m in this structure, a BV of 245V (the dotted line) is obtained at $N_\mathrm{D}$ $=$ 3.5 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$. The dashed line in the figure shows the BV of the SJ SOI-LDMOSFET with a trench oxide.

Figure 4.33: Comparison of the BV of conventional SOI-LDMOSFETs ( $L_\textrm {d}$ $=$ 20.0 $\mu$m and 13.0 $\mu$m) and the SJ SOI-LDMOSFET with a trench oxide ( $L_\textrm {d}$ $=$ 13.0 $\mu$m). $N_\textrm {D}$ of conventional devices are 2.3 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$ (at $L_\textrm {d}$ $=$ 20.0 $\mu$m) and 3.5 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$ (at $L_\textrm {d}$ $=$ 13.0 $\mu$m), respectively, and 6.0 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$ for the SJ SOI-LDMOSFET.

Because of the increased surface path of the device, the BV increases with the trench oxide in the drift region. A BV of 300V is obtained with $L_\mathrm{d}$ $=$ 13.0$\mu$m in this structure. Note that this is the same BV as that of the conventional SOI-LDMOSFET with $L_\mathrm{d}$ $=$ 20.0$\mu$m. Figure 4.34 shows the potential distribution of the RESURF SJ SOI-LDMOSFET which has a trench oxide and $L_\mathrm{d}$ $=$ 13.0$\mu$m. Because of the increased surface path to the vertical direction in the drift region, we can see potential lines at the side wall of the trench oxide. These potential lines help to increase the BV to the maximum value of conventional SOI-LDMOSFETs with $L_\mathrm{d}$ $=$ 20.0$\mu$m. Figure 4.35 shows the electric field distribution of the suggested device at $V_\mathrm{DS}$ = 300V. Note that a higher electric field can be observed at the trench oxide edges. From this figure we can see clearly several peaks of the electric field. The SJ SOI-LDMOSFET with a trench oxide has an additional peak in the middle of the SOI layer below the gate. Figure 4.36 shows a comparison of the electric field at the top surface between conventional SOI-LDMOSFETs with $L_\mathrm{d}$ $=$ 20.0$\mu$m and 13.0$\mu$m. Conventional SOI-LDMOSFETs have the peak electric field at the drain, the field plate, and the gate edge near the top surface of the silicon.

Figure 4.34: Potential distribution of a SJ SOI-LDMOSFET which has a trench oxide in the drift region at $V_\textrm {DS}$ $=$ 300 V.

Figure 4.35: Electric field (unit: V/cm) distribution of a SJ SOI-LDMOSFET which has a trench oxide in the drift region at $V_\textrm {DS}$ $=$ 300 V.

Figure 4.36: Comparison of the electric field at the top surface between the conventional SOI-LDMOSFETs with $L_\textrm {d}$ $=$ 20.0 $\mu$m and 13.0 $\mu$m, respectively.

Figure 4.37: Comparison of the electric field at the bottom of the trench oxide between the SOI-LDMOSFET and the SJ SOI-LDMOSFET. Both structures have a trench oxide in the drift region with $L_\textrm {d}$ $=$ 13.0 $\mu$m.

Figure 4.37 shows a comparison of the electric field at the bottom of the trench oxide between the SOI-LDMOSFET and the SJ SOI-LDMOSFET. $L_\mathrm{d}$ of both structures is 13.0$\mu$m. At the $n$-drift and $p$-body junction both devices show similar trends as those of the conventional device, but the abrupt peak can be seen at the trench oxide edge. Generally, in the middle of the device (along the lateral direction of the device) the conventional SOI-LDMOSFET has a broad range of higher electric field near the $n$-drift and $p$-body junction, and no abrupt peak can be found. For the SJ SOI-LDMOSFET with a trench oxide of $L_\mathrm{d}$ $=$ 13.0$\mu$m and $N_\mathrm{D}$ $=$ 6.0 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$, the optimum electric field distribution is obtained with $N_\mathrm{A}$ of 1.5 $\times$ $10^{16}$ $\mathrm{cm}^{-3}$ (with $W_\mathrm{P}$ $=$ 0.8$\mu$m).

The solid line of Figure 4.38 shows the BV versus $n$-drift doping of the conventional SOI-LDMOSFET with an optimum $L_\mathrm{d}$ $=$ 20.0$\mu$m. To achieve the best trade-off between $R_\mathrm{sp}$ and the BV, a higher drift doping with optimum $L_\mathrm{d}$ is essential. From the figure we can see the optimum $n$-drift doping is 2.3 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$. If the $n$-drift doping is reduced below the optimum value, the maximum electric field is moved towards the drain edge. If it exceeds the optimum value, a high electric field is moved towards the gate edge. Both cases cause a lower BV. $R_\mathrm{sp}$ can be lowered by reducing $L_\mathrm{d}$. This shrinks the surface path of the depletion region at breakdown.

The dotted line of the figure shows the BV of a conventional SOI-LDMOSFET with $L_\mathrm{d}$ $=$ 13.0$\mu$m, where a maximum BV of 245V is obtained at $N_\mathrm{D}$ $=$ 2.0 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$. With the trench oxide in the drift region it is possible to increase the surface path of the depletion region and $L_\mathrm{d}$ can be reduced drastically without degrading the BV. The dashed line shows the BV of the SOI-LDMOSFET which has a trench oxide in the drift region and a maximum BV of 300V is obtained at $L_\mathrm{d}$ $=$ 13.0$\mu$m. Because the reduced $n$-drift area by the trench oxide affects the charge balance condition, the optimum doping $N_\mathrm{D}$ is slightly increased compared to the conventional device.

Figure 4.38: BV versus $n$-drift doping of the conventional SOI-LDMOSFETs and SOI-LDMOSFET with trench oxide (trench depth $=$ 2.7 $\mu$m and $L_\textrm {d}$ $=$ 13.0 $\mu$m).

Figure 4.39: BV versus $p$-column doping of SJ SOI-LDMOSFETs with trench oxide which have $L_\textrm {d}$ $=$ 13.0 $\mu$m and $N_\textrm {D}$ $=$ 6.0 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$.

Figure 4.40: $p$-column doping and $R_\mathrm{sp}$ versus $p$-column width of the SJ SOI-LDMOSFET which has a trench oxide.

The reduced conduction area by the trench oxide increases $R_\mathrm{sp}$. To solve this problem we propose the SJ SOI-LDMOSFET by introducing the buried $p$-column in the $n$-drift region together with a trench oxide. With this structure $R_\mathrm{sp}$ can be lowered effectively. Figure 4.39 shows the BV versus $p$-column doping of the proposed devices with various trench depths. $L_\mathrm{d}$ and $N_\mathrm{D}$ are fixed to 13.0$\mu$m and 6.0 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$, respectively.

The trench depth determines the length of the surface path of the device. If it is below 2.5$\mu$m (solid and dot-dashed lines) the BV is lower than that of conventional devices. With the trench depth over 2.7$\mu$m the same BV is reached as the maximum value of conventional SOI-LDMOSFETs.

To obtain a maximum BV, $Q_\mathrm{n}$, $Q_\mathrm{p}$, and $Q_\mathrm{db}$ in the SJ SOI-LDMOSFET should be balanced for complete depletion of the drift region at breakdown. If the $p$-column width is larger, the doping of this layer should be reduced to fulfill the charge balance. It is also important to minimize the $p$-column width to increase the area of current path at the drift region.

Figure 4.40 shows the $p$-column doping and $R_\mathrm{sp}$ versus the $p$-column width of the proposed SJ SOI-LDMOSFET which has a trench oxide. The $p$-column doping concentration at each point in the figure is optimized to have a maximum BV of 300V. Other device parameters such as the trench oxide depth, the drift length $L_\mathrm{d}$, and the $n$-column doping $N_\mathrm{D}$ are 2.7$\mu$m, 13.0$\mu$m, and 6.0 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$, respectively. The doping of the $p$-column is reduced with the increased $p$-column width.

The dotted line of Figure 4.40 clearly denotes this dependence. With $W_\mathrm{P}$ $=$ 0.3 and 1.3$\mu$m, the optimum $N_\mathrm{A}$ is 4.0 and 1.0 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$, respectively. $Q_\mathrm{p}$ $=$ $N_\mathrm{A}$ $\times$ $W_\mathrm{P}$ of both cases remains approximately constant. The solid line shows the relationship between $W_\mathrm{P}$ and $R_\mathrm{sp}$. With reduced $p$-column width $R_\mathrm{sp}$ is improved by the increased conduction area. With a lower value for $W_\mathrm{P}$ of 0.3$\mu$m it is possible to achieve a minimum $R_\mathrm{sp}$ of 25.4m$\Omega$ $cm^2$. If $W_\mathrm{P}$ is increased to 1.3$\mu$m, $R_\mathrm{sp}$ is increased to 29.3m$\Omega$ $cm^2$.

Figure 4.41: $R_\textrm {SP}$ versus BV comparison with conventional MOSFETs, theoretical Si limit, and proposed SJ SOI-LDMOSFETs (see also Fig. 3.13).

Figure 4.41 shows the $R_\mathrm{sp}$ versus BV comparison with conventional MOSFETs, theoretical Si limit, and proposed SJ SOI-LDMOSFETs. AS shown in the figure lower values of $R_\mathrm{sp}$ can be obtained with the proposed SJ SOI-LDMOSFETs which have a trench oxide in the drift region. This is mainly because of the reduced drift length by the trench oxide.

Table 4.4: DC performance comparison between the conventional SOI-LDMOSFET and the proposed device.

	Conventional SOI-LDMOSFET	Proposed SJ SOI-LDMOSFET
$N_\mathrm{D}$, $\mathrm{cm}^{-3}$	2.3 $\times$ $10^{15}$	6.0 $\times$ $10^{15}$
$L_\mathrm{d}$	20.0$\mu$m	13.0$\mu$m
$R_\mathrm{sp}$	33.4m$\Omega$ $cm^2$	25.4m$\Omega$ $cm^2$
BV	300V	300V

Table 4.4 shows a DC performance comparison of the simulation results between the conventional SOI-LDMOSFET and the proposed SJ SOI-LDMOSFET which has a trench oxide in the drift region. For the proposed device with an n-column doping $N_\mathrm{D}$ of 6.0 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$ and a drift length $L_\mathrm{d}$ of 13.0$\mu$m, a maximum BV of 300V is obtained at the p-column doping $N_\mathrm{A}$ $=$ 4.0 $\times$ $10^{16}$ $\mathrm{cm}^{-3}$ and the p-column width $W_\mathrm{P}$ $=$ 0.3$\mu$m. These results demonstrate that the drift length can be reduced with a trench oxide in the drift region. The on-state characteristics depend on the $p$-column width and the drift doping. $R_\mathrm{sp}$ of the proposed device is 25.4m$\Omega$ $cm^2$. It is about 76% of the corresponding $R_\mathrm{sp}$ of the conventional 300V SOI-LDMOSFET. Even the width of the drift region is reduced by the $p$-column and $R_\mathrm{sp}$ is lower than that of the conventional device by the reduced $L_\mathrm{d}$ and the increased $N_\mathrm{D}$ of the proposed device.