2.3.2 Multi-RESURF in JI

A similar RESURF effect can be achieved in multiple junction devices where an additional layer of opposite doping ($p$-top region in Figure 2.21) is incorporated in the $n$-drift region. The main purpose of this structure is to increase the optimum charge in the drift region without reducing the BV. In such structures the vertical depletion of the $n$-drift region is supported by two (or three) junctions. Because of the multi vertical depletion in the device, the total integrated charge $Q_n$ in the $n$-drift layer can be increased allowing the on-resistance to be decreased compared to single-RESURF devices. In order to maintain a high BV in the multi-RESURF devices, it is required that both the $p$-top and the $n$-drift regions are fully depleted.

Figure 2.21 shows the double RESURF lateral diode which has two vertical junctions at the $p$-substrate/$n$-drift and $p$-top/$n$-drift. Another approach to reduce the on-state resistance is to use dual conducting paths in the $n$-drift.

Figure 2.22 shows the RESURF lateral device which has two current path in the $n$-drift. In this figure a $p$-buried layer is added inside of the $n$-drift in the drift region. With this $p$-buried layer $n$-drift charge can be increased compared to that of the double RESURF structure. To decrease further the on-resistance the highly doped and thin $n$-layer can be added on the top of the buried $p$-layer. But the floating $p$-buried layer will give poor BV performance, it must be grounded by contacting to the $p$-well.

Figure 2.21: Lateral double RESURF structure with $p$-top.

Figure 2.22: Lateral RESURF structure with buried $p$-top.

By adding the $n$-layer the $p$-buried layer doping has to be increased to achieve charge balance in the off-state. This $n$-layer forms a current conduction path which reduces the on-state resistance. For the double-RESURF structure shown in Figure 2.21, it is essential that the doping concentration of the $p$-top region ( $P_\mathrm{top}$) is sufficiently large that $P_\mathrm{top}$ > $N_\mathrm{drift}$ > $C_\mathrm{sub}$.The breakdown point is at the lateral $n^+ / p$-top junction with BV given by

$$BV_\mathrm{ld} = \frac{\varepsilon_{si}\,E_\mathrm{c}^2}{2q\, P_\mathrm{top}}\,.$$ (2.11)

As was stated earlier, in order to achieve a high BV in the double-RESURF structure, full depletion of the $p$-top and $n$-drift regions is required. Here, just like in the case of single-RESURF devices, full depletion should occur before the lateral $n^+ / p$-top junction breaks down. Therefore, in the double-RESURF case, the following conditions must be fulfilled

$$d_\mathrm{ptop} (BV_\mathrm{ld}) \geq t_\mathrm{ptop}\,,$$ (2.12)

$$d_\mathrm{n1} (BV_\mathrm{ld}) + d_\mathrm{n2} (BV_\mathrm{ld}) \geq t_\mathrm{ndrift}\,,$$ (2.13)

where $d_\mathrm{ptop} (BV_\mathrm{ld})$ is the vertical depletion extension into the $p$-top region at $BV_\mathrm{ld}$, $t_\mathrm{ptop}$ is the junction depth of the $p$-top region, respectively. $d_\mathrm{n1}$ is the vertical depletion extension into the $n$-drift region from the $p$-top/$n$-drift junction and $d_\mathrm{n2}$ is the vertical depletion extension into the $n$-drift region from the $p$-substrate/$n$-drift junction.

(2.12) prevents the structure from breaking down prematurely at the lateral $n^+ / p$-top, whereas (2.13) guarantees the prevention of a premature breakdown at the lateral $p^+ / n$-drift junction. With this structure the total charge in the $n$-drift region can be increased twice as much as in single-RESURF structure, leading to a much lower on-resistance.