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 (-top region in Figure 2.21) is incorporated in the -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 -drift region is supported by two (or three) junctions. Because of the multi vertical depletion in the device, the total integrated charge in the -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 -top and the -drift regions are fully depleted.

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

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

**Figure 2.21:** Lateral double RESURF structure with -top.
$\begin{figure} \begin{center} \psfig{file=figures/chapt2/resurf2.eps, width=0.65\linewidth} \end{center} \end{figure}$

**Figure 2.22:** Lateral RESURF structure with buried -top.
$\begin{figure} \begin{center} \psfig{file=figures/chapt2/resurf3.eps, width=0.65\linewidth} \end{center} \end{figure}$

By adding the -layer the -buried layer doping has to be increased to achieve charge balance in the off-state. This -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 -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 -top junction with BV given by

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

(2.11)

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

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

(2.12)

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

(2.13)

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

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

Jong-Mun Park 2004-10-28