2.3.3 RESURF in SOI

SOI (Silicon on Insulator) lateral devices are promising candidates for output power devices in smart power applications. They are suitable devices for high temperature environments, because of a very low leakage current at increased temperature. Several approaches to extend the RESURF concept to SOI devices have been suggested. For SOI structures the RESURF effect is based on the electric field distribution in the lateral depletion layer and the buried oxide.

The breakdown voltage of conventional high-voltage SOI devices is limited by the buried oxide thickness, SOI thickness, and the drift layer length. Most of the voltage is supported by the buried oxide layer. The maximum electric field strength at the interface between silicon and the buried oxide is limited by that of the silicon region at the interface. To obtain a higher voltage, the buried oxide and the SOI thickness must be increased, or SOI thickness must be decreased from the certain critical values depending on the buried oxide thickness. With the substrate at the ground potential, the breakdown voltage of SOI can be expressed as [1]:

$$BV = (\frac{t_{soi}}{2} + \frac{\varepsilon_{si}}{\varepsilon_{ox}} t_{ox})\, E_c\,,$$ (2.14)

Figure 2.23: Critical electric field for the breakdown of silicon.

Figure 2.24: Breakdown voltage versus drift length. Dashed curve shows the calculated BV, and circle and square points show the experimental values for the buried oxide thickness of 4.4 $\mu$m and 1.6 $\mu$m, respectively.

where $t_{soi}$ and $t_{ox}$ are the thicknesses, and $\varepsilon_{si}$ and $\varepsilon_{ox}$ are the dielectric permittivities of silicon and buried oxide, respectively. The critical electric field $E_c$ of the silicon layer increases with the doping increase. Assuming the $n^+ p$ abrupt junction diode which the lightly doped region (doping concentration $N_\mathrm{A}$) is thicker than the maximum depletion region thickness at breakdown, the $E_c$ is expressed as (see (3.31) and (3.32)):

$$E_c = 4123\, {N_\mathrm{A}}^{-\frac{7}{8}} \,[V/cm].$$ (2.15)

Figure 2.23 shows the critical electric field of silicon versus doping concentration. From this figure it is clear that the $E_c$ for break down is a week function of the doping concentration and lies in the range of 1 to 4 $\times$ $10^{5}$ V$/$ $\mathrm{cm}$ at the doping concentration from 1 $\times$ $10^{13}$ $\mathrm{cm^{-3}}$ to 1 $\times$ $10^{16}$ $\mathrm{cm^{-3}}$. Whenever the local electric field approaches the $E_c$, avalanche breakdown can be expected. However, it is important to note that exact breakdown voltage is determined by performing ionization integral through the depletion region.

The doping concentration of the drift region of the lateral device can be increased with decreasing the SOI thickness. This is the reason why the BV below an SOI thickness of 2.0$\mu$m increases with decreasing SOI thickness.

Figure 2.24 shows the breakdown voltage versus drift length with two different buried oxide thickness [1]. Assuming uniform lateral electric field it can be shown that the calculated breakdown voltage is approximately proportional to the length of the drift region, and that the device with thicker buried oxide approaches the ideal case. For the device with the thinner buried oxide the breakdown voltage saturates for longer drift region. The maximum breakdown voltage of the lateral device is determined essentially by the vertical electric field and no longer depends on the drift region.

The buried oxide will breakdown if the electric field there exceeds a critical value typically about 600V$/$$\mu$m. This value is much higher compared to that of silicon. However, a maximum electric field at the interface between silicon and $\mathrm{SiO_2}$ is limited by Gauss's law:

$$\varepsilon_{si}\,E_{n(si)} = \varepsilon_{ox}\,E_{n(ox)}.$$ (2.16)

Figure 2.25 shows the theoretical BV versus SOI thickness for three different values of buried oxide thickness $t_\mathrm{ox}$. The red dashed line in Figure 2.25 (at the BV of 1200V) shows the dielectric breakdown of a 2.0$\mu$m-thick buried oxide. The BV is increased if the SOI thickness is increased over 3.0$\mu$m or decreased approximately below 2.0$\mu$m.

LDMOSFETs on SOI will become important devices in SOI technology. Figure 2.26 shows a view of a conventional $n$-channel SOI-LDMOSFET. It is designed for a BV of 300V with an SOI thickness $t_\mathrm{soi}$ of 7.0$\mu$m and a buried oxide thickness of $t_\mathrm{ox}$ of 2.0$\mu$m. The drift region of the device is doped according to the RESURF principle. Field plates at the drain and gate region will help to reduce the electric field crowding, hence increasing the SOI doping. Generally, large voltage drops over the buried oxide. However, Gauss's law which is the conservation of the electric flux at the interface of the material with different dielectric constants, must be kept in mind. The maximum electric field at the buried oxide is limited by the critical electric field at the silicon side of the interface. Figure 2.27 shows clearly that the maximum electric field at the buried oxide follows Gauss's law. The electric field at this region is about 1.15 $\times$ $10^6$ V$/$ $\mathrm{cm}$ which is about 3.3 times the critical electric field in silicon (about 1.15 $\times$ $10^6$ V$/$ $\mathrm{cm}$ at a doping concentration of 1.0 $\times$ $10^{17}$ $\mathrm{cm^{-3}}$).

Figure 2.25: Theoretical BV versus SOI thickness.

Figure 2.26: Schematic structure of an $n$-channel SOI-LDMOSFET.

Figure 2.27: Electric field strength of the silicon layer ($n$-drift region), buried oxide and substrate at breakdown along the vertical cut line A in Fig. 2.26.