4.1 Superjunction LDMOSFET

Double diffused MOS transistors (DMOSFETs) are commonly used in high-voltage applications for industrial electronics or automotive systems. DMOSFETs can be fabricated using different approaches. A conventional approach is to use a vertical structure [131,132] (VDMOSFET). Another established device is the lateral DMOSFET (LDMOSFET) where the current flows laterally. LDMOSFETs can be fabricated either in bulk silicon or in silicon on insulator (SOI). Thin SOI LDMOSFETs enable perfect isolation and fast switching performance. However, the breakdown voltage is reduced due the possibility of vertical and horizontal breakdown. Moreover, self-heating is much higher than for devices fabricated in bulk silicon due to the thermal isolation of SiO$_2$.

To increase the breakdown voltage of LDMOSFETs several approaches are reported, such as linearly-graded n-epi layers on buried oxide which, however, leads to serious self-heating even if field oxide is used. The main disadvantage of this structure is that serious self-heating takes place. To reduce self-heating and for further enhancement of the breakdown voltage partial SOI technologies have been introduced. Nevertheless, this structure is much more complex.

To increase the breakdown voltage simply the drift length of the devices must be enlarged which, however, would lead to dramatically enlarged chip sizes. Increasing the breakdown voltage while reducing the on-resistance has been one of the main issues. The reduction of the on-resistance is very important for power-devices to limit power dissipation for higher currents.

One of the main limitations for breakdown voltage design is given by the RESURF (REduced SURface Field) effect [133,134,135,136]. In order To increase the breakdown voltage of the device the doping concentration in the drift layer must be the reduced and the drift length increased at the expense of the specific on-resistance ( $R_\mathrm{sp}$), which is the product of on-resistance and active area of the device.

The recently reported most promising concepts to escape this vicious circle make use of superjunctions [134,135,136,137,138,139,K5] and lateral trench gates [68,69,K6]. Superjunctions are alternating p- and n-columns which allow to increase the doping concentration while keeping the breakdown voltage high. These approaches allow further reduction of the on-resistance at a maintained breakdown voltage. For VDMOSFETs several superjunction devices have been reported [131,132,140] too.