Novel Lateral MOS Controlled Power Devices and Technologies for Power Integrated Circuits
Power microelectronics can be considered as a combination of semiconductor high voltage integrated circuits, smart power discrete devices, low voltage circuits, and the fabrication technologies with the feature of high integrated density. It is always desirable that a lateral MOS controlled power device, as a key component in such a monolithic integrated circuit or system, can provide a large current carrying capability, high blocking voltage, fast switching speed, small area of occupation. However, conventional lateral MOS power devices, especially the devices with the operation of the conductivity modulation mode, have to be improved further to meet these requirements. Isolation technology, as an essential technology in PICs, is extremely important to realise the integration of high voltage devices with low voltage circuits. Therefore, this dissertation focuses on developing and investigating novel lateral MOS controlled power devices and new isolation technologies. An isolation technology, DELDI, has been analysed in detail. The REduced VERtical Field (REVERF) effect in the DELDI technology has been revealed for the first time. The results show that this effect dominates the breakdown voltage of lateral MOS power devices with an ideal REduced SURface Field (RESURF) effect. The influence of the physical and structural parameters of devices in the DELDI technology on the breakdown voltage has been investigated. Furthermore, it is found that the Kirk effect in the DELDI technology plays a key role on the on-state performance of devices with a conductivity modulation. This effect has been used to develop devices with the enhanced on-state performance. Two lateral MOS-controlled power devices, Lateral Thyristor-enhanced Insulated Gate Bipolar Transistor (LTIGBT) and Double p-well Lateral Emitter Switched Thyristor (DP-LEST), have been analysed in detail. The LTIGBT demonstrates a superior on-state performance in relation to a conventional LIGBT. At a current density of 150A/cm^ for a gate bias of 6V, the forward drop of the LTIGBT is 0.3 V lower than that of the conventional LIGBT under identical conditions. It has been revealed that the Negative Differential Resistance effect in the LTIGBT improves the latch-up immunity. The analysis has shown that the forward voltage drop and the carrying current density of the DP-LEST are close to those of a conventional LIGBT, but its latch-up current and voltage are higher than those of the LIGBT. A new anode structure is proposed to improve the switching speed of lateral MOS power devices with the conductivity modulation. Its feasibility has been demonstrated by means of various devices, such as the LIGBT, LEST, LTIGBT, DP-LEST, and novel multi-channel lateral MOS power devices. This open base anode exhibits a considerable reduction in the turn-off time and no significant effect on the on-state performance. The turn-off time of the fast switching LIGBT is about one order lower than that of the conventional LIGBT. The turn-off times of the fast switching DP- LEST and the fast switching LTIGBT are half of those with a conventional anode. More impressively, this new anode is also useful to reduce the turn-off time of devices with the multi-cathode cells. A new approach, multi-cathode cell, has been proposed. This approach demonstrates a significant reduction in the forward voltage drop and a substantial enhancement in the current carrying capability of lateral MOS power devices. A unique quasi-vertical mode of operation for additional MOS channels has been revealed. This quasi-vertical mode avoids the limitation in the pinch-off region to the adjacent channel current and enhances the adjacent channel current in the sandwich region. For devices with 2 cathode cells, the carrying current density at the forward voltage drop of 4V is 1.64 times for the MC-LIGBT, 1.65 times for the multi-cell DP-LEST, and 1.87 times for the multi-cell LTIGBT in relation to those of single cathode cell. More impressively, the carrying current density of the multi-cell LTIGBT at above conditions is 1.36 times of the MC-LIGBT with identical structural parameters except additional p-well and n-emitter regions.
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