Shielded gate transistors are preferred for certain applications over conventional transistors, such as conventional trench MOSFETs (metal oxide semiconductor field effect transistors) because they provide several advantageous characteristics. Shielded gate trench transistors (SGTs) exhibit reduced gate-to-drain capacitance Cgd, reduced on-resistance RDSon, and increased breakdown voltage of the transistor. For conventional trench MOSFETs, the placement of many trenches in a channel, while decreasing the on-resistance, also increased the overall gate-to-drain capacitance. The introduction of the shielded gate trench MOSFET structure remedies this issue by shielding the gate from the electric field in the drift region, thereby substantially reducing the gate-to-drain capacitance. The shielded gate trench MOSFET structure also provides the added benefit of higher impurity carrier concentration in the drift region for the device's breakdown voltage and hence lower on-resistance.
Shielded gate trench MOSFET devices are described, e.g., in U.S. Pat. No. 5,998,833 to Baliga.
The improved performance characteristics of the shielded gate trench MOSFET make the technology an excellent choice for power switching applications such as the switching converter commonly referred to as a synchronous buck converter (a type of DC-DC converter in which the output voltage is “stepped-down” compared to an input voltage). The shielded gate trench MOSFET is particularly suitable for the high-side switch in a synchronous buck converter. However, for the low-side switch which operates as a synchronous rectifier, excessive charge during the reverse recovery of the body diode results in increased power dissipation and reduced converter efficiency.
SGT with shielded gate at source potential has advantages of low RDSon. The shield electrodes beneath the gate electrodes reduce gate-drain capacitance.
In a conventional SGT design, the shield electrodes and gate electrodes are formed in a self-aligned process that uses a single mask to form a set of trenches that are used for both the gate electrodes and the shield electrodes. However, the structural requirements of the shield electrodes and the gate electrodes are different. For example, because the shield electrode is at source potential, the shield electrode must be electrically insulated from the semiconductor layer in which the trench is formed. A thick oxide is typically used between epitaxial layer and the shield electrode to sustain breakdown. There is also a mesa between adjacent shield electrodes. When devices are scaled down problems can arise with the mesas getting too close together leaving insufficient room for the thick oxide.
It is within this context that embodiments of the present invention arise.