Exemplary power semiconductor devices include planar-gate MOSFET transistors, vertical gate MOSFET transistors, insulated-gate bipolar transistors (IGBTs), rectifiers, and synchronous rectifiers. Typical implementations of the trench-gate variety of these devices comprise an array of trenches formed in the top surface of the semiconductor die, with each trench filled with a shield electrode and/or a gate electrode, depending upon the type of power device. The trenches define a corresponding array of mesas, each mesa being disposed between adjacent trenches. Depending upon the device implemented on the die, various electrodes and/or doped regions are disposed at the top of the mesa. Each mesa and its adjacent trenches implement a small instance of the device, and the small instances are coupled together in parallel to provide the whole power semiconductor device. The whole device has an ON state where a desired current flows through the device, an OFF state where current flow is substantially blocked in the device, and a breakdown state where an undesired current flows due to an excess off-state voltage being applied between the current conducting electrodes of the device. The voltage at which breakdown is initiated is called the breakdown voltage. Each mesa and its adjacent trenches are configured to provide a desired set of ON-state characteristics and breakdown voltage. There are various tradeoffs in the design of the mesa and trenches between achieving good ON-state characteristics, high breakdown voltage, and improved switching characteristics.
A typical power semiconductor die has an active area where the array of mesas and trenches that implement the device are located, a field termination area around the active area, and an inactive area where interconnects and channel stops may be provided. The field termination area minimizes the electric fields around the active area, and is not intended to conduct current. Ideally, one would like the device's breakdown voltage to be determined by the breakdown processes associated with the active area. However, there are various breakdown processes that can occur in the field termination area and inactive area at significantly lower voltages. These breakdown processes may be referred to as passive breakdown processes.
Much effort has been made in the prior art to design field termination areas that have higher breakdown voltages than the active area. However, such prior art designs often fall short of this goal, often requiring compromises that increase the total die area and cost of the die.
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