In power electronics, MOSFETs (metal–oxide–semiconductor field-effect transistor) with an induced channel of n-type conductivity are mainly used. A power MOSFET is a specific type of a MOSFET designed to handle significant power levels. Compared to the other power semiconductor devices, such as an insulated-gate bipolar transistor (IGBT) or a thyristor, its main advantages are high switching speed and good efficiency at low voltages. It shares with the IGBT an isolated gate that makes it easy to drive. They can be subject to low gain, sometimes to a degree that the gate voltage needs to be higher than the voltage under control.
Energy indicators come to the fore among the operational parameters of a semiconductor device. These are power losses and the efficiency of the operating mode depending on them.
In the switching mode, it is necessary to strive to ensure that the resistance of a power MOSFET in the open state is minimal, then the power loss in the transistor will also be minimal.
Thus, the main requirements for the parameters of power MOSFETs are:
- Reduction of channel resistance, which is provided by the creation of a short channel
- Optimal dimensions and doping of the drift high-resistance layer, providing a given blocking voltage with minimal resistance when the current flows in the open state
The necessity to switch large currents leads to the concept of a multicellular structure when a transistor consists of a large number of cells connected in parallel, each of which is a complete MOSFET. Therefore, the optimal design of the cell is also relevant, providing maximum current with minimal geometric dimensions.
The traditional planar MOSFET structure with the horizontal current flow is not optimal for meeting these requirements, since, firstly, it is not adapted to block high voltage, and secondly, it has a suboptimal cell layout when the metallization of the drain and source is located on the same side of the structure, which greatly increases its geometric dimensions.
In order to increase the blocking voltage of the power MOSFET, a modified cell with a drift n⁻-region is used. In such a structure, the drain p-n junction can be high-voltage. However, the horizontal size of the drift region, where it expands when the voltage space charge region of the p-n junction is applied, should have a well-defined value – the greater the voltage, the greater. This further worsens the compactness of the MOSFET structure cell. The use of MOSFET structures with vertical current flow allows optimizing the cell layout.
Thus, the main features of power MOSFETs are the reduction of the channel length and the appearance of a high-resistance drift drain n⁻-region through which the drift of the charge carriers of the drain current occurs. A simple reduction in the channel length would lead to a decrease in the breakdown voltage between the drain and the gate. The introduction of an additional drift n⁻-region makes it possible to obtain high values of the breakdown voltage of the transistor. The presence of a drift region, however, negatively affects the resistance of an open MOSFET.
A modern high-power silicon MOSFET has a crystal size of up to 15 mm, and up to several hundred thousand elementary cells connected in parallel are placed on the crystal. The use of a polysilicon gate makes it possible to form interlayer insulation between the gate and the metallization of the source, which makes it possible to carry out a very dense packaging of cells on a crystal.
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