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Power Metal-Oxide-Semiconductor Field-Effect Transistor
The operational principle of the metal-oxide-semiconductor field-effect transistor (MOSFET, or sometimes called MOS in short) devices originates from the field-effect concept by Lilienfeld in the 1930s. When the industry eventually learned how to grow the good-quality gate dielectric with acceptable charges, the n-MOS technology and the later CMOS technology took off in a fast path. Today, MOS device technology, namely CMOS or BiCMOS process, becomes prevalent and dominant in integrated circuit (IC) development. At the same time, power MOSFET devices began to emerge and were introduced in 1970s. So far, many structures for power MOSFET devices have been proposed, such as the double-diffusion MOSFET (or DMOS), V-groove MOSFET (or VMOS), and trenched-gate UMOS. Due to many processing difficulties related to the gate dielectrics and the enhanced electric field at the tip of the V-groove (Holmes and Salama, 1973), the VMOS structure is seldom used in production. Instead, the DMOS structure is a preferred choice. For the UMOS structure, it is seen as the preferred choice to provide a low on-state resistance. The introduction of power MOSFET brought about the prediction of the extinction of the bipolar power transistor. However, this prediction did not happen immediately as cost and reliability plagued the power MOSFET. The bipolar power transistor continued to be used in many high-voltage and high-current applications. Only in the mid-1980s, the bipolar power transistor finally met the most critical rival with the introduction of the insulated-gate bipolar transistor (IGBT), which will be described in Chapter 5.
The high gate impedance is a primary feature of the power MOSFET that greatly simplifies its gate drive circuitry. The negative temperature coefficient of the drain current, i.e. the drain current reduces when temperature goes up, in power MOSFET also provides an additional advantage over their bipolar counterparts. In a sense, the bipolar transistor is very susceptible to thermal run-away but not the case for power MOSFET. However, the power MOSFET suffers a higher on-state conduction loss as compared to the bipolar power transistor of similar die sizes because of the absence of conductivity modulation in power MOSFET. In spite of this, the trend of higher switching frequency to increase power density in power electronic system continues, therefore, the choice of using power MOSFET over conventional power bipolar switching transistor is evident.
In this chapter, the operations of the power MOSFET are described. This begins with the basic MOS physics necessary for the understanding of the device. This is then followed by the derivation of current-voltage characteristics. Various power MOSFET structures and their switching characteristics are mentioned along with the presentations of experimental switching waveforms under different operating and load conditions. Gate driver circuits for power MOSFET in various configurations are also discussed. Finally, more advanced gate structures, such as the dummy-gate and folded-gate MOSFET devices and the radio frequency (RF) MOSFET device on the partial silicon on insulator (SOI) platform are introduced.
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