Date of this Version
IEEE Industry Applications Magazine, July/August , www.IEEE.Org/Ias
Historically, Power Semiconductor devices have been divided into three broad categories: diodes, transistors, and thyristors. Although modern devices can be classified in this way, there is an increasing overlap in device design and function. Also semiconductors, such as silicon carbide (SiC), gallium nitride (GaN), and other materials, as well as novel device designs have increased the suitability and broadened the applications of semiconductor switches in megawatt (MW) power conversion circuits and systems.
Transistors include the traditional power bipolar (nonsilicon materials being considered for the future), power metal-oxide-semiconductor field-effect transistors (MOSFETs), and hybrid devices that have some aspect of a control FET element integrated with a bipolar structure, such as an insulated-gate bipolar transistor (IGBT). Because of power limitations, MOSFETs are not used in MW drives and converter circuits. Thyristors are three-terminal devices that have a four-layer structure (typically, three p-n junctions) for the main power handling section of the device. All transistors and thyristor types are controllable and can be switched from a forward-blocking state (little or no current flows) into a forward-conduction state (large forward current flows). Transistors and most modern thyristors [except silicon-controlled thyristors (SCRs)] are also controllable in switching from a forward conduction back into a forward-blocking state. Some thyristors are able to block forward and reverse voltages (symmetric blocking) when compared with others that only block voltage in forward direction (asymmetric blocking). Transistor-type devices are generally asymmetric components, though some work on symmetric IGBTs has also been done .
Basic Design of Power Devices
Modern bipolar power devices are designed around a common theme: the p-i-n structure (approximated as p+-n-- n+), as shown in Figure 1, during the off-state or blocking condition of full applied voltage. One of the desirable attributes of a high-power switch is the ability to withstand (block) large off-state voltages. The relationship between the maximum sustainable blocking voltage, VBlk, and impurity (dopant) concentration in the center region (base), Nbase, is given in
VBlk α N-.75 base