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Power Semiconductor DevicePower semiconductor devices are essentially electronic devices made from semiconductor materials (e.g. silicon, gallium arsenide), used as switches or rectifiers in power electronic circuits (power converters for example). They are also called power devices or power ICs. Few most common power devices are power diode, thyristor, power MOSFET, IGBT (insulated gate bipolar transistor) etc. A power diode or MOSFET, for example, operates on similar principle as that of its small-signal counterpart, but it is able to carry larger amount of current than the small-signal device and also able to support larger reverse bias voltage in the off-state. Though the operating principle remains fundamentally same, structural changes are often made in power devices to accommodate for the necessary features such as high current density or high reverse blocking voltage. For exapmle, the common small-signal MOSFET employs a lateral structure where drain and source of the device reside laterally at the same level but in case of a power MOSFET the drain is pushed to the bottom of the device and a vertical structure results. Naturally, different and extra fabrication steps are needed to achieve this. Most of the power MOSFETs, which are in use today, are fabricated using a double-diffusion (one has to diffuse dopants twice in the semiconductr bulk) process and naturally called DMOSFET. Parameters of power semiconductor devices 1. Breakdown voltage (Often the trade-off is between breakdown voltage rating and on-resistance because pushing up the breakdown voltage by incorporating thicker and lower doped drift region leads to higher on-resistance) 2. On-resistance (Higher the current rating lower the on-resistance due to higher number of parallel cells but this increases overall capacitance and slows down the speed) 3. Rise and fall times for switching 4. Safe-operating area (from thermal dissipation consideration) Common Power Semiconductor Devices Perhaps the true revolution in power electronics started with the discovery and wide-spread use of thyristor. They are able to withstand very high reverse voltage and capable of carrying high current too. One significant problem with thyristors is that once they are 'latched-on' in the conducting state they cannot be turned off by external control. Essentially their turn-off is passive. This is certainly a major disadvantage in switching circuits. After that power bipolar junction transistors (BJT) came into the market. BJT has better switching performance and controllability than thyristors but somewhat less voltage and current ratings. Power MOSFET is currently most popular power device (particularly in low and medium power level applications e.g. switch-mode power supplies, motor drives, UPS). It has the best switching characteristics (due to its unipolar conduction), high input impedance (resulting in very low drive current and simpler gate drive circuits), and ease of paralleling (due to its positive thermal coefficient of resistance). MOSFET suffers from low transconductance and higher on-state voltage drop (compared to BJT). A new device, incorporating features of both MOSFET and BJT, was proposed in 1980s. It is called insulated-gate bipolar transistor or IGBT. It has better current density than MOSFET and better switching characteristics than BJT but slower than MOSFET. IGBT is the primary choice today in high-power (> 10 kW), low to medium frequnecy (up to -30 kHz) applications. IGBT suffers from typical 'current-tail' problem during turn-off. This is due to injection of minority carriers into its thick base region during conducting state by a mechanism called 'base conductivity modulation'. One very interesting device has recently been conceptualized and proposed. It is called "Optically-triggered power transistor" or OTPT for short. It features 2 electrical terminals - source and drain (similar to MOSFET but a third optical window instead of a regular gate terminal. By shining light of suitable wavelength and intensity it may be possible to control the switching of the device. One great advantage of the device is that it is inherently suitable to be made of III-V compound semiconductor materials e.g. Gallium Arsenide instead of conventional silicon. That lends significant electrical performance enhancements because of better carrier dynamics and wider bandgap related properties of these III-V semiconductors compared to silicon. In short, OTPT is expected to have potential of switching at MHz frequency range without sacrificing switching or conduction efficiency or breakdown voltage rating. --128.248.172.38 03:20, 13 Dec 2004 (UTC) Tirthajoyti Sarkar, Univ of Illinois
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