POWER TRANSISTORS

POWER TRANSISTORS - POWER TRANSISTORS Power transistors are...

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Unformatted text preview: POWER TRANSISTORS Power transistors are devices that have controlled turn-on and turn-off characteristics. These devices are used a switching devices and are operated in the saturation region resulting in low on-state voltage drop. They are turned on when a current signal is given to base or control terminal. The transistor remains on so long as the control signal is present. The switching speed of modern transistors is much higher than that of thyristors and are used extensively in dc-dc and dc-ac converters. However their voltage and current ratings are lower than those of thyristors and are therefore used in low to medium power applications. Power transistors are classified as follows Bipolar junction transistors(BJTs) Metal-oxide semiconductor filed-effect transistors(MOSFETs) Static Induction transistors(SITs) Insulated-gate bipolar transistors(IGBTs) BIPOLAR JUNCTION TRANSISTORS The need for a large blocking voltage in the off state and a high current carrying capability in the on state means that a power BJT must have substantially different structure than its small signal equivalent. The modified structure leads to significant differences in the I-V characteristics and switching behavior between power transistors and its logic level counterpart. POWER TRANSISTOR STRUCTURE If we recall the structure of conventional transistor we see a thin p-layer is sandwiched between two n-layers or vice versa to form a three terminal device with the terminals named as Emitter, Base and Collector. The structure of a power transistor is as shown below Collector pnp BJT Emitter Base Collector npn BJT Emitter Base Emitter Base n + 10 19 cm- 3 p 10 16 cm- 3 n 10 14 cm- 3 n + 10 19 cm- 3 Collector 250 m 50-200 m 10 m 5-20 m (Collector drift region) Base Thickness Fig. 1: Structure of Power Transistor The difference in the two structures is obvious. A power transistor is a vertically oriented four layer structure of alternating p-type and n- type. The vertical structure is preferred because it maximizes the cross sectional area and through which the current in the device is flowing. This also minimizes on-state resistance and thus power dissipation in the transistor. The doping of emitter layer and collector layer is quite large typically 10 19 cm-3 . A special layer called the collector drift region (n- ) has a light doping level of 10 14 ....
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This note was uploaded on 11/28/2010 for the course IT CIT340 taught by Professor Na during the Spring '06 term at Excelsior.

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POWER TRANSISTORS - POWER TRANSISTORS Power transistors are...

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