07-05-2012, 05:30 PM
Power Semiconductor Devices
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Introduction
Historically, bipolar semiconductor devices (i.e, diode, transistor, thyristor, thyristor, GTO etc) have been the front runners in the quest for an ideal power electronic switch. Ever since the invention of the transistor, the development of solid-state switches with increased power handling capability has been of interest for expending the application of these devices. The BJT and the GTO thyristor have been developed over the past 30 years to serve the need of the power electronic industry. Their primary advantage over the thyristors have been the superior switching speed and the ability to interrupt the current without reversal of the device voltage. All bipolar devices, however, suffer from a common set of disadvantages, namely, (i) limited switching speed due to considerable redistribution of minority charge carriers associated with every switching operation; (ii) relatively large control power requirement which complicates the control circuit design. Besides, bipolar devices can not be paralleled easily.
The reliance of the power electronics industry upon bipolar devices was challenged by the introduction of a new MOS gate controlled power device technology in the 1980s. The power MOS field effect transistor (MOSFET) evolved from the MOS integrated circuit technology. The new device promised extremely low input power levels and no inherent limitation to the switching speed. Thus, it opened up the possibility of increasing the operating frequency in power electronic systems resulting in reduction in size and weight. The initial claims of infinite current gain for the power MOSFET were, however, diluted by the need to design the gate drive circuit to account for the pulse currents required to charge and discharge the high input capacitance of these devices. At high frequency of operation the required gate drive power becomes substantial. MOSFETs also have comparatively higher on state resistance per unit area of the device cross section which increases with the blocking voltage rating of the device. Consequently, the use of MOSFET has been restricted to low voltage (less than about 500 volts) applications where the ON state resistance reaches acceptable values. Inherently fast switching speed of these devices can be effectively utilized to increase the switching frequency beyond several hundred kHz.
Constructional Features of a Power MOSFET
As mentioned in the introduction section, Power MOSFET is a device that evolved from MOS integrated circuit technology. The first attempts to develop high voltage MOSFETs were by redesigning lateral MOSFET to increase their voltage blocking capacity. The resulting technology was called lateral double deffused MOS (DMOS).
Operating principle of a MOSFET
At first glance it would appear that there is no path for any current to flow between the source and the drain terminals since at least one of the p n junctions (source – body and body-Drain) will be reverse biased for either polarity of the applied voltage between the source and the drain. There is no possibility of current injection from the gate terminal either since the gate oxide is a very good insulator. However, application of a positive voltage at the gate terminal with respect to the source will covert the silicon surface beneath the gate oxide into an n type layer or “channel”, thus connecting the Source to the Drain as explained next.