A power MOSFET is a voltage-controlled device and requires only a small input cur rent. The switching speed is very high and the switching times are of the order of nano seconds. Power MOSFETs find increasing applications in low-power high-frequency converters. MOSFETs do not have the problems of second breakdown phenomenal as do BJTs. However, MOSFETs have the problems of electrostatic discharge and require special care in handling. In addition, it is relatively difficult to protect them under short-circuited fault conditions.
The two types of MOSFETs are (1) depletion MOSFETs and (2) enhancement MOSFETS. An n-channel depletion-type MOSFET is formed on a p-type silicon substrate, with two heavily doped n silicon sections for low resistance connections. The gate is isolated from the channel by a thin oxide layer. The three terminals are called gate, drain, and source. The substrate is normally connected to the source. The gate-to-source voltage Vos could be either positive or negative. If Vos is negative, some of the electrons in the n-channel area are repelled and a depletion region is created below the oxide layer, resulting in a narrower effective channel and a high resistance from the drain to source Rps. If VGs is made negative enough, the chan nel becomes completely depleted, offering a high value of Rps, and no current flows from the drain to source. Ips = 0. The value of VGs when this happens is called pinch off voltage Vp. On the other hand, if VGs is made positive, the channel becomes wider, and Ips increases due to reduction in Rps. With a p-channel depletion-type MOSFET, the polarities of VDs, IDs, and VGs are reversed.
An n-channel enhancement-type MOSFET has no physical channel. If Vas is positive, an induced voltage attracts the electrons from the p-substrate and accumulates them at the surface beneath the oxide layer. If Vis greater than or equal to a value known as threshold voltage V₁. a sufficient number of electrons are accumulated to form a virtual n-channel, as shown by shaded lines t, and the current flows from the drain to source. The polarities of Vps. Its and Vos are reversed for a p-channel enhancement-type MOSFET. Power MOSFETs of various sizes.
Because a depletion MOSFET remains on at zero gate voltage, whereas an enhancement-type MOSFET remains off at zero gate voltage, the enhancement-type MOSFETS are generally used as switching devices in power electronics. In order to reduce the on-state resistance by having a larger current conducting area, the V-type structure is commonly used for power MOSFETS. The cross section of a power MOSFET known as a vertical (V) MOSFET.
When the gate has a sufficiently positive voltage with respect to the source, the effect of its electric field pulls electrons from the n+ layer into the p layer. This opens
a channel closest to the gate, which in turn allows the current to flow from the drain to the source. There is a silicon oxide (SiO₂) dielectric layer between the gate metal and the n+ and p junction. MOSFET is heavily doped on the drain side to create an n+ buffer below the n-drift layer. This buffer prevents the depletion layer from reaching the metal, evens out the voltage stress across then layer, and also reduces the forward voltage drop during conduction. The buffer layer also makes it an asymmetric device with rather low reverse voltage capability.
MOSFETS require low gate energy, and have a very fast switching speed and low switching losses. The input resistance is very high. 10 to 10 . MOSFETS, however, suffer from the disadvantage of high forward on-state resistance, and hence high on-state losses, which makes them less attractive as power devices, but they are excellent as gate amplifying devices for thyristors.
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