Basic Structure

A power MOSFET has three terminals called drain, source and gate in place of the corresponding three terminals collector, emitter and base for BJT. The circuit symbol of power MOSFET is as shown fig (a). Here arrow indicates the direction of electron flow.

A BJT is a current controlled device whereas a power MOSFET is a voltage-controlled device. As its operation depends upon the flow of majority carriers only, MOSFET is a unipolar device.

The control signal, or base current in BJT is much larger than the control signal (or gate current) required in a MOSFET. This is because of the fact that gate circuit impedance in MOSFET is extremely high, of the order of $10^9$ ohm. This large impedance permits the MOSFET gate to be driven directly from microelectronic circuits. BJT suffers from second breakdown voltage whereas MOSFET is free from this problem. Power MOSFETs are now finding increasing applications in low-power high frequency converters.

Power MOSFETs are of two types ; n-channel enhancement MOSFET and p-channel enhancement MOSFET. Out of these two types, n-channel enhancement MOSFET is more common because of higher mobility of electrons.

A simplified structure of n-channel MOSFET of low power rating is shown in Fig. (b). On-p-substrate (or body), two heavily doped $n^+$ regions are diffused as shown. An insulating layer of silicon dioxide (Si02)- is grown on the surface. Now this insulating layer is etched in order to embed metallic source and drain terminals.

When gate circuit is open, no current flows from drain to source and load because of one reverse biased $n^+-p$ junction. When gate is made positive with respect to source, an electric field is established as shown in Fig (b). Eventually, induced negative charges in the p-substrate below $Si0_2$ layer are formed. These negative charges, called electrons, form n-channel and current can flow from drain to source as shown by the arrow. If $V_{GS}$ is made more positive, n-channel becomes more deep and therefore more current flows from D to S. This shows that drain current $I_D$ is enhanced by the gradual increase of gate voltage, hence the name enhancement MOSFET.

Construction and Working

The constructional details of high power MOSFET are shown in below figure.In this figure is shown a planar diffused metal-oxide-semiconductor (DMOS) structure for n -channel which is quite common for power MOSFETs. On $n+$ substrate, high resistivity $n^-$ layer is epitaxially grown. The thickness of $n^-$ layer determines the voltage blocking capability of the device. On the other side of $n^+$ substrate, a metal layer is deposited to form the drain terminal. Now $p^-$ regions are diffused in the epitaxially grown $n^-$ layer. Further, $n^+$ regions are diffused in $p^-$ regions as shown. As before, $Si0_2$ layer is added, which is then etched so as to fit metallic source and gate terminals. A power MOSFET actually consists of a parallel connection of thousands of basic MOSFET cells on the same single chip of silicon.

When gate circuit voltage is zero, and VDD is present , $n^- - p^-$ junctions are reverse biased and no current flows from drain to source. When gate terminal is made positive with respect to source, an electric field is established and electrons form n-channel in the $p^-$ regions as shown. So a current from drain to source is established as indicated by arrows. With gate voltage increased current $I_D$ also increases as expected. Length of n-channel can be controlled and therefore on-resistance can be made low if short length is used for the channel.

Power MOSFET conduction is due to majority carriers, therefore, time delays caused by removal or recombination of minority carriers are eliminated. Thus, power MOSFET can work at switching frequencies in the megahertz range.