ENHANCEMENT-MODE MOSFETs

An enhancement-mode MOSFET is normally off because it requires a gate bias signal to cause current flow because of the high impedance of its substrate source-to-drain channel.In the N-channel enhancement-mode MOSFET shown in Fig. 1a, the substrate is P-type silicon and both the source and drain regions are heavily doped N-type silicon. The metal gate, the insulation layer, and the channel act like a capacitor, so if a bias is placed on the gate, a charge of opposite polarity will appear in the channel below it. For example, if the drain voltage is positive with respect to the source voltage, and the bias on the gate is zero, no current will flow.

But, if the gate is then made positive, negative charge carriers (electrons) are induced in the channel between the source and drain regions. Further increases in positive bias induce more electrons into the channel, where they accumulate to form an N-type channel between source to the drain. The value of drain current depends on channel resistance, so gate voltage controls drain current. Because channel conductivity is enhanced by a positive gate bias, the transistor is called an enhancement-mode MOSFET.

Figure 1b shows the schematic symbol for an N-type enhancement-mode MOSFET. The vertical line connected to the gate pin represents the gate, and the broken lines connected to the drain and source pins indicate that a channel does not exist until a gate voltage is applied. The arrowhead representing conventional current points from the P-type substrate to the induced N-type channel

A P-channel enhancement-mode MOSFET has the same geometry as the N-channel enhancement-mode MOSFET except that both the material dopants and the applied voltage polarities are reversed. Its schematic symbol is identical except that the direction of the arrowhead is reversed.


Figure 1 Enhancement-mode N-channel MOSFET: (a) section view, and (b) symbol.