E-MOSFET

E-MOSFET

As its name indicates, this MOSFET operates only in the enhancement mode and has no depletion mode. It operates with large positive gate voltage only. It does not conduct when the gate-source voltage V_GS = 0. This is the reason that it is called normally-off MOSFET. In these MOSFET’s drain current I_D flows only when V_GS exceeds V_GST [gate-to-source threshold voltage].

When drain is applied with positive voltage with respect to source and no potential is applied to the gate two N-regions and one P-substrate from two P-N junctions connected back to back with a resistance of the P-substrate. So a very small drain current that is, reverse leakage current flows. If the P-type substrate is now connected to the source terminal, there is zero voltage across the source substrate junction, and the^-drain-substrate junction remains reverse biased.

When the gate is made positive with respect to the source and the substrate, negative (i.e. minority) charge carriers within the substrate are attracted to the positive gate and accumulate close to the-surface of the substrate. As the gate voltage is increased, more and more electrons accumulate under the gate. Since these electrons cannot flow across the insulated layer of silicon dioxide to the gate, so they accumulate at the surface of the substrate just below the gate. These accumulated minority charge carriers N -type channel stretching from drain to source. When this occurs, a channel is induced by forming what is termed an inversion layer (N-type). Now a drain current start flowing. The strength of the drain current depends upon the channel resistance which, in turn, depends upon the number of charge carriers attracted to the positive gate. Thus drain current is controlled by the gate potential.

Since the conductivity of the channel is enhanced by the positive bias on the gate so this device is also called the enhancement MOSFET or E- MOSFET.

The minimum value of gate-to-source voltage V_GS that is required to form the inversion layer (N-type) is termed the gate-to-source threshold voltage V_GST. For V_GS below V_GST, the drain current I_D = 0. But for V_GS exceeding V_GST an N-type inversion layer connects the source to drain and the drain current I_D is large. Depending upon the device being used, V_GST may vary from less than 1 V to more than 5 V.

JFETs and DE-MOSFETs are classified as the depletion-mode devices because their conductivity depends on the action of depletion layers. E-MOSFET is classified as an enhancement-mode device because its conductivity depends on the action of the inversion layer. Depletion-mode devices are normally ON when the gate-source voltage V_GS = 0, whereas the enhancement-mode devices are normally OFF when V_GS = 0.

Schematic Symbols of EMOSFET

Schematic symbols for an N-channel E-MOSFET are shown in figure. For zero value of V_GS, the E-MOSFET is OFF because there is no conducting channel between source and drain. Each of schematic symbols shown in figures has broken channel line to indicate this normally OFF condition. As we know that for V_GS exceeding the threshold voltage V_GST, an N-type inversion layer, connecting the source to drain, is created. In each of the schematic symbols, the arrow points to this inversion layer, which acts like an N-channel when the device is conducting. In each case, the fact that the device has an insulated gate is indicated by the gate not making direct contact with the channel. The schematic symbol shown in figure shows the source and substrate internally connected, while the other symbol shown in figure shows the substrate connection brought out separately from the source.

The schematic symbols for a P-channel E-MOSFET are also shown. In these cases the arrow points outwards.