Thus Enhancement Type MOSFET can be operated in either of these regions i.e. The almost vertical components of the curves correspond to the ohmic region, and the horizontal components correspond to the constant current region. The characteristic curves have almost vertical and almost horizontal parts. When V GS is greater than V GST, the device turns on and the drain current I D is controlled by the gate voltage. When V GS is lesser than V GST, I D is approximately zero. Characteristics:ĭrain characteristics of an N-channel E-MOSFET are shown in Fig. 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. E-MOSFET is classified as an enhancement-mode device because its conductivity depends on the action of the inversion layer. JFETs and DE-MOSFETs are classified as the depletion-mode devices because their conductivity depends on the action of depletion layers. Depending upon the device being used, V GST may vary from less than 1 V to more than 5 V. 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. For V GS below V GST, the drain current I D = 0. 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. Since the conductivity of the channel is enhanced by the +ve bias on the gate, this device is called the Enhancement Type MOSFET or E-MOSFET. Thus drain current is controlled by the gate potential. The strength of the drain current depends upon the channel resistance, which in turn depends on the number of charge carriers attracted to the positive gate. When this occurs, a channel is induced by forming what is termed an inversion layer (N-type). These accumulated minority charge carriers make an N-type channel stretching from drain to source. Since these electrons cannot flow across the insulated layer of silicon dioxide to the gate, they accumulate at the surface of the substrate just below the gate. As the gate voltage is increased, more and more electrons accumulate under the gate. minority) charge carriers within the substrate are attracted to the +ve gate and accumulate close to the surface of the substrate. When the gate is made positive with respect to the source and the substrate, negative (i.e. 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. Both the junctions cannot be forward biased at the same time, so only an extremely small drain current i.e. When drain is applied with +ve voltage with respect to source and no potential is applied to the gate, the two N-regions and one P-type substrate form two P-N junctions connected back to back with a resistance of the P-substrate. In these MOSFETs drain current I D flows only when V GS exceeds V GST. This is the reason that it is called normally-off MOSFET. It does not conduct when gate-source voltage V GS = 0. It operates with large positive gate voltage only. Operation of Enhancement Type MOSFET:Īs its name indicates, this MOSFET operates only in the enhancement mode and has no depletion mode. Channels are electrically induced in these MOSFETs, when a positive gate-source voltage V GS is applied to it. The main difference between the construction of DE-MOSFET and that of E-MOSFET is that in E-MOSFET substrate extends all the way to the silicon dioxide (SiO 2) and no channels are doped between the source and the drain. Figure 13.80 shows the construction of an N-channel E-MOSFET.
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