$$\mathrm$$ Schematic Symbol of E–MOSFET N–Channel E–MOSFET The minimum value of VGS of proper polarity that turns on the E–MOSFET is known as Threshold Voltage (V GS (Th)). The E–MOSFET has no physical channel from source to drain since the substrate extends completely to the SiO2 layer.īy the application of gate–source voltage (VGS) of proper magnitude and polarity the device can be made operating. The E–MOSFET can be operated in the enhancement mode only. Hence in this part the D–MOSFET behaves as a constant current source. It can be seen in the curve, the most of the part of the curve is horizontal, showing constant drain current. This is because the negative gate voltage reduces the charge carriers in the n–channel and drain current ID decreases. This increases the drain current and puts the D–MOSFET in the enhancement mode.įor the negative value of VGs, the drain current decreases and the D–MOSFET operates in the depletion mode. Since for n–channel D–MOSFET, the positive gate voltage induces more free electrons in the channel. The drain characteristics of D–MOSFET is the curve between drain current (I D) and Drain–Source Voltage (V DS) for different positive and negative values of gate–source voltage (V GS).įor the positive values of VGS the drain current increases and the D–MOSFET operates in the enhancement mode. Since the operation with positive gate depends upon the enhancing conductivity of the channel, thus the positive gate operation is known as enhancement mode operation. Thus by changing the positive voltage on the gate terminal, the conductivity of channel can be altered. The higher the positive voltage on the gate, grater the current from source to drain. Hence a positive gate voltage increases or enhances the conductivity of the channel. These free electrons are added to those already in the channel, thus the total number of free electrons in the n–channel is increased. Since the gate is positive, it induces negative charges (free electrons) in the channel by the capacitor action. Since the operation with negative gate depends upon depleting or emptying the channel of free electrons, hence the negative gate operation is known as depletion mode operation. Hence, by changing the negative voltage on the gate, resistance of the n-channel can be varied and hence the source to drain current. The higher the negative voltage at the gate terminal, causes the smaller is the current from source to drain. This effect is same as the resistance of channel is increased. Thus lesser number of free electrons are available for the current conduction through the channel. So that the n–channel is depleted of some of its free electrons. These electrons repel the free electrons in the n- channel. Since the gate is at negative potential, hence there are electrons on the gate. When negative voltage is applied to the gate terminal the operation is called depletion mode operation and when positive voltage is applied to gate then operation is called as enhancement mode operation. Since the gate isolated from the channel by the metal–oxide, hence either positive voltage or negative voltage can be applied to the gate terminal. When voltage at the gate terminal is changed, the electric field of the capacitor changes that in turn changes the resistance of the conduction channel. One plate of this capacitor is the gate terminal and the other plate is the channel with the metal oxide layer as the dielectric. The gate of D–MOSFET forms a small capacitor. Only a small passes remains for the flow of holes from source to drain. In this MOSFET, the n–type substrate constricts the channel between the source and drain so that In N–channel D–MOSFET the p–type substrate makes narrower the channel between source and drain so that only small passes remain for the flow of electrons when the drain is made positive with respect to the source. Because of this, it is also known as depletion enhancement MOSFET. The D-MOSFET can be operated in both depletion mode and enhancement mode. Why the Name MOSFET? The gate terminal is insulated from the channel with the help of metal-oxide (SiO2) and hence the name MOSFET Types of MOSFET In other words, An FET that can be operated in the enhancement mode is known as MOSFET. Since its operation depends upon the flow of majority carriers only, hence MOSFET is a unipolar device. The MOSFET is a voltage controlled device. In general, the body terminal is connected with the source thus forming a three terminal device just like an FET. It is a semiconductor device which is used for switching and amplification applications in electronic circuits. A Metal Oxide Semiconductor Field Effect Transistor (MOSFET) has four terminals − Source (S), Gate (G), Drain (D), Body (B).
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