So, from the above discussion, we can conclude that in a DE-MOS, more negative the gate voltage, the less the drain current that flows through the channel. This increases the channel resistance which resultantly reduces the drain current. In a DE-MOSFET when the gate potential is made negative with respect to the substrate, it causes repulsion of negative charge carriers out of the initially formed channel. When the MOSFET is operated with 0 gate voltage it is said that the device is operating in E-mode. The diagram shown below will help you to understand DE-MOS in a better way:ĭE-MOSFET has the ability to work at both positive and negative gate potential. This means that the channel conducts even when V GS = 0. Due to this, current flows in between the source and drain without any gate bias voltage. In a depletion type or DE-MOS, a channel for conduction is already constructed physically. The construction for gate terminal is the same as in case of N-channel MOSFET. In the same way, to construct a P-channel MOSFET, an N-type substrate is taken and is diffused with two highly doped P-type material thus forming source and drain terminal. The area required by the MOSFET is of the order 0.003µm 2 or less and the layer of SiO 2 provides an extremely high input impedance of the order of 10 10 to 10 15 ohms. SiO 2 is a type of insulator referred to as dielectric, which generates an opposing electric field when subjected to an externally applied field. A metal plate is also deposited in between the source and drain terminal which acts as gate terminal for the device. Metals are deposited through holes which resultantly forms drain and source terminal. This lightly doped P-type substrate contains two heavily doped N-type material thus forming source and drain.Ī thin layer of SiO 2 is deposited over the surface and holes are then cut through SiO 2. The constructional detail of enhancement type MOSFET is shown below:Īs we can see in the diagrams shown above for the construction of an N-channel DE-MOSFET and N – channel E-MOS, a P-type substrate is used. The diagram shown below describes the construction of a depletion type MOSFET: This insulated gate feature of MOSFET is responsible for infinite impedance on the practical basis because no flow of current is noticed in between the gate and the channel. It has a gate terminal which is made insulated by an oxide layer so as to prevent direct contact with the substrate.
Constructional detail of a DE-MOSFET and E-MOSFETĪs we have already discussed earlier that MOSFET is a member of the FET family. In the same way, we can construct P – channel depletion and enhancement MOSFET also. Let’s have a look at the N channel depletion and enhancement type MOSFET:
The voltage applied across the gate is needed to create a channel for its conductance. In an enhancement type MOSFET, there is no any pre-constructed channel existence is noticed. In depletion type MOSFET a channel is already constructed physically and gate-source voltage is needed to switch the device “OFF”. Wider channel width provides the better conductivity of the device. The applied voltage changes the channel width. It finds application widely in switching and amplification of electronic signals because of its ability to change conductivity with the applied voltage.ĭue to the small size of MOSFET, it is most commonly used transistor in digital circuits. On the contrary, MOSFET has a smaller value of capacitance and its input impedance is much more than that of FET due to small leakage current. As FET offers large value of drain resistance with moderate input impedance and delayed operation. MOSFET is an advanced FET invented to overcome the disadvantages of FET.
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