Which of the following describes the construction of a MOSFET? A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is constructed so that the gate is separated from the channel with a thin insulating layer. The metal oxide is used in a thin layer to keep the gate from being in direct contact with the channel.
What is the construction of gate terminal in depletion type MOSFET?
Working of an Enhancement type MOSFET – This is a type of MOSFET in which no any channel is doped between the source and drain at the time of construction as you have already seen in the above, In E-MOS, a positive gate to source voltage is required for the channel to induce electrically. When the gate to source voltage is made 0, E-MOS does not conduct. Due to this reason, it is called normally-off MOSFET. When the positive gate voltage exceeds the threshold value then drain current starts to flow through the device. Consider a case when a positive drain to source voltage is applied and the gate terminal is at 0 potential.
- In this case, the P-type substrate and the two N regions behave as two PN junctions connected back to back and P substrate provides the resistance.
- In this condition, both junctions cannot be forward bias simultaneously leading to very small drain current which is a reverse leakage current,
- Let us now move further and consider the case when the gate is made somewhat positive with respect to the source.
The minority charge carriers of p-type substrate i.e., electrons get attracted by the positive potential of the gate. These negative carriers accumulate or gather at the surface of the substrate just below the gate terminal. Any further increase in the V GS will cause more electrons to deposit under the gate.
Since dielectric is used so these electrons cannot be able to flow across the insulating layer of SiO 2, Thus they accumulate at the surface of the substrate itself. Thus, an N-channel is made between source and drain by the accumulation of minority charge carriers, Thus, drain current I D flows through the channel.
The flow of drain current depends on the channel resistance which in turn depends on the charge carriers attracted towards the positive gate terminal. So, by the above discussion, we can conclude that I D is controlled by the gate potential V GS, It is called enhancement MOSFET as the channel conductivity is enhanced by the positive gate potential.
What is the structure of MOSFET?
Construction of a MOSFET – The construction of a MOSFET is a bit similar to the FET. An oxide layer is deposited on the substrate to which the gate terminal is connected. This oxide layer acts as an insulator (sio 2 insulates from the substrate), and hence the MOSFET has another name as IGFET. The voltage at gate controls the operation of the MOSFET. In this case, both positive and negative voltages can be applied on the gate as it is insulated from the channel. With negative gate bias voltage, it acts as depletion MOSFET while with positive gate bias voltage it acts as an Enhancement MOSFET,
How to construct a p-channel MOSFET?
Constructional detail of a DE-MOSFET and E-MOSFET – As we have already discussed earlier that MOSFET is a member of the FET family. It has a gate terminal which is made insulated by an oxide layer so as to prevent direct contact with the substrate. 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. As 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. This lightly doped P-type substrate contains two heavily doped N-type material thus forming source and drain. A thin layer of SiO 2 is deposited over the surface and holes are then cut through SiO 2,
Metals are deposited through holes which resultantly forms drain and source terminal. A metal plate is also deposited in between the source and drain terminal which acts as gate terminal for the device. SiO 2 is a type of insulator referred to as dielectric, which generates an opposing electric field when subjected to an externally applied field.
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, 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.
What do the curves in the MOSFET circuit diagram indicate?
Characteristic Curve of Depletion MOSFET – The drain characteristics of a typical N-channel MOSFET is shown in the diagram below- The bottom curve shows the condition when no gate voltage is applied due to which a negligible value of drain current flows from source to drain. The curve at the upper portion shows the condition when gate voltage V GS is made positive and lower curves indicate the condition for negative gate voltage.