At a Glance of Power MOSFET
In the previous blog, we discussed what is power electronics and the classification of power devices, so from today’s blog we will learn about power devices starting with Power MOSFET.what are the Specification of MOSFET, Structure of MOSFET, Depletion Enhancement MOSFET, Enhancement MOSFET, and the modes of operation such as Depletion & Enhancement mode also the Output characteristics.
Introduction to Power MOSFET
MOSFET is a metal oxide semiconductor field-effect transistor. which is a power device
You might have thought that why do we use this Power word? What is its meaning? Here the power indicates its handling capacity regarding voltage current and power. It is similar to any semiconductor device, but we all know that there is a difference between Christopher Nolan’s Batman and Snyder cut batman. Similarly, it applies to this MOSFET (it is more powerful).
Power MOSFET is a voltage control device and requires only a small input current. The control signal is given to a metal gate electrode.
The Gate terminal is separated from the semiconductor surface by an intervening surface.
MOSFET is evolved from integrated circuit technology in 1970 in response to power transistors control using low gate drive levels.
MOSFET is a unipolar device because current conduction occurs through the transport of majority carriers in the drift region without majority carrier injection required for bipolar transistor operation. And switching speed of MOSFET is high.
Specifications of MOSFET
Operating frequency – 100 kHz
Switching timing – 50 – 100 nanosecond
Operating frequency – 500 kHz
Structure of Power MOSFET
The basic structure of MOSFET contains three terminals drain-source and gate, and according to the mode of operations, the MOSFET is divided.
There are two basic types of MOSFET
- Depletion Enhancement MOSFET
- Enhancement MOSFET
In depletion enhancement MOSFET, there are two modes of operation that is depletion mode and enhancement mode.
In the case of enhancement MOSFET, there is only one mode of operation: enhancement mode.
Depletion Enhancement MOSFET
The drain and source are diffused into the substrate material and then connected by a narrow channel adjacent to the insulated gate.
Again there are two types P- channel and N- channel MOSFET. But for a better explanation, let’s take the example of N – channel MOSFET.
The DE MOSFET has two modes of operation: The depletion mode or the enhancement mode.
However, positive or negative gate voltage can be applied because the gate is insulated from the channel.
When the MOSFET is in depletion mode when a negative gate-to-source voltage is applied, it is said to be operated in enhancement mode when a positive gate-to-source voltage is applied.
Depletion mode:
We all know the construction of capacitor imagines that; the gate is the one plate of parallel plate capacitor the channel. And another plate as a channel. The dielectric material between two plates is a silicon dioxide insulating layer. Whenever we apply a negative voltage to the gate terminal, the conduction electrons are repelled from the channel due to negative charges on the gate. Therefore the positive ions are left behind the place.
However, the N- channel is getting depleted of some of its electrons due to which channel conductivity decreases. The greater the negative voltage on the gate depletion region will also increase.
When the good negative gate to source voltage (VGS) is applied, the drain current will become zero. The channel will be totally depleted. And the DE MOSFET conducts when the value of VGS is above zero.
Enhancement mode:
As the name enhancement suggests an increase in something hence our enhancement mode increases the conductivity of MOSFET.
A MOSFET is said to be in enhancement mode when the positive gate voltage is applied.
The positive voltage applied across the gate causes more conduction electrons racted to the channel; however, this causes increasing (enhancing) channel conductivity.
Enhancement MOSFET
The construction of Enhancement MOSFET is quite different from DE MOSFET because it has no physical channel. And the substrate of enhancement extends entirely to the SiO2 layer.
For the N- channel device, when the positive gate voltage above the threshold value is applied, it induces a channel while creating a thin layer of negative charge in the substrate region adjacent to the Sio2 layer.
The conductivity of the channel can be increased by increasing the gate to source voltage; however, this causes the pulling of more electrons into the channel. If the gate voltage is below the threshold value, then there will be no channel.
The current flowing through MOSFET can be enhanced by increasing the magnitude of gate voltage to form a deep conduction channel.
N- channel enhancement MOSFET is a voltage-controlled current device. However, it is possible to construct a P – channel MOSFET for which the conduction entirely depends on the movement of holes
It is essential to understand that gate contact is electrically insulated and draws a negligible D.C leakage current.
Output characteristics of Power MOSFET
Image source: Google | Image by Circuits Today
The output characteristics are drawn between the drain current, which is the drain function to source voltage VDS and gate to source voltage VGS as a variable parameter.
MOSFET works in three regions cut off, ohmic and active region. It is used as a switch; it should be operated in cut-off and ohmic regions only.
The MOSFET is in the cut-off region when the gate-source voltage VGS is less than the threshold voltage.
The drain to source voltage should be greater than the applied voltage for which the device must withstand the applied voltage.
The device works in the ohmic region when the more enormous gate to source voltage is applied, whereas the drain to source voltage VDS should be small. By minimizing the on-state voltage, the power dissipation can be kept low.
In the active region, the drain current becomes independent of the gate to source voltage. The power dissipation of MOSFET becomes high.
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