MOSFET is a three-terminal device. The terminals are Drain, Source and Gate.

Overdrive voltage, usually abbreviated as VOV, is typically referred to in the context of MOSFET transistors. How to use an N channel MOSFET (a type of transistor) to turn anything on and off! Also, remember to put a 100k resistor between gate and ground if you want.

MOSFET is a short form of Metal Oxide Semiconductor Field Effect Transistor. This device is capable of switching at a high frequency up to 100 kHz. Which is much higher than thepower BJT. It has a good current carrying capacity and higher off-state blocking voltage capacity. A power MOSFET is a voltage-controlled device.

Types of MOSFET

1. Depletion type MOSFET
2. Enhancement type MOSFET

Construction of MOSFET

Power MOSFET has a vertically oriented four-layer structure. The p-type middle layer term as the body. The n^– layer is the drift layer. This layer is lightly dropped as compared to the drain and source layer. The breakdown voltage of power MOSFET determines from the width of the drift layer. First and last both layers are an n⁺ layer. First layer is the source layer and the last layer is the drain layer.

The n+ p n- n+ structure term as enhancement mode n channel MOSFET. A p-channel MOSFET structure has exactly opposite doping profile.

The GATE terminal is not directly connected with p-type, there is an oxide layer between the metal and semiconductor. This oxide layer act as a dielectric layer between the metal and the semiconductor. It forms a MOS (metal oxide semiconductor) capacitance at the input of the MOSFET. This capacitance is large (>1000 pF). The oxide layer gives good insulating property by providing the SiO₂ layer which isolates the gate terminal from the body layer.

Operating principle of MOSFET

Power MOSFET is a minority carrier device. So, conduction takes place only by the electrons. Therefore, the conduction cannot take place through the MOSFET from the drain to source due to the presence of P-layer in between. But this is possible by Inversion layer creation.

The operation of MOSFET divides into two parts

1. Formation of the depletion layer
2. Creation of Inversion Layer

1) Formation of the depletion layer

By connecting a positive voltage to the drain with respect to the source and the gate is positive with respect to the body, the MOSFET works as forward biased. The p-layer has a large number of holes and few electrons. The holes are the majority charge carrier and electrons are minority charge carrier. Due to the positive voltage applied between the gate and the body, these electrons are attracted towards the gate and gather below the oxide layer and produce the depletion layer.

2) Creation of Inversion Layer

The number of electrons below the oxide layer will greater than the number of holes if the positive gate voltage increases further. Hence, n-type of sublayer form below the oxide layer. This process is known as the creation of the inversion layer. The process of generation of an inversion layer due to the extremely applied gate voltage is known as the field effect. This inversion layer is also known as the induced layer.

The resistance of induced layer depends on the magnitude of the gate to body voltage. Higher the gate voltage less the resistance. The resistance decreases with an increase in the gate to body voltage. But after a certain level, the resistance is not decreased even increasing the gate to body voltage. If the maximum specified value of the gate voltage exceeds then the oxide layer will breakdown.

I-V characteristic

It is a graph of drain current I_d versus drain to source voltage V_DS for different values of the gate to source voltage V_GS. It has three regions; saturation, cut-off, and ohmic region. In the application where the MOSFET used as a switch, the device works in the cut-off region and ohmic region when turned OFF and ON respectively. The operation in the saturation region avoided reducing the power dissipation in the on-state.

When the gate-source voltage is less than the threshold voltage, the MOSFET is in the cut-off state. To avoid breakdown the drain to source breakdown voltage should be greater than the applied voltage. The avalanche breakdown takes place.

The power MOSFET goes into the ohmic region when a larger positive gate to source voltage apply and the drain to source voltage is small. In this region, the power dissipation is low.

In the saturation region, the drain current is almost independent of the drain to source voltage. It is only dependent on the gate to source voltage. The gate voltage is greater than the threshold voltage. The drain current increase with the increase in the gate to source voltage.

Merits of a power MOSFET

1. The GATE driving circuit of MOSFET is simple.
2. It can operate at the high switching frequency.
3. It has better thermal stability because the temperature coefficient of MOSFET is positive.
4. easy to turn ON and OFF.
5. On-state resistance is low.
6. The second breakdown does not take place.

Demerits of a power MOSFET

1. The on-state voltage across the MOSFET is very high. So, on-state power dissipation is high.
2. It has the asymmetric blocking capacity. They can block a high forward voltage but they cannot block high reverse voltage. Therefore, we need to connect a diode to protect the MOSFET.

Application of Power MOSFET

1. Uninterrupted Power Supplies (UPS)
2. Switch Mode Power Supplies (SMPS)
3. High-frequency inverter
4. Motor control application
5. Display driver
6. In power amplifier
7. Industrial applications
8. Relay driver

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Power MOSFET

The Power MOSFET is a type of MOSFET. The operating principle of power MOSFET is similar to the general MOSFET. The power MOSFETS are very special to handle the high level of powers. It shows the high switching speed and by comparing with the normal MOSFET, the power MOSFET will work better. The power MOSFETs is widely used in the n-channel enhancement mode, p-channel enhancement mode, and in the nature of n-channel depletion mode. Here we have explained about the N-channel power MOSFET. The design of power MOSFET was made by using the CMOS technology and also used for development of manufacturing the integrated circuits in the 1970s.

What is a Power MOSFET?

A power MOSFET is a special type of metal oxide semiconductor field effect transistor. It is specially designed to handle high-level powers. The power MOSFET’s are constructed in a V configuration. Therefore, it is also called as V-MOSFET, VFET. The symbols of N- channel & P- channel power MOSFET are shown in the below figure.

Basic Statures of Power MOSFET

There is three basic status in the power MOSFET which is following.

• On sate resistance
• Breakdown voltage
• Body diode

On State Resistance

If the power MOSFET is in ON sate, then it produces the resistive behavior in-between the drain & source terminals. We can see in the following figure, that the resistance is the sum of many elementary contributions. The RS resistance is the source resistance. It will show all resistance between the source terminals of the package to the channel of the MOSFET.

The Rch resistance is the channel resistance and this resistance is inversely proportional to the channel width & for a given die size, to the channel density. This resistance is very important contributors to the RDSon of the low voltage MOSFET. The intensive work has done to reduce their cell size with respect to increase the channel density.

The access resistance is represented by the Ra. The access resistance shows the resistance of the epitaxial zone directly to the gate electrode. The current direction is changed from the channel to the vertical.

RJFET is the detrimental effect of the cell size reduction. The P implantation is observed from the gate of a parasitic JFET transistor and it has reduced the width of the current flow.

Rn represents the epitaxial layer and it is used for sustaining the blocking voltage. This resistance is directly related to the voltage rating of the device. The high voltage MOSFET requires a thick low dependent layer which is highly resistive and a low voltage transistor requires a thin layer with the higher doping layer which is very less resistive. This is the main factor for the resistance of high voltage MOSFET.

The RD resistance is the equivalent of resistance of the RS for the drain. The RD resistance, represent the transistor substrate and the package connections.

Break Down Voltage

The power MOSFET is equivalent to the PIN diode, if it is in the OFF state and it is initiated by the P+ diffusion, the N- epitaxial layer and the N+ substrate. This structure is reverse biased when it is highly nonsymmetrical structure and the space charge region extends principally to the lightly doped side, which is the N- layers.

Even though, when the MOSFET is in the ON state, there is a no function of the N- layers. Moreover, it is lightly doped rejoin, intrinsic resistivity is non-negligible and it is added to the MOSFET ON- state drain to source resistance.

There are two important parameters to run both the breakdown voltage and the RDSon of the transistor, which is the doping level and the thickness of the N- epitaxial layer. If the layer is thicker, it has low doping level and the breakdown voltage is high. Similarly, thicker the layer, it has the high doping level and the radon is low. Hence we can observe that there is a trade-off in the design of the MOSFET, between the voltage rating and the ON state resistance.

Body Diode

The body diode can be seen in the following figure that the source metallization is connected to both the N+ and P implantations. Even though the basic principle of the MOSFET requires only that the source should be connected to the N+ zone. Thus, this would result in a floating P zone between the N-doped source and drain. It is equivalent to an NPN transistor with a nonconnected base. Under some conditions like high drain current, in the order of the same volts of an on-state drain to source voltage, this parasitic transistor of NPN should be triggered and make the MOSFET uncontrollable.

The connections of the P implantation to the source metallization short the base terminal of the transistor parasitic to its emitter and it prevents the latching. Hence this solution creates a diode between the cathode & anode of the MOSFET and the current blocks in one direction.

For inductive loads, the body diodes utilize the freewheeling diodes in the configuration of H Bridge & half bridge. Generally, these diodes will have a high forward voltage drop, the current is high. They are sufficient in many applications like reducing part count.

Working of Power MOSFET and Characteristics

The construction of the power MOSFET is in V-configurations, as we can see in the following figure. Thus the device is also called as the V-MOSFET or V-FET. The V- the shape of power MOSFET is cut to penetrate from the device surface is almost to the N+ substrate to the N+, P, and N – layers. The N+ layer is the heavily doped layer with a low resistive material and the N- layer is a lightly doped layer with the high resistance region.

What Is V-mosfet

Both the horizontal and the V cut surface are covered by the silicon dioxide dielectric layer and the insulated gate metal film is deposited on the SiO2 in the V shape. The source terminal contacts with the both N+ and P- layers through the SiO2 layer. The drain terminal of this device is N+.

The V-MOSFET is an E-mode FET and there is no exists of the channel in between the drain & source till the gate is positive with respect to the source. If we consider the gate is positive with respect to the source, then there is a formation of the N-type channel which is close to the gate and it is in the case of the E-MOSFET. In the case of E-MOSFET, the N-type channel provides the vertical path for the charge carriers. To flow between the drain and source terminals. If the VGS is zero or negative, then there is no channel of presence and the drain current is zero.

The following figures show the drain & transfer characteristics for the enhancement mode of N-channel power MOSFET is similar to the E-MOSFET. If there is an increase in the gate voltage then the channel resistance is reduced, therefore the drain current ID is increased. Hence the drain current ID is controlled by the gate voltage control. So that for a given level of VGS, ID is remaining constant through a wide range of VDS levels.

The channel length of the power MOSFET is in the diffusion process, but in the MOSFET the channel length is in the dimensions of the photographic masks employed in the diffusion process. By controlling the doping density and diffusion time, the channel length will become shorter. The shorter channels will give, the more current densities which will contribute again to larger power dissipation. It also allows a larger transconductance gm to be attained in the V-FET.

In the geometry of power MOSFET, there is an important factor which is the presence of lightly doped, N- epitaxial layer which is close to the N+ substrate. If the VGS is at zero or negative, then the drain is positive with respect to the source and there is a reverse biased between the P- layer & N- layer. At the junction the depletion region penetrates into the N- layer, therefore it punch-through the drain to the source are avoided. Hence, relatively high VDS are applied without any danger of device breakdown.

In the power MOSFET, there is available of P-channel. The characteristics are similar to the N-channel MOSFET. The direction of the current and voltage polarities are in reverse direction.

MOSFET Power Amplifier

There are different types of MOSFET power amplifiers like 300W MOSFET power amplifier, 240W MOSFET power amplifier, 160W MOSFET power amplifier and 100W MOSFET power MOSFET amplifier. But here we are explaining about the 100W MOSFET power amplifier with its circuit.

100W MOSFET Power Amplifier Circuit Operation

The circuit operation of 100w MOSFET amplifier consists of PNP transistor from the differential amplifier circuit. The AC input signal is sent to one of the transistors and the other transistor gets the output signal from the feedback. The AC signal is a coupled to the base terminal of the transistor Q1 with the help of the coupling capacitor and the feedback signal. The feedback signal is fed into the base terminal of the second transistor Q2 with the help of resistor R5 & R6. The potentiometer is used to set the output of the amplifier.

The first stage differential output amplifier is fed to the second stage differential amplifier input. In the case of the first differential amplifier, when the input voltage is more than the feedback voltage than the input voltage of the two transistors Q3 and Q4 of second differential amplifier differs from each other. The current mirror circuit of the transistors is Q5 & Q6. This circuit ensures the flowing of output current in the push-pull amplifier circuit is constant.

V Mosfet Is Designed To

Thus, this circuit can be achieved because, when the collector current of transistor Q3 is increased, the collector current of Q4 decreases is to maintain the flow of constant current through the universal point of the emitter terminal of the transistors Q3 &Q4. There is an equal output current in between the current mirror circuit & collector current of the Q3 transistor. The potentiometer R12 protects the applications of DC biasing to each MOSFET because the two MOSFETs are in the balancing to each other.

If the positive voltage is applied to the gate terminal of the Q7 transistor then it will conduct. Correspondingly transistor Q8 will conduct for the negative threshold voltage. To prevent the MOSFET output from the oscillating the gate resistors are used.

Applications of Power MOSFET

V Mosfet Wikipedia

The power MOSFET’s are used in the power supplies

• DC to DC converters
• Low voltage motor controllers
• These are widely used in the low voltage switches which are less than the 200V

Mosfet Vgs

This article will give the information on the working principle of power MOSFET circuit and its applications. We hope by reading this article you have gained some basic knowledge and understanding about the power MOSFET. Here is the question for you, what are the functions of the power MOSFET?