N Channel MOSFET : Circuit, Working, Differences & Its Applications

MOSFET is a kind of transistor and it is also called IGFET (Insulated Gate Field Effect Transistor) or MIFET (Metal Insulator Field Effect Transistor). In a MOSFET, the channel & gate are separated through a thin SiO2 layer and they form a capacitance that changes with gate voltage. So, MOSFET works like a MOS Capacitor that is controlled through the input gate to source voltage. Thus, MOSFET can also be used as a voltage-controlled capacitor. The structure of the MOSFET is similar to the MOS capacitor because the silicon base in this capacitor is p-type.

These are classified into four types p channel enhancement, n channel enhancement, p channel depletion, and n channel depletion. This article discusses one of the types of MOSFET like N channel MOSFET – working with applications.

What is N Channel MOSFET?

A type of MOSFET in which the MOSFET channel is composed of a majority of charge carriers as current carriers like electrons is known as N channel MOSFET. Once this MOSFET is ON, then the majority of charge carriers will move throughout the channel. This MOSFET is a contrast to P-Channel MOSFET.

This MOSFET includes N- the channel region which is located in the middle of the source & drain terminals. It is a three-terminal device where the terminals as G (gate), D(drain), and S (source). In this Transistor, the source & drain is heavily doped n+ region & the body or substrate is of P-type.

Working

This MOSFET includes an N-channel region which is located in the middle of the source & drain terminals. It is a three-terminal device where the terminals are G (gate), D(drain), and S (source). In this FET, the source & drain is heavily doped n+ region & the body or substrate is of P-type.

Here, the channel is created on the arrival of electrons. The +ve voltage also attracts electrons from both the n+ source & drain regions into the channel. Once a voltage is applied in between the drain & sources then current freely flows in between the source & drain and the voltage at the gate simply controls the charge carriers electrons within the channel. Similarly, if we apply –ve voltage at the gate terminal then a hole channel is formed below the oxide layer.

N Channel MOSFET Symbol

The N channel MOSFET symbol is shown below. This MOSFET includes three terminals like source, drain and gate. For the n-channel mosfet, the arrow symbol direction is inward. So, the arrow symbol specifies the channel type like P-channel or N-channel.

N Channel MOSFET Circuit

The circuit diagram for controlling a brushless dc fan using N channel mosfet and Arduino Uno rev3 is shown below. This circuit can be built with an Arduino Uno rev3 board, n channel mosfet, a brushless dc fan, and connecting wires.

The MOSFET used in this circuit is 2N7000 N-channel MOSFET and it is enhancement-type thus we should set the output pin of Arduino to high for providing power to the fan.

The connections of this circuit follow as;

• Connect the source pin of the MOSFET to GND
• The gate pin of the MOSFET is connected to pin 2 of Arduino.
• The drain pin of MOSFET to the black color wire of the fan.
• The red color wire of the brushless dc fan is connected to the breadboard’s positive rail.
• An extra connection needs to be given from the Arduino 5V pin to the breadboard’s positive rail.

Generally, a MOSFET is used for switching & amplifying signals. In this example, this mosfet is used as a switch which includes three terminals like gate, source & drain. The n channel MOSFET is one type of voltage-controlled device and these MOSFETs are available in two types enhancement mosfet and depletion mosfet.

Generally, an enhancement MOSFET is off once the Vgs (gate-source voltage) is 0V, thus a voltage should be provided to the gate terminal so that current flows throughout the drain-source channel. Whereas, depletion MOSFET is generally turned on once the Vgs (gate-source voltage) is 0V so that current flows throughout the drain to the source channel until a +ve voltage is given at the gate terminal.

Code

void setup() {
// put your setup code here, to run once:
pinMode(2, OUTPUT);

}

void loop() {
// put your main code here, to run repeatedly:
digitalWrite(2, HIGH);
delay(5000);
digitalWrite(2, LOW);
delay(5000);
}

Thus, when the 5v supply is given to the gate terminal of mosfet, the brushless dc fan will be turned ON. Similarly, when the 0v is given to the gate terminal of mosfet then the fan will be turned OFF.

Types of N Channel MOSFET

The N channel MOSFET is a voltage-controlled device that is classified into two types enhancement type and depletion type.

N Channel Enhancement MOSFET

An enhancement type N channel MOSFET is generally off once the gate to source voltage is zero volts, thus a voltage should be provided to the gate terminal so that current supplies throughout the drain-source channel.

The working of n channel enhancement MOSFET is the same as enhancement p channel MOSFET except for the construction and operation. In this type of MOSFET, a p-type substrate that is lightly doped can form the device body. The source & drain regions are doped heavily with n-type impurities.

Here the source & body are commonly connected to the ground terminal. Once we apply a positive voltage to the gate terminal then the minority charge carriers of p-type substrate will attract toward the gate terminal because of the positivity of the gate & equivalent capacitive effect.

The majority charge carriers like electrons and minority charge carriers of p-type substrate will be attracted toward the gate terminal so that it forms a negative uncovered ions layer under the dielectric layer by recombining electrons with holes.

If we increase the positive gate voltage continually, the recombination process will get saturated after the threshold voltage level then charge carriers like electrons will start to build up at the place to form a free electrons conductive channel. These free electrons will also come from the heavily doped source and drain the n-type region.

If we apply +ve voltage at the drain terminal then the flow of current will be there throughout the channel. So the channel resistance will depend on the free charge carriers like electrons within the channel & again these electrons will depend on the device’s gate potential within the channel. When the free electrons concentration forms the channel & the flow of current throughout the channel will get enhanced because of the increase in gate voltage.

N Channel Depletion MOSFET

Generally, this MOSFET is activated whenever the voltage at the gate to the source is 0V, therefore current supplies from the drain to the source channel until a positive voltage is applied at the gate (G) terminal. The N channel depletion MOSFET working is different as compared to n channel enhancement MOSFET. In this MOSFET, the substrate used is a p-type semiconductor.

In this MOSFET, both the source & drain regions are heavily doped n-type semiconductors. The gap between both the source & drain regions is diffused through n-type impurities.

Once we apply a potential difference between source & drain terminals, the current flows throughout the n region of the substrate. When we apply a -ve voltage at the gate terminal then the charge carriers like electrons will get repealed & shifted down in the n-region just under the silicon dioxide dielectric layer.

Consequently, there will be positive uncovered ions layers under the SiO2 dielectric layer. So in this way, a depletion of charge carriers will occur within the channel. Thus, the overall channel’s conductivity will get decreased.

In this condition, when the same voltage is applied at the drain terminal, then the current at the drain will get decreased. Here we have observed that the drain current can be controlled by changing the depletion of charge carriers within the channel, so it is known as depletion MOSFET.

Here, the gate is in a -ve potential the drain is in a +ve potential & the source is at ‘0’ potential. As a result, the voltage difference is more between drain to the gate than source to the gate, therefore the depletion layer width is more toward the drain than the source.

Difference between N Channel MOSFET and P Channel MOSFET

The difference between n channel and p channel mosfet includes the following.

How to Test an N Channel MOSFET

The steps involved in testing N channel MOSFET are discussed below.

• To test an n channel MOSFET, an analog multimeter is used. For that, we need to place the knob in the 10K range.
• For testing this MOSFET, first place the black probe on the drain pin of the MOSFET and the red probe on the gate pin to discharge internal capacitance within the MOSFET.
• After that, move the red color probe to the source pin while the black probe is still on the drain pin
• Use the right finger to touch both the gate & drain pins so that we can observe that the pointer of the analog multimeter will turn aside to the center range of the scale of the meter.
• Take away the red probe of the multimeter & also the right finger from the source pin of MOSFET then again place the finger on the red probe & source pin, the pointer will still stay at the center of the multimeter scale.
• For discharging it, we have to take away the red probe & just one-time touch on the gate pin. Finally, this will discharge again the internal capacitance.
• Now, a red probe has to use again to touch the source pin, then the multimeter’s pointer will not deflect at all as previously you have discharged it by simply touching the gate pin.

Characteristics

N channel MOSFET has two characteristics like drain characteristics and transfer characteristics.

Drain Characteristics

The drain characteristics of the N-channel mosfet include the following.

• The drain characteristics of the n channel mosfet are plotted in between the output current & the VDS which is known as Drain to source voltage VDS.
• As we can see in the diagram, for different Vgs values, we plot the current values. So we can see different plots of drain current in the diagram like lowest Vgs value, maximum Vgs values, etc.
• In the above characteristics, the current will stay constant after some drain voltage. Therefore, minimum voltage for the drain to source is required to work MOSFET.
• So, when we increase ‘Vgs’ then the channel width will be increased & which results in more ID (drain current).

Transfer Characteristics

The transfer characteristics of N-channel mosfet include the following.

• The transfer characteristics are also known as the transconductance curve which is plotted in between the input voltage (Vgs) and output current (ID).
• At first, whenever there is no gate to source voltage (Vgs) then very less current will flow like in micro amps.
• Once the gate to source voltage is positive, the drain current gradually enhances.
• Afterward, there is a quick increase within drain current equivalent to increasing in vgs.
• The drain current can be achieved through Id= K (Vgsq- Vtn)^2.

Applications

The applications of n channel mosfet include the following.

• These MOSFETs are frequently used in low voltage device applications like a full bridge, and B6-bridge arrangement using the motor & a DC source.
• These MOSFETs are helpful in switching the negative supply for the motor in the reverse direction.
• An n-channel MOSFET operates in saturation & cut-off regions. then it acts like a switching circuit.
• These MOSFETS are used to switch LAMP or the LED to ON/OFF.
• These are preferred in high current applications.

Thus, this is all about an overview of n channel mosfet – working with applications. Here is a question for you, what is p channel mosfet?