N channel MOSFET

A MOSFET is a three-terminal device in which the gate terminal is insulated from the channel by a silicon dioxide (SiO2) layer, therefore, it is also known as an insulated gate field effect transistor (IGFET), MOSFET works like a MOS capacitor which is controlled by the input gate to source voltage. That’s why MOSFET can also be used as a voltage-controlled capacitor.
In this article, we will discuss what is N channel MOSFET, its types and classification, testing, Applications, advantages and disadvantages, FAQs, Conclusion, etc.

What is N-Channel MOSFET?

The N-Channel MOSFET Is a type of MOSFET. In the N channel MOSFET, the channel is composed of the majority of the electrons as current carriers. When the MOSFET is activated, the majority of current flows due to electrons moving through the channel. While in the other type of MOSFET (P channel MOSFET), the majority of current carriers are holes. Upon the substrate a silicon dioxide layer is also coated to provide electrical insulation and the metal top of the silicon dioxide layer makes the gate, source, and drain regions. It is also called NMOS. In the Next Part we will be going through Enhancement and Depletion type MOSFET.

MOSFET can be classified into two forms.

• N channel MOSFET.
• P Channel MOSFET.

These two parts depend upon the voltage applied to the gate terminal.

N-channel MOSFET

Types of N-Channel MOSFET

On the basis of working, construction and Characteristics, The N-Channel MOSFET is classified in to two parts, Which are given below.

• Enhancement Type N-Channel MOSFET
• Depletion Type N-Channel MOSFET

Enhancement Type N-Channel MOSFET

In the enhancement type N-Channel MOSFET two n-regions are embedded in the p- type substrate. These two regions are Source and Drain electrodes which are heavily doped the regions between the Source and Drain electrodes is the channel region whose length is L and channel’s width is W. A thin silicon dioxide (SiO2) layer is coated on the substrate to provide electrical insulation, it is also covering the area between source and drain. The metal upon the silicon dioxide layer shapes the gate, source and drain electrode.

Enhancement MOSFET

Working of Enhancement Type N-Channel MOSFET

The circuit diagram of enhancement type n channel MOSFET is shown in the figure , in which a positive voltage is applied around the gate terminal, when the applied voltage around gate to source terminal is zero (VGS = 0), then no current will flow due to absence of channel. The main working principle of the NMOS is to control the current flow and voltage flow between the source and drain terminals. That’s why NMOS works like a switch. When VGS is increases in the positive duration then the number of electrons near the silicon dioxide layer are increases and at a particular voltage a measurable current will be flowing due to the formation of channel .

The voltage at which a measurable current is start flowing, is called THRESHOLD VOLTAGE, the conductivity of the channel is enhanced by increasing the gate to source voltage, so it is called Enhancement type MOSFET. In the enhancement type NMOS there is no any path between the drain and source when no voltage is applied in between the gate terminal and source terminal. When we apply a gate to source voltage (VGS) then it enhances the channel and making it to capable of conducting current.

When we continue increasing the gate to source voltage (VGS), a high electric field will developed which is forcing atoms inside the P-Type substrate to break the covalent bond. Now free electrons will generated to fill the holes near the gate region. This way holes will pushed away from the gate terminal and increasing the N-Type behavior at the gate terminal, after some time N-Channel will be created between two N wells. When gate to source voltage is greater than the threshold voltage ( VGS > VTH ) an N-Channel is induced at near the gate terminal.

Symbol Of Enhancement Type N-Channel MOSFET

The symbol of Enhancement type N-Channel MOSFET is shown below.

Enhancement Type MOSFET

Characteristics Of N-Channel Enhancement Type MOSFET

The transfer characteristics and Output characteristics are shown below.

Transfer Characteristics Of N-channel Enhancement Type MOSFET

• Drain current will be zero for the VGS is greater or equal to VTH for the N-Channel Enhancement Type MOSFET.
• The transfer characteristics of N-Channel Enhancement type MOSFET is drawn between VGS (gate to source voltage) and ID (drain current).
• The transfer characteristics will always be in positive region and zero till (VGS = VTH).
• The relation between ID (drain current) and VGS (gate to source voltage) is [ ]. This expression is non-linear and is valid only for when (VGS > VTH), that’s why the transfer characteristics will also be non-linear.

Transfer Characteristics of N-Channel Enhancement Type MOSFET

Drain (Output) Characteristics Of N-Channel Enhancement Type MOSFET

• The Output characteristics of N-Channel Enhancement Type MOSFET is drawn between ID (drain current) and VDS (drain to source voltage), for various value of VGS (gate to source voltage).
• When drain to source voltage will be Equal to the voltage difference between the gate to source voltage and THRESHOLD voltage ( VDS = VGS – VTH). This voltage is called pinch off voltage and thus current increases slowly with increase the voltage.
• When drain to source voltage will be greater than the voltage difference between the gate to source voltage and THRESHOLD voltage [ VDS > (VGS – VTH) ], then the drain current become constant and saturation occurred (ID = Constant).

Output characteristics of enhancement type NMOS

Depletion Type N-Channel MOSFET

In the Depletion type N-Channel MOSFET two n-regions are embedded in the p- type substrate. these two regions are Source and Drain electrodes, which are heavily doped, the regions between the Source and Drain electrodes is the channel region, whose length is L and width is W is made on the time of manufacturing the Depletion type MOSFET That’s why in Depletion type N-Channel MOSFET Channel Region is predefined. A thin silicon dioxide (SiO2) layer is coated on the substrate to provide electrical insulation and it is also covering the area between source and drain. The metal upon the silicon dioxide layer shapes the gate, source and drain electrode.

Depletion MOSFET

Working Of N-Channel Depletion Type MOSFET

The circuit diagram of N-channel Depletion type MOSFET is shown in the figure, in which the gate and source terminal are connected to the negative pole of the battery and drain is connected with the positive pole of the battery. When free electrons of source terminal are attached towards the positive potential of the drain terminal and drain current will be start flowing. If a negative potential is applied on the gate terminal is increases the gate terminal repel the conduction electrons of the channel and attract holes from P- type substrate and recombination takes place at the junction therefore the number of free electrons at the junction reduces. Therefore current is also reduces.

The depletion type MOSFET is operated by applying a more negative gate to source voltage (VGS) than threshold voltage (VTH), due to which effect of “Shutting off” or “Depletion” occurs and the majority of the current carriers performed beneath channel at the gate. The Depletion region is a layer where the charge flow is decreases. Depletion region acts as a barrier which opposes electron flow from the N-Side to the P-Side of the semiconductor diode. Width of this Depletion region depends upon the number of impurities atoms added in to the semiconductor.

NOTE :- If a positive potential is applied across the gate terminal then the gate terminal attracted an additional electron from the substrate and conductivity of the channel is increases hence, Depletion type MOSFET behaves as a Enhancement type MOSFET.

Symbol Of Depletion Type N-Channel MOSFET

The symbol of Depletion type N-Channel MOSFET is shown below.

Depletion Type MOSFET

Characteristics Of N-Channel Depletion Type MOSFET

The transfer characteristics and Output characteristics of N-Channel Depletion Type MOSFET is shown below.

Transfer Characteristics Of N-Channel Depletion Type MOSFET

The Transfer characteristics of N-Channel Depletion type MOSFET is given below which contains several steps.

• The transfer characteristics of N-Channel Depletion Type MOSFET is drawn between ID (drain current) and VGS (gate to source voltage).
• In the transfer characteristics indicated that current will flow also when VGS (gate to source voltage) is zero, that’s mean that depletion type MOSFET will conduct even when gate terminal is left unbiased. VGS = 0 volts.

Transfer Characteristics of N-Channel Depletion Type MOSFET

Drain Characteristics Of N-Channel Depletion Type MOSFET

• The drain curve shows the relation between the drain current and drain to source voltage (VDS) at constant gate to source voltage (VGS).
• In the curve when VGS is zero and negative, the MOSFET operates in the depletion mode.
• If the VGS is zero and positive the MOSFET operates in the enhancement mode.
• In the output characteristics we can clearly see that when value of VGS is less or equal to zero then NMOS is operating in depletion mode.
• When the value of VGS is greater than zero then NMOS is working in enhancement mode.

Output characteristics of depletion type NMOS

Testing Of N-Channel MOSFET

For the testing N-Channel MOSFET, we will connect Source with positive pin of multimeter and Drain will connected with the negative pin of the multimeter. In this condition the MOSFET is in forward bias state. Now the reading of the multimeter will be between 0.4 volt to 0.9 volt. If this value is not obtained or obtained value is zero or no value then our MOSFET is defective.

Now when we will revert the pins of multimeter the reading of multimeter must be zero. If multimeter is showing any positive or negative value other than zero then our MOSFET is defective.

Difference Between N-Channel AND P-Channel MOSFET

N-CHANNEL MOSFET	P-CHANNEL MOSFET
It use electron flow as the charge carrier.	It use hole flow as the charge carrier.
Less heat produced.	More heat produced.
It has low input noise.	It has high input noise.
Drain current is positive.	Drain current is negative.
It has n-channel between source and drain.	It has p-channel between source and drain.

Applications Of N-Channel MOSFET

There are some applications of N-Channel MOSFET.

• It can be operates in both saturation region and cut-off region.
• An N-channel MOSFET can be work like a switching circuit.
• N-channel MOSFETs are used in high current application.
• It can be used as power converters in modern electric vehicles.
• It can be used in battery powered motors and in motor control also.

Advantages And Disadvantages Of N-Channel Enhancement Type MOSFET

Advantages and Disadvantages of N-Channel Enhancement Type MOSFET are given below.

Advantages Of N-Channel MOSFET

• In the n-channel MOSFET mobility of electrons is greater the mobility of holes in the p-channel MOSFET.
• It has better signal integrity.
• It can handle the higher frequency operation.
• It reduces the cross over distortion.
• The NMOS is only transistor in which no isolation-island are required, that’s why high packing density is suitable for all logic families including VLSI, ULSI etc.
• NMOS has large Fanout.

Disadvantages Of N-Channel MOSFET

• It has a short life time.
• If we will apply overload voltage then it will be unstable.
• It requires very well designed circuit to perform good.
• The speed of operation is slow because MOS devices are also the capacitor and it’s capacitive loading is very high.
• On every gate it has large propagation delay.
• Power dissipation is very high as compare to CMOS.

Conclusion

Now in the final statement we can say that N-channel MOSFET is better than the P-channel MOSFET because n-channel MOSFET is much faster than the p-channel MOSFET. In the n-channel MOSFET mobility of electrons is greater than the mobility of holes in p-channel MOSFET. N-channel MOSFET is used in various field like electric vehicles, motor control applications, current application etc. It is produces less heat and drain current is positive in the N-CHANNEL MOSFET.

FAQs on N channel MOSFET

How the drain current is modeled for the MOSFET ?

By using Shockley equations and we can modeled drain current for the MOSFET. It is calculated by using formula – (LD = VGS – VTH)VDS. Where “K” is transconductance parameters. “VGS” is gate to source voltage. “VDS” is drain to source voltage. “VTH” is threshold voltage.

How MOSFET differ from BJT?

MOSFET has no requirement of input current only voltage is making them highly efficient. They are more sensitive than BJT, BJT are operated by controlling the flow of current between emitter and collector, whether the MOSFETs controls the flow of current between source and drain.

How a MOSFET works and it’s application ?

A MOSFET is a type of Field effect transistor, it is use for amplification of circuits, switching of electronic signals. The MOSFET is operated by varying the width of channel along which charge carriers are flow. MOSFET is used for high current application.