Definition: MOSFET is an acronym for Metal Oxide Semi-Conductor Field Effect Transistor. It is a device in which the variation in the voltagedetermines theconductivity of the device. It is a semiconductor device that belongs to FET family.

• Enhancement Mosfet Construction And Working
• Enhancement Mosfet Vs Depletion Mosfet
• P Channel Enhancement Mode Mosfet

MOSFET is also known as IGFET i.e., insulated gate field effect transistor. But usually, the word MOSFET is used because most devices are made using Si for semiconductors and gate electrode of metal oxide. It is a three terminal device which has a source, a drain and a gate terminal. These are voltage controlled devices, in which the current flowing between source and drain is proportional to the provided input voltage.

MOSFET is an advanced FET invented to overcome the disadvantages of FET. As FET offers large value of drain resistance with moderate input impedance and delayed operation. On the contrary, MOSFET has a smaller value of capacitance and its input impedance is much more than that of FET due to small leakage current.

It finds application widely in switching and amplification of electronic signals because of its ability to change conductivity with the applied voltage.

Enhancement Mode MOSFET Transistor. In Enhancement-mode MOSFET or eMOSFET, the conducting channels are doped to a level so that it becomes non-conductive. This results in a transistor that no conduction at zero voltage. Additionally, you can say that an eMOSFET is closed or “OFF” by default because of no existing channel. The Depletion-mode MOSFET, which is less common than the enhancement mode types is normally switched “ON” (conducting) without the application of a gate bias voltage. That is the channel conducts when V GS = 0 making it a “normally-closed” device. N-Channel Enhancement MOSFET. Products (14,608) Datasheets (11,713) Images (1,009) Newest Products -Results: 14,608. Smart Filtering As you select one or more parametric filters below, Smart Filtering will instantly disable any unselected values that would cause no results to be found.

Due to the small size of MOSFET, it is most commonly used transistor in digital circuits. The applied voltage changes the channel width. Wider channel width provides the better conductivity of the device.

MOSFETs are of two types:

1. Depletion type MOSFET
2. Enhancement only MOSFET

In depletion type MOSFET a channel is already constructed physically and gate-source voltage is needed to switch the device “OFF”.

In an enhancement type MOSFET, there is no any pre-constructed channel existence is noticed. The voltage applied across the gate is needed to create a channel for its conductance.

Let’s have a look at the N channel depletion and enhancement type MOSFET:

In the same way, we can construct P – channel depletion and enhancement MOSFET also.

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.

The diagram shown below describes the construction of a depletion type MOSFET:

The constructional detail of enhancement type MOSFET is shown below:

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 SiO₂ is deposited over the surface and holes are then cut through SiO₂. 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.

SiO₂ 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 0.003µm² or less and the layer of SiO₂ provides an extremely high input impedance of the order of 10¹⁰ to 10¹⁵ 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.

The construction for gate terminal is the same as in case of N-channel MOSFET.

Working of a Depletion-type MOSFET

In a depletion type or DE-MOS, a channel for conduction is already constructed physically. Due to this, current flows in between the source and drain without any gate bias voltage.

This means that the channel conducts even when V_GS = 0.

The diagram shown below will help you to understand DE-MOS in a better way:

DE-MOSFET has the ability to work at both positive and negative gate potential. When the MOSFET is operated with 0 gate voltage it is said that the device is operating in E-mode.

In a DE-MOSFET when the gate potential is made negative with respect to the substrate, it causes repulsion of negative charge carriers out of the initially formed channel. This increases the channel resistance which resultantly reduces the drain current.

So, from the above discussion, we can conclude that in a DE-MOS, more negative the gate voltage, the less the drain current that flows through the channel.

In the case when the gate terminal is made positive with respect to the substrate, more number of electrons gets attracted towards the channel. Thus, causing more current to flow through the channel.

Enhancement Mosfet Construction And Working

A pinch-off condition also arises in DE-MOS when a much negative gate voltage is applied.

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.

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 figure.

In E-MOS, a positive gate to source voltage is required for the channel to induce electrically. It requires large positive gate voltage for its operation.

E-MOS has its wide application in digital electronics field and computers.

The below-shown diagram indicates the working of an E-MOSFET:

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 SiO₂. 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_Dis controlled by the gate potential V_GS. It is called enhancement MOSFET as the channel conductivity is enhanced by the positive gate potential.

Characteristics Curve of E – MOS

The characteristic curve shows various values of V_GS for which variation in I_D is shown-

As we are already aware of the fact that a gate potential above the threshold value causes drain current I_D to flow. So when V_GS is less than V_GST then approximately 0 drain current flows and when V_GS is greater than V_GST then device turns ON.

Advantages of MOSFET :

1. The operational speed of MOSFET is higher than that of JFET.
2. Input impedance is much higher as compared to JFET.
3. It can be easily used in case of high current applications.
4. These devices provide an easy manufacturing process.

Disadvantages of MOSFET :

1. It is a delicate device and is easily destroyable.
2. Excessive application of gate to source voltage V_GS may destroy the thin SiO₂ layer.

E-MOS is better suited in case of power devices because a positive potential at the gate is required to start the conduction of the device. The applied gate voltage increases the conductivity of the device.

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FETs have a few disadvantages like high drain resistance, moderate input impedance and slower operation. To overcome these disadvantages, the MOSFET which is an advanced FET is invented.

MOSFET stands for Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. This is also called as IGFET meaning Insulated Gate Field Effect Transistor. The FET is operated in both depletion and enhancement modes of operation. The following figure shows how a practical MOSFET looks like.

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 (sio₂ insulates from the substrate), and hence the MOSFET has another name as IGFET. In the construction of MOSFET, a lightly doped substrate, is diffused with a heavily doped region. Depending upon the substrate used, they are called as P-type and N-type MOSFETs.

The following figure shows the construction of a MOSFET.

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.

Classification of MOSFETs

Depending upon the type of materials used in the construction, and the type of operation, the MOSFETs are classified as in the following figure.

After the classification, let us go through the symbols of MOSFET.

The N-channel MOSFETs are simply called as NMOS. The symbols for N-channel MOSFET are as given below.

The P-channel MOSFETs are simply called as PMOS. The symbols for P-channel MOSFET are as given below.

Now, let us go through the constructional details of an N-channel MOSFET. Usually an NChannel MOSFET is considered for explanation as this one is mostly used. Also, there is no need to mention that the study of one type explains the other too.

Construction of N- Channel MOSFET

Let us consider an N-channel MOSFET to understand its working. A lightly doped P-type substrate is taken into which two heavily doped N-type regions are diffused, which act as source and drain. Between these two N+ regions, there occurs diffusion to form an Nchannel, connecting drain and source.

A thin layer of Silicon dioxide (SiO₂) is grown over the entire surface and holes are made to draw ohmic contacts for drain and source terminals. A conducting layer of aluminum is laid over the entire channel, upon this SiO₂ layer from source to drain which constitutes the gate. The SiO₂ substrate is connected to the common or ground terminals.

Because of its construction, the MOSFET has a very less chip area than BJT, which is 5% of the occupancy when compared to bipolar junction transistor. This device can be operated in modes. They are depletion and enhancement modes. Let us try to get into the details.

Working of N - Channel (depletion mode) MOSFET

For now, we have an idea that there is no PN junction present between gate and channel in this, unlike a FET. We can also observe that, the diffused channel N (between two N+ regions), the insulating dielectric SiO₂ and the aluminum metal layer of the gate together form a parallel plate capacitor.

If the NMOS has to be worked in depletion mode, the gate terminal should be at negative potential while drain is at positive potential, as shown in the following figure.

When no voltage is applied between gate and source, some current flows due to the voltage between drain and source. Let some negative voltage is applied at V_GG. Then the minority carriers i.e. holes, get attracted and settle near SiO₂ layer. But the majority carriers, i.e., electrons get repelled.

With some amount of negative potential at V_GG a certain amount of drain current I_D flows through source to drain. When this negative potential is further increased, the electrons get depleted and the current I_D decreases. Hence the more negative the applied V_GG, the lesser the value of drain current I_D will be.

The channel nearer to drain gets more depleted than at source (like in FET) and the current flow decreases due to this effect. Hence it is called as depletion mode MOSFET.

Enhancement Mosfet Vs Depletion Mosfet

Working of N-Channel MOSFET (Enhancement Mode)

The same MOSFET can be worked in enhancement mode, if we can change the polarities of the voltage V_GG. So, let us consider the MOSFET with gate source voltage V_GG being positive as shown in the following figure.

When no voltage is applied between gate and source, some current flows due to the voltage between drain and source. Let some positive voltage is applied at V_GG. Then the minority carriers i.e. holes, get repelled and the majority carriers i.e. electrons gets attracted towards the SiO₂ layer.

With some amount of positive potential at V_GG a certain amount of drain current I_D flows through source to drain. When this positive potential is further increased, the current I_D increases due to the flow of electrons from source and these are pushed further due to the voltage applied at V_GG. Hence the more positive the applied V_GG, the more the value of drain current I_D will be. The current flow gets enhanced due to the increase in electron flow better than in depletion mode. Hence this mode is termed as Enhanced Mode MOSFET.

P - Channel MOSFET

The construction and working of a PMOS is same as NMOS. A lightly doped n-substrate is taken into which two heavily doped P+ regions are diffused. These two P+ regions act as source and drain. A thin layer of SiO₂ is grown over the surface. Holes are cut through this layer to make contacts with P+ regions, as shown in the following figure.

Working of PMOS

When the gate terminal is given a negative potential at V_GG than the drain source voltage V_DD, then due to the P+ regions present, the hole current is increased through the diffused P channel and the PMOS works in Enhancement Mode.

When the gate terminal is given a positive potential at V_GG than the drain source voltage V_DD, then due to the repulsion, the depletion occurs due to which the flow of current reduces. Thus PMOS works in Depletion Mode. Though the construction differs, the working is similar in both the type of MOSFETs. Hence with the change in voltage polarity both of the types can be used in both the modes.

This can be better understood by having an idea on the drain characteristics curve.

Drain Characteristics

The drain characteristics of a MOSFET are drawn between the drain current I_D and the drain source voltage V_DS. The characteristic curve is as shown below for different values of inputs.

Actually when V_DS is increased, the drain current I_D should increase, but due to the applied V_GS, the drain current is controlled at certain level. Hence the gate current controls the output drain current.

Transfer Characteristics

P Channel Enhancement Mode Mosfet

Transfer characteristics define the change in the value of V_DS with the change in I_D and V_GS in both depletion and enhancement modes. The below transfer characteristic curve is drawn for drain current versus gate to source voltage.

Comparison between BJT, FET and MOSFET

Now that we have discussed all the above three, let us try to compare some of their properties.

TERMS	BJT	FET	MOSFET
Device type	Current controlled	Voltage controlled	Voltage Controlled
Current flow	Bipolar	Unipolar	Unipolar
Terminals	Not interchangeable	Interchangeable	Interchangeable
Operational modes	No modes	Depletion mode only	Both Enhancement and Depletion modes
Input impedance	Low	High	Very high
Output resistance	Moderate	Moderate	Low
Operational speed	Low	Moderate	High
Noise	High	Low	Low
Thermal stability	Low	Better	High

So far, we have discussed various electronic components and their types along with their construction and working. All of these components have various uses in the electronics field. To have a practical knowledge on how these components are used in practical circuits, please refer to the ELECTRONIC CIRCUITS tutorial.