The Enhancement Mosfet Is

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.

In this tutorial, we will have a brief introduction to MOSFET i.e., the Metal Oxide Semiconductor Field Effect Transistor. We will learn about different types of MOSFET (Enhancement and Depletion), its internal structure, an example circuit using MOSFET as a Switch and a few common applications.

Enhancement MOSFET, or eMOSFET, can be classed as normally-off (non-conducting) devices, that is they only conduct when a suitable gate-to-source positive voltage is applied, unlike Depletion type mosfets which are normally-on devices conducting when the gate voltage is zero. However, due to the construction and physics of an enhancement type. 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.

Introduction

Transistors, the invention that changed the World. They are semiconductor devices that act as either an electrically controlled switch or a signal amplifier. Transistors come a variety of shapes, sizes and designs but essentially, all transistors fall under two major families. They are:

  • Bipolar Junction Transistors or BJT
  • Field Effect Transistors or FET

To learn more about a basics of transistor and its history, read the Introduction to Transistors tutorial.

There are two main differences between BJT and FET. The first difference is that in BJT, both the majority and minority charge carriers are responsible for current conduction whereas in FETs, only the majority charge carriers are involved.

The other and very important difference is that a BJT is essentially a current controlled device meaning the current at the base of the transistor determines the amount of current flowing between collector and emitter. In case of a FET, the voltage at the Gate (a terminal in FET equivalent to Base in BJT) determines the current flow between the other two terminals.

FETs are again divided into two types:

Enhancement mosfet vs depletion mosfet
  • Junction Field Effect Transistor or JFET
  • Metal Oxide Semiconductor Field Effect Transistor or MOSFET

Let us focus on MOSFET in this tutorial.

Metal Oxide Semiconductor FET

The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is one type of FET transistor. In these transistors, the gate terminal is electrically insulated from the current carrying channel so that it is also called as Insulated Gate FET (IG-FET). Due to the insulation between gate and source terminals, the input resistance of MOSFET may be very high such (usually in the order of 1014 ohms.

Like JFET, the MOSFET also acts as a voltage controlled resistor when no current flows into the gate terminal. The small voltage at the gate terminal controls the current flow through the channel between the source and drain terminals. In present days, the MOSFET transistors are mostly used in the electronic circuit applications instead of the JFET.

MOSFETs also have three terminals, namely Drain (D), Source (S) and Gate (G) and also one more (optional) terminal called substrate or Body (B). MOSFETs are also available in both types, N-channel (NMOS) and P-channel (PMOS). MOSFETs are basically classified in to two forms. They are:

  • Depletion Type
  • Enhancement Type
Channel Construction of MOSFET

Depletion Type

The depletion type MOSFET transistor is equivalent to a “normally closed” switch. The depletion type of transistors requires gate – source voltage (VGS) to switch OFF the device.

What is enhancement mode in mosfet

The symbols for depletion mode of MOSFETs in both N-channel and P-channel types are shown above. In the above symbols, we can observe that the fourth terminal (substrate) is connected to the ground, but in discrete MOSFETs it is connected to source terminal. The continuous thick line connected between the drain and source terminal represents the depletion type. The arrow symbol indicates the type of channel, such as N-channel or P-channel.

In this type of MOSFETs a thin layer of silicon is deposited below the gate terminal. The depletion mode MOSFET transistors are generally ON at zero gate-source voltage (VGS). The conductivity of the channel in depletion MOSFETs is less compared to the enhancement type of MOSFETs.

Enhancement Type

The Enhancement mode MOSFET is equivalent to “Normally Open” switch and these types of transistors require a gate-source voltage to switch ON the device. The symbols of both N-channel and P-channel enhancement mode MOSFETs are shown below.

Here, we can observe that a broken line is connected between the source and drain, which represents the enhancement mode type. In enhancement mode MOSFETs, the conductivity increases by increasing the oxide layer, which adds the carriers to the channel.

Generally, this oxide layer is called as ‘Inversion layer’. The channel is formed between the drain and source in the opposite type to the substrate, such as N-channel is made with a P-type substrate and P-channel is made with an N-type substrate. The conductivity of the channel due to electrons or holes depends on N-type or P-type channel respectively.

Structure of MOSFET

The basic structure of the MOSFET is shown in the above figure. The construction of the MOSFET is very different when compared to the construction of the JFET. In both enhancement and depletion modes of MOSFETs, an electric field is produced by gate voltage, which changes the flow charge carriers, such as electrons for N-channel and holes for P-channel.

Here, we can observe that the gate terminal is situated on top of thin metal oxide insulated layer and two N-type regions are used below the drain and source terminals.

In the above MOSFET structure, the channel between drain and source is an N-type, which is formed opposite to the P-type substrate. It is easy to bias the MOSFET gate terminal for the polarities of either positive (+ve) or negative (-ve).

If there is no bias at the gate terminal, then the MOSFET is generally in non-conducting state so that these MOSFETs are used to make switches and logic gates. Both the depletion and enhancement modes of MOSFETs are available in N-channel and P-channel types.

Depletion Mode

The depletion mode MOSFETs are generally known as ‘Switched ON’ devices, because these transistors are generally closed when there is no bias voltage at the gate terminal. If the gate voltage increases in positive, then the channel width increases in depletion mode.

As a result the drain current ID through the channel increases. If the applied gate voltage more negative, then the channel width is very less and MOSFET may enter into the cutoff region. The depletion mode MOSFET is a rarely used type of transistor in the electronic circuits.

The following graph shows the Characteristic Curve of Depletion Mode MOSFET.

The V-I characteristics of the depletion mode MOSFET transistor are given above. This characteristic mainly gives the relationship between drain- source voltage (VDS) and drain current (ID). The small voltage at the gate controls the current flow through the channel.

The channel between drain and source acts as a good conductor with zero bias voltage at gate terminal. The channel width and drain current increases if the gate voltage is positive and these two (channel width and drain current) decreases if the gate voltage is negative.

Enhancement Mode

The Enhancement mode MOSFET is commonly used type of transistor. This type of MOSFET is equivalent to normally-open switch because it does not conduct when the gate voltage is zero. If the positive voltage (+VGS) is applied to the N-channel gate terminal, then the channel conducts and the drain current flows through the channel.

If this bias voltage increases to more positive then channel width and drain current through the channel increases to some more. But if the bias voltage is zero or negative (-VGS) then the transistor may switch OFF and the channel is in non-conductive state. So now we can say that the gate voltage of enhancement mode MOSFET enhances the channel.

Enhancement mode MOSFET transistors are mostly used as switches in electronic circuits because of their low ON resistance and high OFF resistance and also because of their high gate resistance. These transistors are used to make logic gates and in power switching circuits, such as CMOS gates, which have both NMOS and PMOS Transistors.

The V-I characteristics of enhancement mode MOSFET are shown above which gives the relationship between the drain current (ID) and the drain-source voltage (VDS). From the above figure we observed the behavior of an enhancement MOSFET in different regions, such as ohmic, saturation and cut-off regions.

MOSFET transistors are made with different semiconductor materials. These MOSFETs have the ability to operate in both conductive and non-conductive modes depending on the bias voltage at the input. This ability of MOSFET makes it to use in switching and amplification.

N-Channel MOSFET Amplifier

When compared to BJTs, MOSFETs have very low transconductance, which means the voltage gain will not be large. Hence, MOSFETs (for that matter, all FETs) are generally not used in amplifier circuits.

But, none the less, let us see a single-stage ‘class A’ amplifier circuit using N-Channel Enhancement MOSFET. The N-channel enhancement mode MOSFET with common source configuration is the mainly used type of amplifier circuit than others. The depletion mode MOSFET amplifiers are very similar to the JFET amplifiers.

The input resistance of the MOSFET is controlled by the gate bias resistance which is generated by the input resistors. The output signal of this amplifier circuit is inverted because when the gate voltage (VG) is high the transistor is switched ON and when the voltage (VG) is low then the transistor is switched OFF.

The general MOSFET amplifier with common source configuration is shown above. This is an amplifier of class A mode. Here the voltage divider network is formed by the input resistors R1 and R2 and the input resistance for the AC signal is given as Rin = RG = 1MΩ.

The equations to calculate the gate voltage and drain current for the above amplifier circuit are given below.

VG = (R2 / (R1 + R2))*VDD

ID = VS/ RS

Where,

VG = gate voltage

The

VS = input source voltage

VDD = supply voltage at drain

RS = source resistance

Enhancement Depletion Mosfet

R1 & R2 = input resistors

The different regions in which the MOSFET operates in their total operation are discussed below.

Cut-off Region: If the gate-source voltage is less than the threshold voltage then we say that the transistor is operating in the cut-off region (i.e. fully OFF). In this region drain current is zero and the transistor acts as an open circuit.

What Is Enhancement Mode In Mosfet

VGS < VTH => IDS = 0

Ohmic (Linear) Region: If the gate voltage is greater than threshold voltage and the drain-source voltage lies between VTH and (VGS – VTH) then we say that the transistor is in linear region and at this state the transistor acts as a variable resistor.

VGS > VTH and VTH < VDS < (VGSVGS – VTH) => MOSFET acts as a variable Resistor

Saturation Region: In this region the gate voltage is much greater than threshold voltage and the drain current is at its maximum value and the transistor is in fully ON state. In this region the transistor acts as a closed circuit.

VGS >> VTH and (VGS – VTH) < VDS < 2(VGS – VTH) => IDS = Maximum

The gate voltage at which the transistor ON and starts the current flow through the channel is called threshold voltage. This threshold voltage value range for N-channel devices is in between 0.5V to 0.7V and for P-channel devices is in between -0.5V to -0.8V.

The Enhancement Mosfet Is Mcq

The behavior of a MOSFET transistor in depletion and enhancement modes depending on the gate voltage is summarized as follows.

MOSFET Type
VGS = +ve
VGS = 0
VGS = -ve
N-Channel Depletion
ON
ON
OFF
N-Channel Enhancement
ON
OFF
OFF
P-Channel Depletion
OFF
ON
ON
P-Channel Enhancement
OFF
OFF
ON

Applications

  • MOSFETs are used in digital integrated circuits, such as microprocessors.
  • Used in calculators.
  • Used in memories and in logic CMOS gates.
  • Used as analog switches.
  • Used as amplifiers.
  • Used in the applications of power electronics and switch mode power supplies.
  • MOSFETs are used as oscillators in radio systems.
  • Used in automobile sound systems and in sound reinforcement systems.

Conclusion

A complete beginner’s guide to introduction of MOSFET. You learned the structure of a MOSFET, different types of MOSFET, their circuit symbols, an example circuit using a MOSFET to control an LED and also few areas of applications.

Related Posts:

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.

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.

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.

Enhancement Mosfet Vs Depletion 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 VGS = 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.

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 VGS 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 VGS 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 ID 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 IDis controlled by the gate potential VGS. 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 VGS for which variation in ID is shown-

Mosfet

As we are already aware of the fact that a gate potential above the threshold value causes drain current ID to flow. So when VGS is less than VGST then approximately 0 drain current flows and when VGS is greater than VGST 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.

E Mosfet

Disadvantages of MOSFET :

  1. It is a delicate device and is easily destroyable.
  2. Excessive application of gate to source voltage VGS 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.

Mosfet Enhancement Mode

Related Terms: