Enhancement MOSFET : Working, Differences & Its Applications

A MOSFET (metal–oxide–semiconductor FET) is one kind of field-effect transistor with an insulated gate that is mainly used for amplifying or switching signals. Now in analog and digital circuits, MOSFETs are used more frequently as compared to BJTs. MOSFETs are mainly used in amplifiers because of their infinite input impedance so it allows the amplifier to capture nearly all the incoming signal. The main benefit of MOSFET as compared to BJT is, that it requires nearly no input current for controlling the load current. MOSFETs are classified into two types enhancement MOSFET and depletion MOSFET. So this article provides brief information on the enhancement MOSFET – working with applications.

What is Enhancement Type MOSFET?

The MOSFET which works in enhancement mode is known as E-MOSFET or enhancement mosfet. Enhancement mode means, that whenever the voltage toward the gate terminal of this MOSFET increases, then the current flow will be increased more from drain to source until it reaches the highest level. This MOSFET is a three-terminal voltage-controlled device where the terminals are a source, gate, and drain.

The features of these MOSFETs are low power dissipation, simple manufacturing, and small geometry. So these features will make them used within integrated circuits. There is no pathway in between the drain (D) and source (S) of this MOSFET when no voltage is applied in between the gate & source terminals. So, applying a voltage at gate-to-source will enhance the channel, making it capable of conducting current. This property is the main reason to call this device an enhancement-mode MOSFET.

Enhancement MOSFET Symbol

The enhancement MOSFET symbols for both P-channel & N-channel are shown below. In the below symbols, we can notice that a broken line is simply connected from the source to the substrate terminal, which signifies the enhancement mode type.

The conductivity in EMOSFETs enhances by increasing the oxide layer, which adds the charge carriers toward the channel. Usually, this layer is known as the Inversion layer.

The channel in this MOSFET is formed in between the D (drain) and S (source). In the N-channel type, the P-type substrate is used whereas in the P-channel type, the N-type substrate is used. Here the channel conductivity because of the charge carriers mainly depends on P-type or N-type channels correspondingly.

 

Enhancement Mosfet Working Principle

Enhancement type MOSFETS are normally off which means when an enhancement-type MOSFET is connected, there will be no flow of current from the terminal drain (D) to the source (S) when no voltage is given to its gate terminal. This is the reason to call this transistor a normally off device.

Similarly, if the voltage is given to the gate terminal of this MOSFET, then the drain-source channel will become very less resistive. When the voltage from gate to source terminal increases then the flow of current from drain to source terminal will also increase until the highest current is supplied from drain terminal to source.

Construction

The construction of enhancement MOSFET is shown below. This MOSFET includes three layers gate, drain, and source. The body of MOSFET is known as a substrate that is connected internally to the source. In the MOSFET, the metallic gate terminal from the semiconductor layer is insulated through a silicon dioxide layer otherwise a dielectric layer.

This EMOSFET is constructed with two materials like P-type and N-type semiconductors.  A substrate gives physical support to the device. A thin SiO layer and an outstanding electrical insulator simply cover the region in between the source & drain terminals. On the oxide layer, a metallic layer forms the gate electrode.

In this construction, the two N regions are separated through some micrometers distance over a lightly doped p-type substrate. These two N-regions are performed like the source and drain terminals. On the surface, a thin insulation layer is developed which is known as silicon dioxide. The charge carriers like holes made on this layer will establish aluminum contacts for both the source & the drain terminals.

This conduction layer works like the terminal gate which is laid on the SiO2 as well as the complete area of the channel. However for conduction, it doesn’t contain any physical channel In this kind of enhancement MOSFET, the p-type substrate is extended on the whole SiO2 layer.

Working

The working of EMOSFET is when VGS is 0V then there is no channel that will connect the source & drain. The p-type substrate has only a small number of thermally produced minority charge carriers like free electrons thus the drain current is zero. Because of this reason, this MOSFET will be normally OFF.

Once the gate (G) is positive (+ve), then it attracts minority charge carriers like electrons from p–substrate where these charge carriers will combine through the holes under the layer of SiO2. Further VGS is increased then the electrons will have enough potential to over come and bonding  and  more charge carriers i.e. electrons get deposited in the channel.

Here, the dielectric is used to prevent the electron’s movement across the silicon dioxide layer. This accumulation will result in the n-channel formation between Drain and Source terminals. So this can lead to the generated drain current flow throughout the channel. This drain current is simply proportional to the channel’s resistance which depends further on the charge carriers attracted to the +ve terminal of the gate.

Types of Enhancement Type MOSFET

They are available in two types N Channel Enhancement MOSFET and P Channel Enhancement MOSFET.

In the N channel enhancement type, the lightly doped p-substrate is used and two heavily doped n-types regions will make the source & drain terminals. In this type of E-MOSFET, the majority of charge carriers are electrons. Please refer to this link to know more about – N-channel MOSFET.

In the P channel type, the lightly doped N-substrate is used and two heavily doped p-types regions will make the source & drain terminals. In this type of E-MOSFET, the majority of charge carriers are holes. Please refer to this link to know more about – P-channel MOSFET.

Characteristics

The VI and drain characteristics of n channel enhancement MOSFET and p channel enhancement are discussed below.

Drain Characteristics

The N channel enhancement mosfet drain characteristics are shown below. In these characteristics, we can observe the drain characteristics plotted between the Id and Vds for different Vgs values as shown in the diagram, As you can see that when the Vgs value is increased, then the current ‘Id’ will also be increased.

The parabolic curve on the characteristics will show the locus of VDS where the Id(drain current) will get saturated. In this graph, the linear or ohmic region is shown. In this region, the MOSFET can function as a voltage-controlled resistor. So, for the fixed Vds value, once we change the Vgs voltage value, then the channel width will be changed or we can say that the resistance of the channel will change.

The ohmic region is a region where the current ‘IDS’ raises with an increase in the VDS value. Once MOSFETs are designed to work in the ohmic region, then they can be utilized as amplifiers.

The gate voltage at which point the transistor turns ON & starts flowing current throughout the channel is known as threshold voltage (VT or VTH). For N-channel, this threshold voltage value ranges from 0.5V – 0.7V whereas for P-channel devices it ranges from -0.5V to -0.8V.

Whenever the Vds<Vgs-Vt & Vgs > Vt then, in this case, the MOSFET will operate in a linear region. So in this region, it can function as a voltage-controlled resistor.

In the cut-off region, when the voltage Vgs <VT then the current throughout the MOSFET is zero otherwise we can say that the MOSFET will stay in the OFF condition.

Whenever the mosfet is operated on the right side of the locus then we can say that it is operated in a saturation region. So, mathematically, whenever the Vgs voltage is > or = Vgs-Vt then it is operating in a saturation region. So this is all about the drain characteristics in different regions of enhancement mosfet.

Transfer Characteristics

The transfer characteristics of the N channel enhancement mosfet are shown below. The transfer characteristics show the relationship between the input voltage ‘Vgs’and output drain current ‘Id’. These characteristics basically show how the ‘Id’ changes when Vgs values change. So from these characteristics, we can observe that the drain current ‘Id’ is zero upto the threshold voltage. After that, when we increase the Vgs value, then the ‘Id’ will increase.

The relationship between the current ‘Id’ and Vgs can be given as Id = k(Vgs-Vt)^2. Here, the ‘K’ is the device constant which depends on the device’s physical parameters. So by using this expression, we can find out the drain current value for the fixed Vgs value.

P Channel Enhancement MOSFET

The P channel enhancement mosfet drain characteristics are shown below. Here, the Vds and Vgs will be negative. The drain current ‘Id’ will supply from the source to the drain terminal. As we can notice from this graph, when Vgs become more negative then the drain current ‘Id’ will also increase.

When the Vgs >VT, then this MOSFET will operate in the cut-off region. Similarly, if you observe the transfer characteristics of this MOSFET then it will be a mirror image of the N-channel.

Applications

Biasing of Enhancement MOSFET

Generally, Enhancement MOSFET (E-MOSFET) is biased either with voltage divider bias otherwise drain feedback bias. But the E-MOSFET cannot be biased with self-bias & zero bias.

Voltage Divider Bias

The voltage divider bias for N channel E-MOSFET is shown below. Voltage divider bias is similar to the divider circuit using BJTs. In fact, the N-channel enhancement MOSFET needs the gate terminal which is higher than its source just like the NPN BJT needs a base voltage that is higher as compared to its emitter.

In this circuit, the resistors like R1 & R2 are used to make the divider circuit for establishing the gate voltage.

When the source of E-MOSFET is directly connected to the GND then VGS = VG. So, the potential across resistor R2 needs to be set above VGS(th) for proper operation with E-MOSFET characteristic equation like I_D = K (V_GS-V_GS(th))^2.

By knowing the VG value, the characteristic equation of E-MOSFET is used to establish the drain current. But the device constant ‘K’ is the only missing factor that can be calculated for any particular device depending on the VGS (on), and ID (on) coordinate pair.

The constant ‘K’ is derived from the characteristic equation of E-MOSFET like K = I_D/(V_GS-V_GS(th))^2.

K = I_D/(V_GS-V_GS(th))^2.

So, this value is used for other biasing points.

Drain Feedback Bias

This biasing uses the “on” operating point on the characteristic curve above mentioned. The idea is to set up a drain current through a suitable selection of the power supply & drain resistor. The drain feedback circuit prototype is shown below.

This is a quite simple circuit that uses some basic components. This operation is understood by applying KVL.

V_DD = V_RD + V_RG + V_GS

V_DD = I_DR_D + I_GR_G + V_GS

Here, Gate current is insignificant so the above equation will become

V_DD=I_DR_D+V_GS

and also V_{DS =}V_GS

Thus,

V_GS =V_DS = V_DD − I_DR_D

This equation can be utilized as the basis for the bias circuit design.

Enhancement MOSFET Vs Depletion MOSFET

The difference between enhancement mosfet and depletion mosfet includes the following.

Enhancement MOSFET	Depletion MOSFET
Enhancement MOSFET is also known as E-MOSFET.	Depletion MOSFET is also known as D-MOSFET.
In enhancement mode, the channel initially does not exist and is formed by the voltage applied to the gate terminal.	In depletion mode, the channel is permanently fabricated at the construction time of the transistor.
Normally it is OFF device at zero Gate (G) to Source (S) voltage.	It is normally an ON device at zero Gate (G) to Source (S) voltage.
This MOSFET cannot conduct current at OFF condition.	This MOSFET can conduct current at OFF condition.
To turn ON this MOSFET, it requires positive gate voltage.	To turn ON this MOSFET, it requires negative gate voltage.
This MOSFET has a diffusion & leakage current.	This MOSFET doesn’t have a diffusion & leakage current.
It has no permanent channel.	It has a permanent channel.
The voltage at the gate terminal is directly proportional to the current at the drain terminal.	The voltage at the gate is Inversely proportional to the current at Drain.

Please refer to this link to know more about – Depletion Mode MOSFET.

The applications of Enhancement MOSFET include the following.

• Generally, enhancement MOSFETs are used in switching, amplifier, and inverter circuits.
• These are used in different motor drivers, digital controllers & power electronics ICs.
• It is used in digital electronics.

Thus, this is all about an overview of an Enhancement MOSFET – working with applications. The E-MOSFET is obtainable in both high and low-power versions which operate in only enhancement mode. Here is a question for you, what is depletion MOSFET?