Enhancement Mode Fet

4.3.2 N-Channel Enhancement Mode MOSFET Operation. MOSFET (IGFET) Operation. The gate has a voltage applied to it that makes it positive with respect to the source. This causes holes in the P type layer close to the silicon dioxide layer beneath the gate to be repelled down into the P type substrate, and at the same time this positive. Enhancement MOSFET or E-MOSFET is a type of MOSFET where there is no channel constructed during its fabrication but it is induced in the substrate using the gate voltage. The E-MOSFET does not conduct when there is no gate voltage i.e.

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.

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:

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. Civilization v - civ and scenario pack: polynesia download.

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 (V_GS) to switch OFF the device.

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 I_D 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 (V_DS) and drain current (I_D). 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 (+V_GS) 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 (-V_GS) 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 (V_G) is high the transistor is switched ON and when the voltage (V_G) 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.

V_G = (R₂ / (R₁ + R₂))*VDD

I_D = V_S/ R_S

Where,

V_G = gate voltage

V_S = input source voltage

V_DD = supply voltage at drain

R_S = source resistance

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.

Enhancement Mode Fetal

V_GS < V_TH => I_DS = 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.

V_GS > V_TH and V_TH < V_DS < (V_GSVGS – V_TH) => 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.

V_GS >> V_TH and (V_GS – V_TH) < V_DS < 2(V_GS – V_TH) => I_DS = 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 behavior of a MOSFET transistor in depletion and enhancement modes depending on the gate voltage is summarized as follows.

MOSFET Type	V_GS = +ve	V_GS = 0	V_GS = -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

N-channel Enhancement Mode Fet

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.

Enhancement and depletion mode MOS Transistor

Metal oxide semiconductor field effect transistor (MOSFET): It is a transistor for amplifying and switching electronic signals. It is also called Unipolar device because the current is produced by any one of the charge carrier either electrons or holes depending upon n channel or p channel. MOSFET is a four terminal device with source (S), drain (D), gate (G) and body (B) or substrate terminal. The body (B) of the MOSFET is connected to the source terminal and is treated as 3 terminal device similar to FET. Actually these two terminals are internally short circuited and it appears like a 3 terminal device in electric diagrams.

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This diagram shows all the terminals and the gate is separated from the body by an insulating material.

Electric circuit

Enhancement mode MOSFET:

The Metal Oxide Silicon FET (MOSFET) or Metal Oxide Silicon Transistor (M.O.S.T.) has an even higher input resistance (typically 10¹² to 10¹⁵ ohms) than that of the JFET. In the MOSFET device the gate is completely insulated from the rest of the transistor by a very thin layer of metal oxide (Silicon dioxide SiO₂). Hence the general name applied to any device of this type, is the IGFET or Insulated Gate FET.

Construction of a N Channel Enhancement Mode MOSFET

The basic construction of a MOSFET is shown in diagram. The body or substrate of P type silicon is used, then two heavily doped N type regions are diffused into the upper surface, to form a pair of closely spaced strips.

A very thin (about 10⁻⁴ mm) layer of silicon dioxide is then evaporated onto the top surface forming an insulating layer. Parts of this layer are then etched away above the N type regions using a photographic mask to leave these regions uncovered. On top of the insulating layer, between the two N type regions, a layer of aluminium is deposited. This acts as the GATE electrode. Metal contacts are also deposited on the N type regions, which act as the SOURCE and DRAIN connectors

Enhancement Mode Operation.

Overlord™. The gate has a voltage applied to it that makes it positive with respect to the source. This causes holes in the P type layer close to the silicon dioxide layer beneath the gate to be repelled down into the P type substrate, and at the same time this positive potential on the gate attracts free electrons from the surrounding substrate material. These free electrons form a thin layer of charge carriers beneath the gate electrode (they can’t reach the gate because of the insulating silicon dioxide layer) bridging the gap between the heavily doped source and drain areas. This layer is sometimes called an “inversion layer” because applying the gate voltage has caused the P type material immediately under the gate to firstly become “intrinsic” (with hardly any charge carriers) and then an N type layer within the P type substrate.

Depletion Mode Transistor

Any further increase in the gate voltage attracts more charge carriers into the inversion layer, so reducing its resistance, and increasing current flow between source and drain. Reducing the gate source voltage reduces current flow. When the power is switched off, the area beneath the gate reverts to P type once more.

As well as the type described above, devices having N type substrates and P type (inversion layer) channels are also available. Operation is identical, but of course the polarity of the gate voltage is reversed.

This method of operation is called “ENHANCEMENT MODE” as the application of gate source voltage makes a conducting channel “grow”, therefore it enhances the channel. Other devices are available in which the application of a bias voltage reduces or “depletes” the conducting channel.

Circuit Symbols for Enhancement Mode MOSFETs (IGFETs)

Threshold voltage : It is abbreviated as Vth and is defined as the gate voltage where an inversion layer forms at the interface between the insulating layer (oxide) and the substrate (body) of the transistor. The formation of the inversion layer allows the flow of electrons through the gate-source junction.

For an enhancement-mode, n-channel MOSFET, the three operational modes are:

1. Cut-off , sub threshold or weak-inversion mode:

When V_GS < V_th:

where is gate-to-source bias and is the threshold voltage of the device.

According to the basic threshold model, the transistor is turned off, and there is no conduction between drain and source. A more accurate model considers the effect of thermal energy on the Boltzmann distribution of electron energies which allow some of the more energetic electrons at the source to enter the channel and flow to the drain. This results in a subthreshold current that is an exponential function of gate–source voltage. While the current between drain and source should ideally be zero when the transistor is being used as a turned-off switch, there is a weak-inversion current, sometimes called subthreshold leakage.

In weak inversion the current varies exponentially with as given approximately by:

where = current at , the thermal voltage and the slope factor n is given by

with = capacitance of the depletion layer and = capacitance of the oxide layer. In a long-channel device, there is no drain voltage dependence of the current once , but as channel length is reduced drain inducedbarrier lowering introduces drain voltage dependence that depends in a complex way upon the device geometry (for example, the channel doping, the junction doping and so on). Frequently, threshold voltage V_th for this mode is defined as the gate voltage at which a selected value of current I_D0 occurs, for example, I_D0 = 1 μA, which may not be the same V_th-value used in the equations for the following modes.

Enhancement Vs Depletion Mode

2. Triode mode or linear region (also known as the ohmic mode

When V_GS > V_th and V_DS < ( V_GS – V_th )

The transistor is turned on, and a channel has been created which allows current to flow between the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. As here, the voltage between transistor gate and source ( V_GS) exceeds the threshold voltage(V_th), it is known as Overdrive Voltage. The current from drain to source is modeled as:

where is the charge-carrier effective mobility, is the gate width, is the gate length and is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest.

3. Saturation or active mode

When V_GS > V_th and V_DS > ( V_GS – V_th )

The switch is turned on, and a channel has been created, which allows current to flow between the drain and source. Since the drain voltage is higher than the gate voltage, the electrons spread out, and conduction is not through a narrow channel but through a broader, two- or three-dimensional current distribution extending away from the interface and deeper in the substrate. The onset of this region is also known as pinch-off to indicate the lack of channel region near the drain. The drain current is now weakly dependent upon drain voltage and controlled primarily by the gate–source voltage, and modeled approximately as:

The additional factor involving λ, the channel-length modulation parameter, models current dependence on drain voltage due to the Early effect, or channel length modulation. According to this equation, a key design parameter, the MOSFET transconductance is:

where the combination V_ov = V_GS – V_th is called the overdrive voltage, and where V_DSsat = V_GS – V_th (which Sedra neglects) accounts for a small discontinuity in which would otherwise appear at the transition between the triode and saturation regions.

Another key design parameter is the MOSFET output resistance r_out given by:

r_out is the inverse of g_DS where . I_D is the expression in saturation region.

If λ is taken as zero, an infinite output resistance of the device results that leads to unrealistic circuit predictions, particularly in analog circuits.

As the channel length becomes very short, these equations become quite inaccurate. New physical effects arise. For example, carrier transport in the active mode may become limited by velocity saturation. When velocity saturation dominates, the saturation drain current is more nearly linear than quadratic in V_GS. At even shorter lengths, carriers transport with near zero scattering, known as quasi-ballistic transport. In addition, the output current is affected by drain-induced barrier lowering of the threshold voltage.

Enhancement Mode Gan Fet

Cross section of a MOSFET operating in the linear (Ohmic) region; strong inversion region present even near drain

Depletion Type Mosfet

Cross section of a MOSFET operating in the saturation (active) region; channel exhibits pinch-off near drain

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