For an N-channel enhancement MOSFET V GS(th) is above 0 V. Therefore, even at V GS of 0 V, a depletion MOSFET can conduct current. To turn off a depletion MOSFET the V GS should be lower than the (negative) V GS(th). This is clearly shown in schematic symbols for both. Figure 3 below shows the schematic symbols for. The LND150 is a high voltage N-channel depletion mode (normally-on) transistor utilizing lateral DMOS technology. The gate is ESD protected. The LND150 is ideal for high voltage applications in the areas of normally-on switches, precision constant current sources, voltage ramp generation and amplification.

N-Channel MOSFET is a type of metal oxide semiconductor field-effect transistor that is categorized under the field-effect transistors (FET). MOSFET transistor operation is based on the capacitor. This type of transistor is also known as an insulated-gate field-effect transistor (IGFET). Sometimes it is also known as a metal-insulator field-effect transistor (MIFET). This type of transistor is further classified as p-type and n-type. These p-types and n-type MOSFET’s are further classified as enhancement and depletion based MOSFETs. This classification is based on the formation of the channel in prior or the operation induced due to the existing channel. These transistors also consist of three terminals which referred to as the source, drain, and the gate. The functionality of the MOSFET’s is dependent on these terminals.

What is N-Channel MOSFET?

The MOSFET formed in which the conduction is due to the channel of majority charge carriers called electrons. When this MOSFET is activated as ON this condition results in the maximum amount of the current flow through the device. This type of MOSFET is defined as N-channel MOSFET.

Depletion devices are on with Vgs = 0, and need more negative (for N channel) or more positive (for P channel) to turn them off. Enhancement FETs are easier to use since the supply voltage used for the drain can also be used for the gate. Depletion mode FETs may need an extra supply voltage. Most MOSFETs are enhancement mode. N Channel Depletion Type MOSFET. In an N Channel Depletion Type MOSFET transistor, the semiconductor part is formed by the N-type substrate. While the source and the drain are heavily doped with P-type impurities. Moreover, the voltage applied to the gate is positive which makes the channel depleted due to its free holes. Compared to enhancement mode transistors, known as normally-off, depletion MOSFETs are in an on-state at zero voltage of gate-to-source (V GS), normally-on. This makes N-channel depletion mode MOSFETs a perfect constant current source.

Symbols for N-channel Depletion and Enhancement Types

These n-channel MOSFETs are further classified as

1. N-Channel with Enhancement MOSFET and
2. N-Channel with Depletion MOSFET

Working

N Channel Depletion Mosfet

The working of the n-channel MOSFET is based on the majority of the carriers that are electrons. These electrons move in the channel is responsible for the flow of current in the transistor. The p-substrate material is required in the formation of the gate terminals.

(1) N-Channel with Enhancement MOSFET

In n-channel MOSFET’s the body that is formed due to the p-substrate material that is technically referred to as the substrate. The n-type material is required for the formation of the terminals called source and the drain. Here the p substrate impurities are doped with light concentration whereas n-type is doped heavily.

N-Channel Enhancement MOSFET

The device body that is formed due to p-type and the terminal source are connected to a common ground. A positive polarity of the voltage is applied to the terminal gate. Because of this positivism, it corresponds to an effect of the capacitor. Hence in the p substrate, the minority carriers that are free electrons get attracted and move towards the terminal gate.

Due to this a layer that is because of uncovered ions is formed bellow the layer of dielectric where the combinations of the holes with electrons occur. As the positive voltage applied gradually increases and crosses the minimum threshold the electrons which are minority carriers would be able to overcome the recombination with the holes and they form the channel between the two p type material .

Further application of the positive voltage value at the drain leads to the flow of current through the transistor. The concentrations of the electrons are dependent on the potential applied. These concentrations of the electrons are responsible for the formation of the channel and the application of the voltage at gate enhances the flow of the current. Hence it is termed as N- channel MOSFET of enhancement type.

(2) N-Channel Depletion MOSFET

The construction is similar to the enhancement MOSFET but the working is different in comparison to it. The space that is present in between the terminals of drain and the terminal source is composed of the impurities of n-type. A difference in potential applied at the drain and the terminal source leads to the flow of current through the region n.

A voltage value with the negative polarity is applied at the gate. The electrons present in it get repelled and settles down at the dielectric layer. This is the reason due to which the depletion of the charge carriers occurs and results in the reduction of the overall conductance. At this situation after applying the same value of the voltage at the terminal drain still, the current value gets reduced.

By making the variations at the depletion charge carriers the flow of current at drain can be controlled. This is the reason it is defined as depletion MOSFET. Here the potential value at the drain is positive and the gate has negative polarity and the terminal source remains at the potential value of zero.

The difference of the values of potential is more at the terminals drain and the gate in comparison to the terminals of the source, and the gate. The depletion width will be evident more at the drain in comparison with the source.

N-Channel Characteristics

In n-channel enhancement mode, no current flows through the transistor until the voltage at the gate and terminal source exceed the minimum voltage cut in value. If the voltage at the drain and the terminal source is applied then even there is no evident flow of the current.

After the discussion on the basis of n-channel MOSFET can you tell the importance of n-channel enhancement over depletion type?

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

N Channel Depletion Mosfet Datasheet

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.

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

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

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

P Channel Enhancement Mode Mosfet

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.

N Channel Depletion Mosfet Rate

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.

N Channel Depletion Mosfet Meaning

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.

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.

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.

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.