JFET and MOSFET are the two most popular field effect transistors commonly used in electronic circuits. Both JFET and MOSFET are voltage-controlled semiconductor devices used to amplify weak signals using an electric field effect. The name itself hints at the attributes of the device. At room temperature, JFET gate current (the reverse leakage of the gate-to-channel junction) is comparable to that of a MOSFET (which has insulating oxide between gate and channel), but much less than the base current of a bipolar junction transistor. MOSFET vs JFET JFETs can only be operated in the depletion mode whereas MOSFETs can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward biased, excess- carrier injunction occurs and the gate current is substantial.

Junction Field Effect Transistor (JEFT)

A field effect transistor is a voltage controlled device i.e. the output characteristics of the device are controlled by input voltage. There are two basic types of field effect transistors:

1. Junction field effect transistor (JFET)
2. Metal oxide semiconductor field effect transistor (MOSFET)

Junction Field Effect Transistor (JFET)

A JFET is a three terminal semiconductor device in which current conduction is by one type of carrier i.e. electrons or holes.

The current conduction is controlled by means of an electric field between the gate and the conducting channel of the device.

The JFET has high input impedance and low noise level.

Construction Details:

A JFET consists of a p-type or n-type silicon bar containing two pn junctions at the sides as shown in fig.1.

Fig.1(i) Fig.1 (ii)

The bar forms the conducting channel for the charge carriers.

If the bar is of p-type, it is called p-channel JFET as shown in fig.1(i) and if the bar is of n-type, it is called n-channel JFET as shown in fig.1(ii).

The two pn junctions forming diodes are connected internally and a common terminal called gate is taken out.

Other terminals are source and drain taken out from the bar as shown in fig.1.

Thus a JFET has three terminals such as , gate (G), source (S) and drain (D).

JFET Polarities

Fig.2 (i) shows the n-channel JFET polarities and fig.2 (ii) shows the p-channel JFET polarities.

Fig.2 (i)

Fig.2 (ii)

In each case, the voltage between the gate and source is such that the gate is reverse biased.

The source and the drain terminals are interchangeable.

The following points may be noted:

1. The input circuit ( i.e. gate to source) of a JFET is reverse biased. This means that the device has high input impedance.
2. The drain is so biased w.r.t. source that drain current I_D flows from the source to drain.
3. In all JFETs, source current I_S is equal to the drain current i.e I_S = I_D.

Principle and Working of JFET

Principle of JEFT

Fig.3 shows the circuit of n-channel JFET with normal polarities.

The two pn junctions at the sides form two depletion layers.

The current conduction by charge carriers (i.e. electrons) is through the channel between the two depletion layers and out of the drain.

The width and hence resistance of this channel can be controlled by changing the input voltage V_GS.

The greater the reverse voltage V_GS, the wider will be the depletion layer and narrower will be the conducting channel.

The narrower channel means greater resistance and hence source to drain current decreases.

Reverse will happen when V_GS decreases.

Thus JFET operates on the principle that width and hence resistance of the conducting channel can be varied by changing the reverse voltage V_GS.

In other word, the magnitude of drain current I_D can be changed by altering V_GS.

Working of JEFT

The working of JFET can be explained as follows:

Case-i:

When a voltage V_DS is applied between drain and source terminals and voltage on the gate is zero as shown in fig.3(i), the two pn junctions at the sides of the bar establish depletion layers.

Fig.3 (i)

The electrons will flow from source to drain through a channel between the depletion layers.

The size of the depletion layers determines the width of the channel and hence current conduction through the bar.

Case-ii:

When a reverse voltage V_GS is applied between gate and source terminals, as shown in fig.3(ii), the width of depletion layer is increased.

Fig.3 (ii)

This reduces the width of conducting channel, thereby increasing the resistance of n-type bar.

Consequently, the current from source to drain is decreased.

On the other hand, when the reverse bias on the gate is decreased, the width of the depletion layer also decreases.

This increases the width of the conducting channel and hence source to drain current.

A p-channel JFET operates in the same manner as an n-channel JFET except that channel current carriers will be the holes instead of electrons and polarities of V_GS and V_DS are reversed.

Schematic Symbol of JFET

Fig.4 shows the schematic symbol of JFET.

Fig.4

Difference Between JFET and BJT

The JFET differs from an ordinary BJT in the following ways:

1. In a JFET, there is only one type of carrier,i.e. holes in p-type channel and electrons in n-type channel. For this reason it is also called unipolar transistor.However, in an ordinary BJT, both electrons and holes play role in conduction. Therefore, it is called as bipolar transistor.
2. As the input circuit of a JFET is reverse biased, therefore, it has a high input impedance. However, the input circuit of a BJT is forward biased and hence has low input impedance.
3. The primary functional difference between the JFET and BJT is that no current enters the gate of JFET. However, in typical BJT base current might be a few µA.
4. A BJT uses the current into its base to control a large current between collector and emitter. Whereas a JFET uses voltage on the gate terminal to control the current between drain and source.
5. In JFET, there is no junction. Therefore, noise level in JFET is very small.

Advantages of JFET

A JFET is a voltage controlled, constant current device in which variation in input voltage control the output current. Some of the advantages of JFET are:

1. It has a very high input impedance. This permits high degree of isolation between the input and output circuits.
2. The operation of a JFET depends upon the bulk material current carriers that do not cross junctions. Therefore, the inherent noise of tubes and those of transistors are not present in a JFET.
3. A JFET has a negative temperature co-efficient of resistance. This avoids the risk of thermal runaway.
4. A JFET has a very high power gain. This eliminates the necessity of using driver stages.
5. A JFET has a smaller size, longer life and high efficiency

You Might Like The Following Articles

• Basic Electronics Tutorial

• Electronic Components

• Resistors

• Capacitors

• Inductors
• Transformers
• Diodes
• Transistors
• Basic Electronics Useful Resources
• Selected Reading

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.

Jeet And Mosfet Are One

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.

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

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.

Jeet And Mosfet Are Two

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.