The Jfet Is

1. As a JFET is a device that controls the amount of current going through it via an input voltage, the first application circuit is obvious: a switch. In our effect pedal kits we take advantage of the 3PDT to turn the led on and off mechanically, but a circuit like the following one could perfectly be used instead. JFET as a switch circuit. If we apply a voltage Vg.
2. Explain why the JFET is so sensitive, and comment on what advantages and disadvantages this gives the JFET as an electronic device. Reveal answer The gate-channel PN junction of a JFET normally operates in reverse-bias mode, whereas bipolar transistors require a forward current for emitter-collector conduction.

The crucial difference between BJT and JFET is that BJT is a bipolar device while JFET is a unipolar device. It is so because the operation of BJT is dependent on injection and collection of minority charge carriers that includes both electrons and holes. As against JFET is majority carrier device, thus termed as unipolar.

Another major difference between BJT and JFET is that BJT falls under current controlled device category whereas JFET falls under voltage controlled device category.

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We will discuss some other major differences between BJT and JFET but before proceeding further have a look at the contents to be discussed under this article.

Content: BJT Vs JFET

Comparison

N Channel Jfet Symbol

Definition

High Current Jfet

BJT

BJT is the short form used for bipolar junction transistor. It is a 3 terminal device that is used for switching or amplification purpose.

The figure below shows the basic construction of a bipolar transistor consisting of 3 terminals emitter, base and collector.

It is formed by fusing two p-n junction diodes, that shares a common terminal. The common terminal can be either p or n as we can see in the figure shown above.

It is a current regulating device which controls the current flowing through it. There exist two different configurations of a BJT, i.e., NPN or PNP. Both hold the same operating principle but the difference between the two is their biasing and polarity of power supply voltage.

Let's have a look at the basic operation of NPN bipolar transistor.

In normal operating conditions, EB junction is always forward biased whereas CB junction is always reverse biased as we can see in the figure shown above.

Due to the forward applied voltage V_EB, the electrons in the N region experiences repulsive force and drifts across the lightly doped base region after overcoming the barrier potential. As the base region is lightly doped, only some of the drifted electrons recombine with the holes in the base region.

Now, the increased concentration of electrons in the base region causes more electrons to move across the collector region. As this region is reversed biased, the electrons are immediately collected by this region.

Thus, a proper flow of current is noticed and hence the emitter current is the summation of base and collector current.

JFET is the short form used for Junction Field Effect Transistor. It is a 3 terminal unipolar device that controls the flow of current through the device by the applied input voltage. Here, the 3 terminals are termed as the source, gate and drain.

It is known so because the output current of the device is controlled by the field associated with depletion region.

It can be either n channel JFET or p channel JFET.

As it is a voltage controlled device thus the applied input potential allows the movement of electrons hence causing current to flow through the device.

The figure below shows an n channel JFET with the positive voltage at the drain terminal.

In the absence of any applied input voltage, the two depletion region around the PN junctions is equally wide and symmetrical. However, on applying a positive potential to the drain wrt source, electrons start flowing from source to drain. Thus, causing the drain current to flow through the drain to source.

Another condition exists when the gate terminal is applied with negative potential and drain is positively biased as we can see below:

This reverse biasing of the p-n junction allows a considerable increase in the width of the depletion region. Resultantly, this narrows the channel length and drain current is decreased due to an increase in resistance.

Any additional increase in the gate voltage will cause the drain current to cut-off completely. Conversely, by lowering the negative biasing of the gate terminal, the width of the depletion region reduces.

Key Differences Between BJT and JFET

The points given below describes the difference between BJT and JFET:

1. The key difference between BJT and JFET is that BJT is a device in which output current is controlled by the base current. On the contrary, JFET is a device whose output current is controlled by the input voltage applied to it.
2. BJT possess low to medium input impedance whereas when we talk about JFET, it possesses high input impedance.
3. Whenever there is a need for high gain and fast response then BJT's are preferred while JFET's are low gain devices.
4. A BJT is a device that possesses low thermal stability whereas JFET possesses high thermal stability.
5. Another key difference that exists between BJT and JFET is that BJT is preferred in low current applications while JFET is preferred in low voltage applications.

The Jfet Is One

Conclusion

Although both BJT and JFET belong to transistor family, their operating principle differentiates the two. The larger size of BJT and more power consumption as compared to JFET sometimes proves as its disadvantage.

Table of Contents

Construction of JFET

N channel JFET consists of (i) N-type semiconductor bar which forms the channel and (ii) two heavily doped p-type regions formed by diffusion or alloying on two sides of the n-type bar.

n channel JFET shown in the figure. In the case of p channel JFET, the channel is formed by a P-type semiconductor bar and too heavily doped n-type regions are formed on the two sides of the p-type bar forming the channel.

In FET, current flows through the channel from the left-hand end (called source) to the right-hand end (called a drain). In the case of N channel FET, the current is carried by electrons ( majority Carriers ) and in the case of p channel FET, the current is carried by holes (majority carriers).

Source

The source S is the terminal ( at the left end of the Bar ) through which the majority carrier enters the bar. The current constituted by the majority carriers entering the bar at S is represented by I_S. The electric current I_S is taken as positive if it entered the source terminal.

Drain

Drain the drain D is the terminal ( at the right end of the bar ) through which the majority carriers leave the bar. The conventional current entering the channel at drain D is represented by I_D.

The electric current I_D is taken as positive if it enters the drain current. The drain-to-source voltage is V_DS.V_DS is positive if the drain D is positive w.r.t he sources. Here V_DS = V_DD.

Gate

The gate G is formed by two heavily doped p-type regions on the upper and lower sides of the n-type bar. The voltage between the gate and the source is V_GS = – V_GG is applied to make the PN junction reverse Biased. The electric current entering the gate is represented by I_{G .} The electric current is taken as positive if this current enter the gate.

Channel

The channel of the n-type bar is a portion between to get reasons. The majority of carriers move from the source (S) to the drain (D) through the channel.

Note. The source of JFET corresponds to the emitter of the BJT, gate corresponds to the base of the BJT, and the drain corresponds to the collector of the BJT.

JFETWorking

When a voltage V_DS = – V_DD is applied between the drain and source terminal but the gate is kept at zero potential, then the depletion layer of PN junction at the side of the bar is established. The region between these two depletion layers is known as the channel. The current carrier electron in n channel flow from source to the drain. The width of the channel determines the flow of current through the channel.

When the reverse voltage V_GS = – V_GG is applied between the gate and the source terminal, the depletion layer increase in size, and hence the width of the channel decrease. Consequently, the effective conducting cross-section of the channel decreases. As a result of this, the flow of current from the source to the drain decreases.

The flows of current from the source to the train goes on decreasing with the increase in the reverse voltage between the gate and the source. On the other hand, the flow of current from the source to the drain increases when the reverse voltage between the gate and the source decreases.

For a p channel JFET, the current careers in the channel are holes, and the polarities of V_GS = – V_GG and V_DS = – V_DD is reversed. The schematic diagram of N channel JFET and p channel JFET is shown in the figure respectively.

Thus the current from the source to the drain can be controlled by the applied voltage ( or electric field ) on the gate. Since the output current ( gate current ) of this type of transistor is controlled by the applied electric field, hence it is known as Field Effect Transistor (FET).

JFET Static Characteristics

The static or common source-drain characteristics of a JFET are the graphical representation of drain current ( I_D ) responding to the change in the drain-to-source voltage ( V_GS ), when gate-to-source voltage is kept constant.

It can be either n channel JFET or p channel JFET.

As it is a voltage controlled device thus the applied input potential allows the movement of electrons hence causing current to flow through the device.

The figure below shows an n channel JFET with the positive voltage at the drain terminal.

In the absence of any applied input voltage, the two depletion region around the PN junctions is equally wide and symmetrical. However, on applying a positive potential to the drain wrt source, electrons start flowing from source to drain. Thus, causing the drain current to flow through the drain to source.

Another condition exists when the gate terminal is applied with negative potential and drain is positively biased as we can see below:

This reverse biasing of the p-n junction allows a considerable increase in the width of the depletion region. Resultantly, this narrows the channel length and drain current is decreased due to an increase in resistance.

Any additional increase in the gate voltage will cause the drain current to cut-off completely. Conversely, by lowering the negative biasing of the gate terminal, the width of the depletion region reduces.

Key Differences Between BJT and JFET

The points given below describes the difference between BJT and JFET:

1. The key difference between BJT and JFET is that BJT is a device in which output current is controlled by the base current. On the contrary, JFET is a device whose output current is controlled by the input voltage applied to it.
2. BJT possess low to medium input impedance whereas when we talk about JFET, it possesses high input impedance.
3. Whenever there is a need for high gain and fast response then BJT's are preferred while JFET's are low gain devices.
4. A BJT is a device that possesses low thermal stability whereas JFET possesses high thermal stability.
5. Another key difference that exists between BJT and JFET is that BJT is preferred in low current applications while JFET is preferred in low voltage applications.

The Jfet Is One

Conclusion

Although both BJT and JFET belong to transistor family, their operating principle differentiates the two. The larger size of BJT and more power consumption as compared to JFET sometimes proves as its disadvantage.

Table of Contents

Construction of JFET

N channel JFET consists of (i) N-type semiconductor bar which forms the channel and (ii) two heavily doped p-type regions formed by diffusion or alloying on two sides of the n-type bar.

n channel JFET shown in the figure. In the case of p channel JFET, the channel is formed by a P-type semiconductor bar and too heavily doped n-type regions are formed on the two sides of the p-type bar forming the channel.

In FET, current flows through the channel from the left-hand end (called source) to the right-hand end (called a drain). In the case of N channel FET, the current is carried by electrons ( majority Carriers ) and in the case of p channel FET, the current is carried by holes (majority carriers).

Source

The source S is the terminal ( at the left end of the Bar ) through which the majority carrier enters the bar. The current constituted by the majority carriers entering the bar at S is represented by I_S. The electric current I_S is taken as positive if it entered the source terminal.

Drain

Drain the drain D is the terminal ( at the right end of the bar ) through which the majority carriers leave the bar. The conventional current entering the channel at drain D is represented by I_D.

The electric current I_D is taken as positive if it enters the drain current. The drain-to-source voltage is V_DS.V_DS is positive if the drain D is positive w.r.t he sources. Here V_DS = V_DD.

Gate

The gate G is formed by two heavily doped p-type regions on the upper and lower sides of the n-type bar. The voltage between the gate and the source is V_GS = – V_GG is applied to make the PN junction reverse Biased. The electric current entering the gate is represented by I_{G .} The electric current is taken as positive if this current enter the gate.

Channel

The channel of the n-type bar is a portion between to get reasons. The majority of carriers move from the source (S) to the drain (D) through the channel.

Note. The source of JFET corresponds to the emitter of the BJT, gate corresponds to the base of the BJT, and the drain corresponds to the collector of the BJT.

JFETWorking

When a voltage V_DS = – V_DD is applied between the drain and source terminal but the gate is kept at zero potential, then the depletion layer of PN junction at the side of the bar is established. The region between these two depletion layers is known as the channel. The current carrier electron in n channel flow from source to the drain. The width of the channel determines the flow of current through the channel.

When the reverse voltage V_GS = – V_GG is applied between the gate and the source terminal, the depletion layer increase in size, and hence the width of the channel decrease. Consequently, the effective conducting cross-section of the channel decreases. As a result of this, the flow of current from the source to the drain decreases.

The flows of current from the source to the train goes on decreasing with the increase in the reverse voltage between the gate and the source. On the other hand, the flow of current from the source to the drain increases when the reverse voltage between the gate and the source decreases.

For a p channel JFET, the current careers in the channel are holes, and the polarities of V_GS = – V_GG and V_DS = – V_DD is reversed. The schematic diagram of N channel JFET and p channel JFET is shown in the figure respectively.

Thus the current from the source to the drain can be controlled by the applied voltage ( or electric field ) on the gate. Since the output current ( gate current ) of this type of transistor is controlled by the applied electric field, hence it is known as Field Effect Transistor (FET).

JFET Static Characteristics

The static or common source-drain characteristics of a JFET are the graphical representation of drain current ( I_D ) responding to the change in the drain-to-source voltage ( V_GS ), when gate-to-source voltage is kept constant.

Set of static characteristics of N channel JFET is shown in the figure.

Discussion of the curves

When V_GS = 0 V, the channel between the gate junction is fully open. When V_DS = 0, then no electric field is applied and the majority carrier ( electrons ) are not attracted from the source to the drain. Hence the drain current I_D = 0.. When the value of V_DS increases from zero, then the electric field is set up between drain and source.

Now the current Carriers electrons flow from source to the drain and hence drain current increases linearly. As the value of I_D increases, the voltage drop takes place in the bar. This voltage drop reverse biases the gate and hence the width of the channel decreases.

The narrowing of the channel is not uniform as the voltage drop takes place along the length of the channel. The width of the channel goes on decreasing as we move away from the source. Ultimately a critical value of V_DS is reached when the channel is pinched off as shown in the figure.

The Jfet Is Part

This value of voltage V_DS is known as pinch-off voltage V_P. At this voltage, the value of the gate current I_D also becomes constant.

However, the channel is not completely blocked. If the channel is completely blocked at the Pinch off voltage, then I_D would have been reduced to zero.

The Jfet Is Also Called

As the value of V_DS increases, the reverse voltage between the channel and the gate becomes, hence the breakdown of the gate junction takes place, resulting into a sharp increase in the drain current.

When V_GS is negative, the reverse bias between the channel and the gate increases. As a result of this, the resistance of the channel increases, the drain current decreases. The Pinch Of takes place at a lower value of V_{DS .} Similarly, the break down of the gate junction also takes place at lower value of V_DS.

Jfet Vs Mosfet

Since that gate current is controlled by the gate bias beyond Pinch off, so the amplification of the signal can be achieved by changing the gate bias.