A N-Channel JFET is a JFET whose channel is composed of primarily electrons as the charge carrier. This means that when the transistor is turned on, it is primarily the movement of electrons which constitutesthe current flow.
2N4119/A JFET’s are targeted for ultra high input impedance applications for mid to high frequency designs. Gate leakages are typically 1pA at room temperatures. The 2N4117 has a cutoff voltage of less than 1.8V ideal for low-level power supplies. The TO-72 package is hermetically sealed and suitable for military applications.
This is in contrast to P-Channel JFETs, whose channel is composed primarily of holes, which constitute the current flow.
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A N-Channel JFET is composed of a gate, a source and a drain terminal.
It is made with an N-type silicon channel that contains 2 P-type silicon terminals placed on either side. The gate lead is connected to the p-type terminals, while the drain and source leads are connected to either ends of the N-type channel.
When no voltage is applied to the gate of a N-Channel JFET, current flows freely through the central N-channel. This is why JFETs are referred to as 'normally on' devices. Without any applied to the gate terminal of the transistor,they conduct current across from drain-source region.
How a N-Channel JFET Works
This is a typical diagram you would see of voltage biasing of a N-channel JFET. This diagram also servesto show you all the parts of a N-channel JFET.
How to Turn on an N-Channel JFET
To turn on an N-channel JFET, apply a positive voltage +VDD to the drain terminal of the transistor with no voltage applied to the gate terminal of thetransistor. This will allow a current to flow through the drain-source channel. If the gate voltage, VG, is 0V, the drain current is at its largest value for safe operation, and the JFET is in the ON active region.
So with a sufficient positive voltage, VDD, and no voltage (0V) applied to the base, the N-channel JFET is in maximum operation and has the largest current.
How to Turn Off an N-Channel JFET
To turn off the N-channel JFET, there are 2 steps you can take. You can either cut off the bias positivevoltage, VDD, that powers the drain. Or you can apply a negative voltage to the gate. When a negative voltage is applied to the gate, the drain current is reduced. As the gate voltage, VG, becomes more negative, the current lessens until cutoff, which is when then JFET is in the OFF region.
Characteristics Curve of a N-Channel JFET
The characteristics curve of an N channel JFET transistor shown below is the the graph of the drain current, ID versusthe gate-source voltage, VGS.
This curve represents the transconductance, or simply the gain, of the transistor.
The transconductance of a transistor really means the gain of the transistor.
So this transconductance characteristics below shows the gain of the transistor, how much current the transistor outputs based on the voltage input into the gate terminal. Remember that gain is the the output over the input. The input is how much voltage is fed to the gate terminal. The output is how much current the transistor outputs.
You can see based on this N channel JFET transconductance curve that as the negative voltage to the gate increases, the gain decreases. You can see that the gain, the current ID output by the transistor, is highest when the voltage fed to the gate terminal is 0V. As we increase this voltage (negatively), again, as stated, the gain decreases.
This transconductance curve is important because it shows the operation of a N channel JFET.
You can also see that the transconductance curve, as for all semiconductor devices, is nonlinear, for most of the curve,meaning changes to VGSdo not directly (linearly) increase or decrease drain current, ID, even though this is a lesser issue.
The big point is that, an N-Channel JFET turns on by having a positive voltage applied to the drain terminal of the transistor and ideally no voltage applied to the gate terminal. The transistor circuitshuts off by taking in a negative gate voltage, VGS, greater than about -4V or so. The transistor is in its fully conductive state and is in maximum operation when the voltage at the gate terminal is 0V. As we increase the amount of negative voltage the gate terminal receives, the transistor becomes less conductive. Once the negative voltage reachesa certain threshold, the N channel JFET circuit stops conducting altogether across the drain-source terminal.
The Regions that make up a transconductance curve are the following:
Cutoff Region- This is the region where the JFET transistor is off, meaning no drain current, ID flows from drain to source.
Ohmic Region- This is the region where the JFET transistor begins to show some resistance to the drain current, Id that is beginning to flow from drain to source. This is the only region in the curvewhere the response is linear.
Saturation Region- This is the region where the JFET transistor is fully operation and maximum current, for the voltage, VGS, that is supplied is flowing. During this region, the JFET is On and active.
Breakdown Region- This is the region where the voltage, VDD that is supplied to the drain of the transistor exceeds the necessary maximum. At this point, the JFET loses its ability to resist current because too much voltage is applied across its drain-source terminals. The transistor breaks down and current flowsfrom drain to source.
Related Resources
How to Build an N Channel JFET Switch Circuit
P-Channel JFET Basics
N-Channel MOSFET Basics
P-Channel MOSFET Basics
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The JFET is abbreviated as Junction Field Effect Transistor. JFET is just like a normal FET. The types of JFET are n-channel FET and P-channel FET. A p-type material is added to the n-type substrate in n-channel FET, whereas an n-type material is added to the ptype substrate in p-channel FET. Hence it is enough to discuss one type of FET to understand both.
N-Channel FET
The N-channel FET is the mostly used Field Effect Transistor. For the fabrication of Nchannel FET, a narrow bar of N-type semiconductor is taken on which P-type material is formed by diffusion on the opposite sides. These two sides are joined to draw a single connection for gate terminal. This can be understood from the following figure.
These two gate depositions (p-type materials) form two PN diodes. The area between gates is called as a channel. The majority carriers pass through this channel. Hence the cross sectional form of the FET is understood as the following figure.
Ohmic contacts are made at the two ends of the n-type semiconductor bar, which form the source and the drain. The source and the drain terminals may be interchanged.
Operation of N-channel FET
Before going into the operation of the FET one should understand how the depletion layers are formed. For this, let us suppose that the voltage at gate terminal say VGG is reverse biased while the voltage at drain terminal say VDD is not applied. Let this be the case 1.
In case 1, When VGG is reverse biased and VDD is not applied, the depletion regions between P and N layers tend to expand. This happens as the negative voltage applied, attracts the holes from the p-type layer towards the gate terminal.
In case 2, When VDD is applied (positive terminal to drain and negative terminal to source) and VGG is not applied, the electrons flow from source to drain which constitute the drain current ID.
Let us now consider the following figure, to understand what happens when both the supplies are given.
The supply at gate terminal makes the depletion layer grow and the voltage at drain terminal allows the drain current from source to drain terminal. Suppose the point at source terminal is B and the point at drain terminal is A, then the resistance of the channel will be such that the voltage drop at the terminal A is greater than the voltage drop at the terminal B. Which means,
VA>VB
Hence the voltage drop is being progressive through the length of the channel. So, the reverse biasing effect is stronger at drain terminal than at the source terminal. This is why the depletion layer tends to penetrate more into the channel at point A than at point B, when both VGG and VDD are applied. The following figure explains this.
Now that we have understood the behavior of FET, let us go through the real operation of FET.
Jfet N-channel Switch
Depletion Mode of Operation
As the width of depletion layer plays an important role in the operation of FET, the name depletion mode of operation implies. We have another mode called enhancement mode of operation, which will be discussed in the operation of MOSFETs. But JFETs have only depletion mode of operation.
Let us consider that there is no potential applied between gate and source terminals and a potential VDD is applied between drain and source. Now, a current ID flows from drain to source terminal, at its maximum as the channel width is more. Let the voltage applied between gate and source terminal VGG is reverse biased. This increases the depletion width, as discussed above. As the layers grow, the cross-section of the channel decreases and hence the drain current ID also decreases.
When this drain current is further increased, a stage occurs where both the depletion layers touch each other, and prevent the current ID flow. This is clearly shown in the following figure.
P Channel Jfet Operation
The voltage at which both these depletion layers literally “touch” is called as “Pinch off voltage”. It is indicated as VP. The drain current is literally nil at this point. Hence the drain current is a function of reverse bias voltage at gate.
Since gate voltage controls the drain current, FET is called as the voltage controlled device. This is more clearly understood from the drain characteristics curve.
Drain Characteristics of JFET
Let us try to summarize the function of FET through which we can obtain the characteristic curve for drain of FET. The circuit of FET to obtain these characteristics is given below.
When the voltage between gate and source VGS is zero, or they are shorted, the current ID from source to drain is also nil as there is no VDS applied. As the voltage between drain and source VDS is increased, the current flow ID from source to drain increases. This increase in current is linear up to a certain point A, known as Knee Voltage.
The gate terminals will be under reverse biased condition and as ID increases, the depletion regions tend to constrict. This constriction is unequal in length making these regions come closer at drain and farther at drain, which leads to pinch off voltage. The pinch off voltage is defined as the minimum drain to source voltage where the drain current approaches a constant value (saturation value). The point at which this pinch off voltage occurs is called as Pinch off point, denoted as B.
As VDS is further increased, the channel resistance also increases in such a way that ID practically remains constant. The region BC is known as saturation region or amplifier region. All these along with the points A, B and C are plotted in the graph below.
The drain characteristics are plotted for drain current ID against drain source voltage VDS for different values of gate source voltage VGS. The overall drain characteristics for such various input voltages is as given under.
N Channel Jfet Symbol
As the negative gate voltage controls the drain current, FET is called as a Voltage controlled device. The drain characteristics indicate the performance of a FET. The drain characteristics plotted above are used to obtain the values of Drain resistance, Transconductance and Amplification Factor.