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The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers or holes (p-type), or of negative carriers or electrons (n-type). Ohmic contacts at each end form the source (S) and the drain (D). 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. In this video, the construction and working of n-channel JFET and p-channel JFET are explained. By watching this video, you will learn the following topics:1.
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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.
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.
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.
Jfet N-channel Characteristics
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.
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.
In this project, we will go over how to connect an N-Channel JFET to a circuit for it to function as an electronic switch to power on a load.
A JFET is a transistor which is normally on. This means that the JFET will conduct current across from the drain to the source without any voltage input into the gate terminal. JFETs don't need any biasing voltage at the base to turn on. It will conduct across without gate voltage. To turn off an N-Channel JFET, this is when we need to apply voltage- we apply sufficient negative voltage to the gate of the transistor to turn the JFET. This is why JFETs are referred to as being normally on.
The type of JFET we will use specifically in this circuit is a MPF102. This is a pretty popularN-Channel JFET.
The pinout for the MPF102 is shown below:
If you deal with BJTs often, it's easy to get into the habit of thinking that the middle pin is the gate, like for a BJT, the middle pin is normally the base, but many times for FETs, this is not the case. The gate can be in the middle or be the 3rd pin, so it's always best to check the datasheet for the pinouts of all JFETs. Otherwise, the circuit will be wiredwrong and it won't work.
So that you know which pin, we can go over all components needed for us to build this circuit.
Components Needed for JFET Switch Circuit
- MPF102 JFET
- LED
- 270Ω resistor
- DC Power Supply or 4 'AA' batteries
The MPF102 can be obtained from a number of different electronics online retailers, such as Tayda Electronics.
The datasheet for the MPF102 can be found at the following link: MPF102 N Channel JFET Datasheet.
In this circuit, we power on an LED. We use a 270Ω resistor to act as a current-limiting resistor to the LED.
You can really use any other output device you want such as a buzzer. It doesn't have to be an LED.
N-Channel JFET Switch Circuit Schematic
The schematic diagram of the N-Channel JFET Switch Circuit we will build is shown below.
This circuit is very basic in setup and in operation.
When there is sufficient voltage supplied to the LED and no voltage at the gate terminal, the JFET is fully operational and conducts across from drain to source, turning on and lighting the LED.
To stop current flow, all that is needed is sufficient negative voltage supplied to the gate terminal, about 3-4V. With 3 to 4 volts of negative voltage, the JFET will no longer conduct from drain to source and the LED will turn off.
If using batteries, 2 'AA' batteries (3V) should be enough to turn off the JFET. However, 3 'AA' will give 4.5Vwhich will definitely shut off the JFET, impeding all current flow. So you have the choice of using 2 or 3 batteries at the gate terminal to stop current flow.
N Jets
And this is how an N-Channel JFET works.
To see how this circuit works in real life, see the video below.
Related Resources
How to Connect a Transistor as a Switch in a Circuit
How to Connect a (NPN) Transistor in a Circuit
Types of Transistors
Bipolar Junction Transistors (BJTs)
Junction Field Effect Transistors (JFETs)
Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)
Unijunction Transistors (UJTs)
What is Transistor Biasing?
How to Test a Transistor
Jfet N Channel Symbol