We also applied a voltage across the Drain and Source. In the above image, a JFET is biased through a variable DC supply, which will control the V GS of a JFET. JFET only works in the depletion mode, whereas MOSFETs have depletion mode and enhancement mode. For a P-type JFET, we need to provide positive V GS. If we want to switch off a JFET we need to provide a negative gate to source voltage denoted as V GS for an N-type JFET. In the below image we can see the saturation mode and pinch off mode and we will be able to understand the depletion layer became wider and the current flow becomes less. But the reverse thing happens when a voltage is applied between gate and source in reverse polarity, that makes the P-N junction reversed biased and makes the channel narrower by increasing the depletion layer and could put the JFET in cut-off or pinch off region. When there is no voltage across gate and source, the channel becomes a smooth path which is wide open for electrons to flow. If we interchange the hose with a JFET and the water flow with a current and then construct the current-carrying channel, we could control the current flow. If we squeeze the hose the water flow will be less and at a certain point if we squeeze it completely there will be zero water flow. Suppose a garden hose is providing a water flow through it. One best example to understand the working of a JFET is to imagine the garden hose pipe. A P-N junction is formed in one or both side of the channel. In the long channel of semiconductor material, Ohmic contacts at each end are created to form the Source and Drain connections. Depending on the construction process, if the JFET contains a great number of positive charge carriers (refers as holes) is a P-type JFET, and if it has a large number of negative charge carriers (refers as electrons) is called N-type JFET. JFET is constructed using the long channel of semiconductor material. The N-Channel JFET consists of P-type material in N-type substrate whereas N-type materials are used in the p-type substrate to form a P channel JFET. In the above image, we can see the basic construction of a JFET. For the N channel JFET, the Gate voltage is negative and for the P channel JFET the Gate voltage is positive. The current flowing through the Drain and Source is dependable on the voltage applied to the Gate terminal. Interestingly, an English mnemonic is this, that arrow of an N- Channel device indicates “Points i n”. This arrow also indicates the polarity of P-N junction, which is formed between the channel and the gate. The arrow showing to the gate denotes that the JFET is N-channel and on the other hand the arrow from the gate denotes P-channel JFET. N channel JFET and P channel JFET schematic model are shown in the image above. Same like MOSFET it has two subtypes- N Channel JFET and P Channel JFET. We already discussed about MOSFET in previous tutorial, here will learn about JFET. There are different types of Transistor, in FETs family, there are two subtypes: JFET and MOSFET.
It also provides very high input impedance which is a major advantage over a BJTs. JFET provides low power consumption and fairly low power dissipations, thus improving the overall efficiency of the circuit. It is also an energy efficient version to replace the BJTs. We can use JFET as voltage controlled resistors or as a switch, or even make an amplifier using the JFET. JFET is an essential component for precision level voltage operated controls in analog electronics. Same like MOSFETs, as we have seen in our previous tutorial, JFET has three terminals Gate, Drain, and Source. Normal transistor is a current controlled device which needs current for biasing, whereas JFET is a voltage controlled device. JFET is Junction gate field-effect transistor.
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