The drain-source voltage (V DS) is plotted along the horizontal axis. JFET Output Characteristic CurvesĪ JFET characteristic curve shows the operating characteristics of a JFET. The power consumption of the JFET (in standby operation) is thousands of times less than that of a bipolar transistor controlling the same function. The major advantage of this is good control using voltage rather than current. In a JFET, the junction must be reverse biased so that the junction impedance is high and there is little current flow in the junction. In a bipolar transistor, the junction must be forward biased so that the junction impedance is extremely low and there is current flow in the junction. This is quite different from a bipolar transistor. This gate-to-source voltage (V GS) controls the drain current and must always provide a reverse-bias voltage. When VGS decreases, the depletion region decreases. When V GS increases, the depletion region increases. The size of the depletion region is controlled by the gate-to-source voltage (V GS). The depletion regions merge at a particular VGS of somewhere between 1 V and 8 V. If the gate is made negative with respect to the channel, the diodes (formed by the P-type and N-type materials) become reverse biased and the depletion regions increase.Īt a large enough gate-to-source voltage (VGS), the channel is effectively “ pinched off” because of the depletion regions touch. Each section has its associated electrical field (depletion region). The two sections of P-type material constitute the gate. With a slightly positive voltage, the channel height opens further, allowing maximum current flow. If a DC potential is connected between the source and the drain, the current should flow in the external circuit and through the channel.Īt zero gate voltage, channel height is maximum and channel resistance is minimum, resulting in current flow. This common material constitutes the channel of the JFET. The source and drain of the JFET are connected to a common N-type material. The output current of a JFET is controlled by an electrical field created by the input voltage. The JFET is considered a voltage-driven device rather than a current-driven device like the bipolar junction transistor.įigure 2. The gate voltage creates an electrical field, or depletion region, within the device. The output current of a JFET is controlled by the voltage on the gate. JFET OperationĪ JFET is a unipolar device, which differs in operation from a bipolar junction transistor. JFETs are fully compatible with other semiconductor devices, such as standard bipolar transistors, silicon- controlled rectifiers, triacs, and ICs. JFETs are even found in voltage regulators and current limiters. The arrow points outward for a P-channel JFET. The arrow in a schematic symbol for an N- channel JFET points inward. The gate is a control element, while the drain and source provide the same function as the emitter and collector on a bipolar junction transistor. The two types of JFETs include the N-channel and P-channel.Ī JFET, like all FETs, contains a gate (G), drain (D), and source (S). A junction field-effect transistor (JFET) is a simple FET with a PN junction in which output current is controlled by an input voltage.
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