Relation between different currents in a transistor.
#FET TRANSISTOR LECTURE FULL#
Applications of diode: Half wave Rectifier and Full Wave Rectifier.It can be expressed as,Īmplification Factor (u) − It is the ratio of change in drain-source voltage (ΔV DS) to the change in gate source voltage (ΔV GS) constant drain current (ΔI D). Transconductance (g fs) − It is the ratio of change in drain current (ΔI D) to the change in gate source voltage (ΔV GS) at constant drain-source voltage. Parameters of JFETĪC drain resistance (R d) − It is the ratio of change in the drain source voltage (ΔV DS) to the change in drain current (ΔI D) at constant gate-source voltage. Therefore, drain current (I D) remains constant above pinch-off voltage. Above pinch-off voltage, the channel width becomes so narrow that it allows very small drain current to pass through it. Initially, the drain current (I D) rises rapidly with drain source voltage (V DS) however suddenly becomes constant at a voltage known as pinch-off voltage (V P). The output characteristics of JFET are drawn between drain current (I D) and drain source voltage (V DS) at constant gate source voltage (V GS) as shown in the following figure. With a small change in gate voltage, JFET can be controlled anywhere between full conduction and cutoff state. The size of the P-N junction depletion layer depends upon fluctuations in the values of reverse biased gate voltage. Normally for general operation, the gate terminal is made positive with respect to the source terminal. Rest of the construction details are similar to that of N- channel JFET. At the end of the channel and the gate, lead wires are attached. The gate is formed on top of the P channel with N type material. The following figure shows the crystal structure and schematic symbol of an N-channel JFET. It has a thin layer of P type material formed on N type substrate. Essentially, the amount of bias voltage applied at ID, controls the flow of current carriers passing through the channel of a JFET. It is clear that even when the gate is open full current conduction will take place in the channel. The amount of channel current flow will be determined by the value of V DD and the internal resistance of the channel.Ī typical value of source-drain resistance of a JFET is quite a few hundred ohms. The same amount of current will flow from the source and the drain terminals. When a DC voltage source is connected to the source and the drain leads of a JFET, maximum current will flow through the channel. At the end of the channel and the gate, lead wires are attached and the substrate has no connection. Then the gate is formed on top of the N channel with P type material. Following figure shows the crystal structure and schematic symbol of an N-channel JFET. It has a thin layer of N type material formed on P type substrate.
There are two types of JFETs commonly used in the field semiconductor devices: N-Channel JFET and P-Channel JFET. Source − It is the entry point for majority carriers through which they enter into the semiconductor bar.ĭrain − It is the exit point for majority carriers through which they leave the semiconductor bar.Ĭhannel − It is the area of N type material through which majority carriers pass from the source to drain. Gate − By using diffusion or alloying technique, both sides of N type bar are heavily doped to create PN junction. Following are some important points to remember about FET − Basically, JFETs consist of an N type or P type silicon bar containing PN junctions at the sides. The functioning of Junction Field Effect Transistor depends upon the flow of majority carriers (electrons or holes) only.
By appearance JFET and bipolar transistors are very similar.
Its operation is based on a controlled input voltage. A Field Effect Transistor (FET) is a three-terminal semiconductor device.
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