## Field Effect TransistorFET or Let's discuss the ## StructureThe Junction Field Effect Transistor has three terminals, ## Terminals of JFETThe three terminals of JFET are as follows: ## SourceThe source is the terminal at the bottom of the transistor, as shown above. The majority carriers' ## DrainA drain terminal is present at the top of the transistor. The majority carriers' electrons (in the case of n-channel) and holes (in the case of p-channel) exit from the transistor through the drain terminal. The current generated due to the majority charge carriers through this terminal is the The direction of current in JFET is determined in the opposite direction to the flow of electrons and in the same direction to the flow of holes. The voltage between the drain and source is known as V ## GateGate consists of the The voltage between the gate and source is known as V ## ChannelThe part of the transistor where the n-type doping is present in large amounts is known as the channel. It is defined as the region between the two gate terminals that possess the movement of majority carriers from the source to the drain. ## VoltagesThere are two voltage sources V
It is the voltage across the terminals D and S, i.e., drain and source. The connection of V It can be calculated as: V
It is the voltage across the terminals G and S, i.e., gate and source. It is also known as the ## Types of JFETJFET is further categorized as n-channel JFET and p-channel JFET. The working of both the JFET are similar expect the majority carriers. ## N-channel JFETN-channel refers to the JFET with center channel doped with the N-type or Pentavalent impurities and the two side slabs doped with the P-type or trivalent impurities.
The structure of NPN JFET is shown below: Here, D represents the Drain terminal, S represents the source, and G represents the gate terminal. V The (+) terminal of the V Let's discuss its working. ## Working- When the voltage across the V
_{DS}is switched ON, the electrons flow towards S. The gate voltage remains at zero volts. The electrons (majority carriers) of the n-channel start moving towards the D. It is because same charge repels each other. - Some electrons deposits on the two p-type slabs at the corner, because opposite charge attracts. The depletion region is formed at the two slabs, as shown below:
- The electrons will flow towards the D through the space between the two depletion regions. It results in the current from the D to S.
- When the voltage across the V
_{GS}is switched ON, the more charge is provided at the gate terminals that increase the depletion width of the two p -type layers. It is shown below: - The flow of electrons from S decreases to the reduced gap between the depletion regions.
- The decrease in the voltage at V
_{DS}and V_{GS}also reduces the width of the depletion region. - The large negative bias will further increase the depletion width and the current starts decreasing due to the block of movement of electrons towards the drain. It is shown below:
## P-channel JFETP-channel refers to the JFET with center channel doped with the P-type impurities and the two side slabs doped with the N-type or Pentavalent impurities.
The structure of PNP JFET is shown below: The working of the p-channel JFET is the same as that of the n-channel except for the majority charge carriers in both the Field-Effect Transistors. The charge carriers constituting the current flow in the p-channel are holes, and in the n-channel are electrons. The concept of the connection of the voltages across the three terminals is already discussed above. The depletion width in the p-channel after applying the voltage across the G, D, and S terminals will appear as: The (+) terminal of the V ## Note: The connection of both the JFET is opposite due to the difference of majority carriers' concentration in n-channel and p-channel.## JFET Formula- The formula to calculate the drain current is given by:
**I**_{D}= I_{DSS}(1 - V_{GS}/V_{P})^{2} Where, V_{P}is the pinch-off voltage I_{DSS}is the drain current only, but in the case of zero bias The drain can also be calculated using the formula given by:**I**_{D}= (V_{DD}- V_{D}) / R_{D}**V**is also known as V_{DD}_{GS}. - The formula to calculate the R
_{GS}(G - S resistance) is given by:**R**_{GS}= V_{GS}/ I_{G} - The transconductance of JFET is given by:
**G**_{m}= ? I_{D}/ ? V_{GS} - The drain resistance (R
_{D}) of JFET is given by:**R**_{D}= 1/ Gm = ? V_{GS}/ ? I_{D} - The amplification factor (u) of JFET is defined as the ratio of change in voltages of V
_{DS}and V_{GS}. It is given by:**U = ? V**_{DS}/ ? V_{GS} - The pinch-off voltage can be calculated using the formula:
**V**_{P}= V_{GS}- V_{DS}**V**_{P}= qN_{D}a^{2}/2ε
We will discuss two examples based on the above formula later in the topic. ## Pinch-off voltageIt is defined state of the transistor to turn OFF when the voltage falls below the threshold voltage V ## CharacteristicsHere, we will discuss the characteristics of the n-channel JFET. The characteristics with the D-S voltage on the X-axis with the drain current on the y-axis are shown below: Consider the below JFET circuit. ## Case 1:In case of V At a point, the charge concentration will be high at the drain regions. It is a ## Case 2:If V The reverse bias voltages across the gate increase the drain voltage, increasing the effective voltage across the G junction. The n-channel JFET requires the negative voltage at the S to allow electrons to move towards the D. The D junction requires the positive voltage to attract more electrons to constitute the current. The p-channel requires positive voltage at the S to allow holes to move towards the D. The D junction requires the negative voltage to attract more holes to constitute the current. We can use either end of the channel as the source or drain, and we only need to consider the polarities applied to both channels. ## Mathematical modelWe have already discussed that V Area of A = 2bw Where, 2b is the channel width W is the channel dimension perpendicular to the width The drain current based on the Ohm's law can be written as: I Putting the value of A = 2bw, we get: I Where, N We know ε = V So, I ## FET as a Voltage Variable ResistorFET can work as a VVR in a specific region, and voltage Variable Resistor is also known as ## FET vs. BJTLet's discuss the differences between the Field Effect Transistor and Bipolar Junction Transistor.
## Numerical ExamplesLet's discuss two examples of JFET.
Given: V We need to find the R We know, R Putting the values in the given formula, we get: R R R Or R
We know the formula to find the drain current is given by: I Putting the given values in the above equation, we get: I I I I Or I Thus, we can say that I Example 3: Find the pinch-off voltage of the n-channel silicon JFET with the following parameters: N A = 0.40um ε = 8.854 x 10 Solution: We know, the formula to calculate pinch-off voltage is given by: V ε q is the charge of an electron = 1.6 x 10 Putting the given values in the above equation, we get: V
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