BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) PDF Download

The Field Effect Transistor (FET)

  • FETs is a three-terminal device like the BJT but operates by a different principle. 
  • The three terminals are called the source, drain, and gate
  • The voltage applied to the gate controls the current flowing in the source-drain channel. No current flows through the gate electrode, thus the gate is essentially insulated from the source-drain channel. 
  • Because no current flows through the gate, the input impedance of the FET is extremely large (in the range of 1010–1015 Ω). The large input impedance of the FET makes them an excellent choice for amplifier inputs. 
  • The two common families of FETs, the junction FET (JFET) and the metal oxide semiconductor FET (MOSFET) differ in the way the gate contact is made on the source-drain channel. 
  • In the JFET the gate-channel contact is a reverse biased pn junction. The gate-channel junction of the JFET must always be reverse biased otherwise it may behave as a diode. 
  • All JFETs are depletion mode devices—they are on when the gate bias is zero (VGS = 0). 
  • In the MOSFET the gate-channel contact is a metal electrode separated from the channel by a thin layer of insulating oxide. MOSFETs have very good isolation between the gate and the channel, but the thin oxide is easily damaged (punctured!) by static discharge through careless handling. 
  • MOSFETs are made in both depletion mode (on with zero biased gate, VGS = 0) and in enhancement mode (off with zero biased gate). 

In this Document we will focus on JFETs. 

Schematic Symbols

  • Two versions of the symbols are in common use. The symbols in the top row depict the source and drain as being symmetric. This is not generally true. 
  • Slight asymmetries are built into the channel during manufacturing which optimize the performance of the FET. Thus, it is necessary to distinguish the source from the drain. 
  • In this EduRev document, we will use the asymmetric symbols found on the bottom row, which depict the gate nearly opposite the source. The designation n-channel means that the channel is n doped and the gate is p doped. The p-channel is complement of n-channel.

BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Common Nomenclature (n-channel FET example)

BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Regions of JFET Operation


  • Cut-off region: The transistor is off. There is no conduction between the drain and the source when the gate-source voltage is greater than the cut-off voltage. (I= 0 for VGS > VGS,off
  • Active region (also called the Saturation region): The transistor is on. The drain current is controlled by the gate-source voltage (VGS) and relatively insensitive to VDS. In this region the transistor can be an amplifier.
    BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)
  • Ohmic region: The transistor is on, but behaves as a voltage controlled resistor. When VDS is less than in the active region, the drain current is roughly proportional to the source-drain voltage and is controlled by the gate voltage. BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Common Specifications

IDSS is the drain current in the active region for VGS = 0. (ID source shorted to gate) 

VGS,off is the minimum VGS where ID = 0. VGS,off is negative for n-channel and positive 

for p-channel.

gm is the transconductance, the change in ID with VGS and constant VDS

BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)


Common Circuit Applications


Voltage Controlled Switch. For the on state the gate voltage VGS = 0 and for the off

state |VGS| > |VGS,off| (of great magnitude than VGS,off and with the same sign). The sign of 

the voltage depends on the type of FET, negative for n-channel and positive for p-channel. 

BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Current Source: The drain current is set by RS such that VGS = IDRS. Any value of 

current can be chosen between zero and IDSS (see the ID vs VGS graph for the JFET).

BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)


Source Follower: The simple source follower is shown below. The improved version is shown at the right. The lower JFET forms a current source. The result is that VGS is held constant, removing the defects of the simple circuit. 

BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Voltage-Controlled Resistor: VGS must be between zero and VGS,off.

BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

JFET Diode: The JET pn gate junction can be used as a diode by connecting the source and the drain terminals. This is done if very low reverse leakage currents are required. The leakage current is very low because the reverse leakage current scales with the gate area. Small gate areas are designed into JFETs because it decreases the gate-source and the gate-drain capacitances.

BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

The document BJT & FET - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Electrical Engineering SSC JE (Technical).
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FAQs on BJT & FET - 2 - Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

1. What is the schematic symbol for a field effect transistor (FET)?
Ans. The schematic symbol for a field effect transistor (FET) typically consists of three elements: a vertical line representing the channel, an arrow pointing inward or outward indicating the type of FET (N-channel or P-channel), and a gate terminal connected to the channel.
2. What is the common nomenclature used for an n-channel FET?
Ans. The common nomenclature used for an n-channel FET includes terms such as "enhancement mode" or "depletion mode" to describe its behavior, as well as the specific type of FET, such as MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) or JFET (Junction Field Effect Transistor).
3. What are some common specifications of FETs?
Ans. Common specifications of FETs include parameters such as maximum drain current (ID), maximum voltage ratings (VDS and VGS), transconductance (gm), threshold voltage (Vth), and input and output capacitances (Ciss, Coss, Crss). These specifications determine the performance and operating characteristics of the FET in different circuit applications.
4. What are some common circuit applications of FETs?
Ans. FETs find extensive use in various circuit applications, including amplifiers, switches, oscillators, voltage regulators, and voltage-controlled devices. They are particularly suitable for applications that require high input impedance, low power consumption, and low noise characteristics.
5. What are some frequently asked questions about the differences between BJT and FET?
Ans. 1. What is the main difference between a BJT (Bipolar Junction Transistor) and a FET (Field Effect Transistor)? - The main difference lies in the way they control the flow of current. BJT controls current using both majority and minority carriers, while FET controls current using an electric field. 2. Which transistor type, BJT or FET, is more suitable for high-frequency applications? - FET is generally more suitable for high-frequency applications due to its high input impedance and lower capacitances, which result in better high-frequency response. 3. Are BJT or FET more prone to thermal runaway? - BJTs are more prone to thermal runaway compared to FETs. This is because BJTs have positive temperature coefficients, which means their current increases with temperature, potentially leading to thermal instability. 4. Which transistor type, BJT or FET, has a higher gain? - BJTs typically have higher current gain (hFE or β) compared to FETs. However, FETs can have higher voltage gain due to their high input impedance. 5. Can BJTs and FETs be used interchangeably in all circuit applications? - No, BJTs and FETs have different characteristics and operating principles, so they cannot be used interchangeably in all circuit applications. The choice between them depends on the specific requirements and constraints of the circuit.
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