Junction Field Effect Transistor Important Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. A JFET can operate in

1. Only enhancement mode
2. Only depletion mode
3. Neither depletion nor enhancement modes
4. Both depletion and enhancement modes

Ans. The correct answer is b. Only depletion mode

Explanation: JFETs are only capable of operating in depletion mode. Depletion-mode JFETs are normally on, meaning that they conduct current with zero gate bias.

Ques. The characteristics of JFET are similar to

1. SCR
2. Pentode
3. MOSFET
4. Triode

Ans. The correct answer is b. Pentode

Explanation: JFETs and pentodes are both unipolar field-effect devices, meaning that their operation is based on the movement of charge carriers of a single type (either electrons or holes) in an electric field.

Ques. Comparing the size of BJT and FET, choose the correct statement.

1. BJT is smaller than the FET
2. BJT is larger than the FET
3. Depends on the application
4. Both are of the same size

Ans. The correct answer is b. BJT is larger than the FET

Explanation: BJTs are typically much larger than FETs. This is because BJTs require a thicker base region in order to achieve sufficient current gain. FETs, on the other hand, can be made much smaller because they do not require a base region.

Ques. What is the value of current when the gate-to-source voltage is less than the pinch-off voltage?

1. 100 A
2. 1 A
3. 0 A
4. 5 A

Ans. The correct answer is c. 0 A

Explanation: When the gate to source voltage is less than the pinch-off voltage, both junctions are reverse biased, and no current flows.

Ques. Which of the following statements is true about FET?

1. It has a high input impedance
2. It has a high output impedance
3. It has a low input impedance
4. It does not offer any resistance

Ans. The correct answer is a. It has a high input impedance

Explanation: The high input impedance of FETs is due to the insulating layer of silicon dioxide (SiO₂) that is located between the gate and the channel. This insulating layer prevents current from flowing between the gate and the channel, which means that the gate draws very little current from the input signal source.

Ques. FET is a voltage-controlled device.

1. True
2. False

Ans. The correct answer is a. True

Explanation: The Field Effect Transistors are voltage-controlled devices; the drain current can be controlled by applying a voltage between the gate and the source. To control the operations of a FET, the gate to drain voltage is changed to operate the FET in different operating zones.

Ques. What is the main advantage of FET which makes it more useful in industrial applications?

1. Semiconductor device
2. Less cost
3. Voltage-controlled operation
4. Small size

Ans. The correct answer is d. Small size

Explanation: Because of their small size, IC chips can be made even smaller, reducing wear and tear. The FET is made more advantageous by using process technology constant a, which is the ratio of Width and Length.

Ques. For a p-channel FET, What is the direction of current flow?

1. Gate to source
2. Source to drain
3. Drain to source
4. Gate to drain

Ans. The correct answer is b. Source to drain

Explanation: When the voltage is less than the pinch-off, current flows from the source to the drain. The forward bias drain and gate are responsible for the electron flow from drain to source; because conventional current flows in the opposite direction of electron flow, current will flow from source to drain.

Ques. For an n-channel FET, What is the direction of the current flow?

1. Gate to drain
2. Source to drain
3. Gate to source
4. Drain to source

Ans. The correct answer is d. Drain to source

Explanation: When a voltage larger than the pinch-off is applied, current flows from the drain to the source.

Ques. To use FET as a voltage-controlled resistor, in which region it should operate?

1. Ohmic region
2. cut-off and saturation
3. Saturation
4. Active region

Ans. The correct answer is a. Ohmic region

Explanation: A FET should operate in the ohmic zone when used as a voltage-controlled resistor. The drain current is directly proportional to the drain-source voltage in the ohmic zone. This indicates that the FET's resistance can be controlled by changing the voltage given to the gate.

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Short Answers Questions [2 Marks Questions]

Ques. What is meant by junction field effect transistor?

Ans. The Junction Field Effect Transistor is a unipolar device in which the action of an electric field at a reverse-biased pn-junction controls current flow between its two electrodes. It is a three-terminal unipolar voltage-controlled semiconductor device that is available in N-channel and P-channel configurations.

Ques. What are the two types of junction field effect transistors?

Ans. Junction field effect transistors are classified into two types based on the current flow source. These are

• n-channel junction field effect transistor (n-channel JFET)
• p-channel junction field effect transistor (p-channel JFET)

Ques. What is a field-effect transistor?

Ans. A field-effect transistor is a type of transistor that controls the flow of current in a semiconductor by using an electric field.

Ques. What are the types of field effect transistors?

Ans. There are two types of field effect transistors (FETs)

• Junction field effect transistor or JFET
• Metal oxide semiconductor field effect transistor or MOSFET

Ques. What is a semiconductor?

Ans. Semiconductors are materials that have conductivity between conductors, which are often metals, and non-conductors or insulators, such as ceramics. Gallium arsenide, cadmium selenide, silicon, and germanium are a few examples.

Ques. How does the depletion layer become broader and narrower in JFET?

Ans. The applied gate voltage directly affects the width of the depletion layer. When the gate voltage of an n-channel JFET increases in the negative direction, the depletion layer expands, reducing the effective channel width and, as a result, the drain current. When gate voltage approaches zero, the depletion layer reduces, allowing more charge carriers to flow across the channel and increasing the drain current.

Ques. How many diodes do junction field effect transistors (JFET) contain?

Ans. The junction field effect transistor contains two diodes. One diode formed between the gate and the channel region, and another diode formed between the source and the drain region. This diode is called the parasitic diode.

Ques. When was the junction field-effect transistor invented?

Ans. In 1953, the first working model of junction field-effect transistors was developed.

Ques. What is a transistor?

Ans. A transistor is a semiconductor device that is used to increase or switch electrical signals and power. It is a basic component of modern electronics. It is typically made of semiconductor material and has at least three terminals for connecting to an electronic circuit.

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Long Answers Questions [3 Marks Questions]

Ques. What are the advantages of junction field effect transistors?

Ans. The advantages of junction field effect transistors are

• The input impedance of the JFET is very high.
• The JFET can be fabricated in a small size area.
• Because it is a majority charge carrier device, it has less noise.
• It has a low power consumption.
• Because of its reduced size, it takes up less space in circuits.
• Because it has a negative temperature coefficient of resistance, it provides greater temperature stability.

Ques. What are the disadvantages of junction field effect transistors?

Ans. The following are the disadvantages of junction field effect transistors

• The main drawback of junction field effect transistors (JFETs) is their relatively low gain bandwidth product.
• Due to feedback from internal capacitance, JFET performance decreases as frequency increases.
• Because transconductance is low, voltage gain is also low.
• When compared to BJT, FET has slower switching times. High delay times are caused by the internal junction capacitance of FETs.

Ques. Why is FET preferred over BJT?

Ans. FET is preferred over BJT due to the following reasons

• When compared to BJTs, field-effect transistors have a higher input impedance. The gain of FETs is lesser as compared with BJTs.
• FETs produce less noise.
• FET has a lower radiation effect.
• Because the offset voltage of a FET is zero at zero drain current, it makes a good signal chopper.
• FETs have a higher thermal stability.

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Very Long Answers Questions [5 Marks Questions]

Ques. What are the applications of junction field effect transistors?

Ans. The following are the applications of junction field effect transistors

• The JFET is used as a constant current source.
• The JFET is used as a high-impedance wide-band amplifier.
• The JFET is used as a buffer amplifier.
• It is used as a High Input Impedance Amplifier.
• The JFET is used as an electronic switch.
• The JFET is used as a phase shift oscillator.
• The JFET is used as a chopper.
• It is used as a chopper.
• It is used as a voltage variable resistor (VVR) or voltage development resistor (VDR).

Ques. What is the difference between JFET and MOSFET?

Ans. The following are the differences between JFET and MOSFET

Ques. Describe the working of junction field effect transistors.

Ans. When there is no voltage applied across the source and gate, the channel acts as an open passage for carriers to travel through. 

• When the bias is reversed, the channel narrows by increasing the depletion layer, putting the JFET in the cut-off or pinch-off region.
• Maximum saturation current will flow through the channel and be limited by the small depletion region surrounding the junctions when no Gate voltage is supplied and a small voltage is put between the Drain and the Source.
• When a small negative voltage is supplied to the Gate, the size of the depletion zone expands, reducing the total effective area of the channel and hence decreasing the current flowing through it. 
• When a reverse bias voltage is applied, the width of the depletion region expands, causing the conduction current to decrease through the channel.
• A small current flows into the gate connection when the PN junction is reverse-biased. 
• When the Gate voltage is reduced to a negative value, the channel width decreases until no current flows through it, and the FET is said to be "pinched-off." 
• The voltage at which the channel nearly closes is referred to as the "pinch-off voltage."

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