Transistor viva Questions and Answers

• What is a transistor?

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit.

• Can you explain the different types of transistors?

There are two main types of transistors: Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET).

• • Bipolar Junction Transistors (BJT): These are made up of 3 layers of semiconductor material and can be either NPN or PNP type. They are called bipolar because they use both electrons and holes for conduction.
    • NPN Transistor: In this type, a thin layer of p-type semiconductor is sandwiched between two n-type semiconductors.
    • PNP Transistor: In this type, a thin layer of n-type semiconductor is sandwiched between two p-type semiconductors.
  • Field Effect Transistors (FET): These have a different structure and are divided into Junction Gate Field-Effect Transistors (JFETs) and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs).
    • Junction Gate Field-Effect Transistors (JFETs): These are the simplest type of FET and are sometimes just referred to as FETs. They can be used as a voltage controlled resistance or as an electronically controlled switch.
    • Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs): These are the most commonly used type of FETs. They have four terminals: source, gate, drain, and body. The voltage at the gate terminal controls the current flow from the source to the drain. MOSFETs are further classified into:
      • Depletion Mode MOSFET: In this type, the device is normally ON at zero gate voltage, and a gate voltage can deplete the device to turn it OFF.
      • Enhancement Mode MOSFET: In this type, the device is normally OFF at zero gate voltage, and a gate voltage can enhance the device to turn it ON.

• What is the function of the base in a transistor?

The base in a transistor is the terminal that controls the flow of current between the emitter and the collector. It is the region where the amplification action takes place.

• What is transistor biasing?

Transistor biasing is the process of setting a transistor’s DC operating voltage or current conditions to the correct level so that any AC input signal can be amplified correctly by the transistor.

• What is the difference between an NPN and a PNP transistor?

The main difference between an NPN and a PNP transistor is the direction of current flow. In an NPN transistor, current flows from the collector to the emitter, whereas in a PNP transistor, current flows from the emitter to the collector.

• What is the role of a transistor in a circuit?

In a circuit, a transistor can serve several different functions such as signal amplification, regulation of electrical signals, and acting as a switch.

• What is the significance of the Emitter, Base, and Collector in a transistor?

The Emitter is heavily doped and it emits or injects electrons into the base. The Base is lightly doped and it controls the number of electrons that can pass through to the collector. The Collector collects the electrons from the base.

• What is the operating point of a transistor and why is it important?

The operating point of a transistor, also known as the bias point, quiescent point, or Q-point, is the DC voltage or current at a certain point of a transistor circuit. It’s important because it’s the point around which the signal varies.

• What is transistor saturation?

Transistor saturation is the state in which both junctions are forward-biased. In this state, the transistor allows maximum current to flow through it.

• What is transistor cut-off?

Transistor cut-off is the state in which both the junctions are reverse biased. In this state, the transistor behaves like an open switch and does not allow any current to flow through it.

• What is the difference between a transistor and a diode?

A diode is a two-terminal electronic component that conducts current primarily in one direction; it has low resistance in one direction, and high resistance in the other. A transistor, on the other hand, is a three-terminal device. It’s used to amplify or switch electronic signals and electrical power, thus it can control the output.

• What is a Darlington Transistor?

A Darlington transistor is a compound structure of a particular design made by two bipolar transistors connected in such a way that the current amplified by the first transistor is amplified further by the second one. This configuration gives a much higher current gain than each transistor taken separately.

• What is a Transistor Transconductance?

Transconductance is the parameter of a transistor, given the symbol ‘gm’. It is defined as the change in the output current (drain current for FETs, or collector current for BJTs) divided by the small change in the input voltage (gate-source voltage for FETs, or base-emitter voltage for BJTs) which is causing it.

• What is the Early effect in transistors?

The Early effect in transistors, also known as base-width modulation, is the variation in the width of the base in a BJT due to a variation in the applied base-to-collector voltage.

• What is the difference between enhancement type and depletion type MOSFET?

In an enhancement type MOSFET, the channel is established by the application of a voltage to the gate. In a depletion type MOSFET, the channel is established by doping, and the application of a gate voltage can deplete this channel.

• What is a power transistor?

A power transistor is a type of transistor that is used to amplify or switch electronic signals but with higher power ratings. They are used in power amplification, regulation, and electrical switching applications.

• What is a phototransistor?

A phototransistor is a type of transistor that uses light to control the flow of current. It is essentially a regular transistor with a built-in photodiode.

• What is the difference between a JFET and a MOSFET?

The main difference between a JFET and a MOSFET is the way they control the flow of current. A JFET uses a junction (a PN junction) to control the flow of current, while a MOSFET uses a metal-oxide-semiconductor structure for this purpose.

• What is the Miller effect in transistors?

The Miller effect in transistors is an increase in the equivalent input capacitance of an amplifier due to amplification of the effect of input capacitance by the amplifier gain.

• What is the role of a transistor in a digital circuit?

In a digital circuit, transistors are used as basic building blocks to create logic gates which perform boolean algebra. These gates are then used to create more complex digital circuits and systems.