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Electronics Tutorial

Transistor

At its core, a transistor is a three-layer semiconductor device consists of emitter , base and collector, typically made of silicon.

Modern electronics rely heavily on transistors, which are essential semiconductor devices. It serves as an amplifier, a switch, and a signal modulator. Transistors are found in nearly every electronic device, from smartphones and computers to televisions and radios. Here’s everything you need to know about transistors:

There are two main types of transistor NPN and PNP.

 

NPN Transistor

This type consists of three layers: N (Negative) – P (Positive) – N (Negative). The middle layer is thinner than the outer layers and it is lightly doped while the outer layers are heavily doped. It is used for amplification and switching.

npn transistor

Basic Principle

  • When a small current flows from base to emitter, it allows electrons to flow from emitter(E) to base(B).
  • Then the electrons diffuse through the base(B) towards the collector(C) terminal.
  • Then the collector(C) terminal collects the electrons and allows a much larger current of the electrons to flow from collector(C) to emitter(E).

Modes of Operation

Active Mode (amplification)
  • Base-Emitter Junction: In this mode, a small forward bias voltage is applied across the base-emitter junction of the NPN transistor. This biasing results in a small base current (IB) flowing into the base terminal.
  • Collector-Base Junction: The collector-base junction is reverse-biased, which prevents significant current flow from collector to base.
  • Transistor Action: The small base current (IB) controls a much larger collector current (IC). This relationship between IB and IC makes NPN transistors suitable for amplification.
  • Purpose: The active mode is the primary mode of operation for NPN transistors and is used for amplifying weak input signals in various electronic applications.
Cut Off Mode (switch off)
  • Base-Emitter Junction: In the cutoff mode, there is no forward bias applied to the base-emitter junction, meaning the base current (IB) is essentially zero.
  • Collector-Base Junction: The collector-base junction is typically reverse-biased.
  • Transistor State: With no base current, there is virtually no collector current (IC).
  • Purpose: The cutoff mode turns the transistor off, effectively making it behave like an open switch
Saturation Mode (switch on)
  • Base-Emitter Junction: In saturation mode, a sufficiently large forward bias voltage is applied across the base-emitter junction, resulting in a significant base current (IB).
  • Collector-Base Junction: The collector-base junction may be forward-biased or only slightly reverse-biased.
  • Transistor State: In this mode, the transistor is fully turned on, allowing the maximum possible collector current (IC) to flow.
  • Purpose: Saturation mode is used when you want the transistor to act as a closed switch, allowing maximum current flow between collector and emitter.

In summary, the mode of operation of an NPN transistor is determined by the biasing conditions applied to its terminals, particularly the base-emitter junction. Proper biasing is crucial to ensure the transistor functions as an amplifier or a switch, as required by the specific circuit or application.

PNP Transistor

This type also has three layers: P (Positive) – N (Negative) – P (Positive). The middle layer is thinner than the outer layers and it is lightly doped while the outer layers are heavily doped. Like the NPN transistor, it can be used for amplification and switching but with opposite polarities.

pnp transistor

Basic Principle

  • When a small current flows from emitter(E) to base(B), this flow of current consists of “holes” moving from P type emitter(E) to N type base(B).
  • Then the “holes” diffuse through the base(B) towards the collector(C) terminal.
  • Then the collector(C) terminal collects the holes and allows a much larger current to flow from collector(C) to emitter(E).

Modes of Operation

A PNP transistor operates in modes similar to those of an NPN transistor, but with differences in the direction of current flow and biasing. The primary modes of operation for a PNP transistor are the active mode (amplification mode) and the cutoff mode. Like NPN transistors, there is also a saturation mode, which is less commonly used. Here’s an overview of each mode of operation for a PNP transistor:

Active Mode (amplification)
  • Base-Emitter Junction: In the active mode, a small forward bias voltage is applied across the base-emitter junction of the PNP transistor. This biasing results in a small base current (IB) flowing out of the base terminal.
  • Collector-Base Junction: The collector-base junction is reverse-biased, which prevents significant current flow from base to collector.
  • Transistor Action: The small base current (IB) controls a much larger collector current (IC), just as in an NPN transistor. This characteristic makes PNP transistors suitable for amplification.
  • Purpose: The active mode is the primary mode of operation for PNP transistors and is used for amplifying weak input signals in various electronic applications
Cut Off Mode (switch off)
  • Base-Emitter Junction: In the cutoff mode, there is no forward bias applied to the base-emitter junction of the PNP transistor, resulting in an essentially zero base current (IB).
  • Collector-Base Junction: The collector-base junction may be reverse-biased.
  • Transistor State: With no base current, there is virtually no collector current (IC).
  • Purpose: The cutoff mode turns the PNP transistor off, effectively making it behave like an open switch.
Saturation Mode (switch on)
  • Base-Emitter Junction: In saturation mode, a sufficiently large forward bias voltage is applied across the base-emitter junction, resulting in a significant base current (IB) flowing out of the base terminal.
  • Collector-Base Junction: The collector-base junction may be forward-biased or only slightly reverse-biased.
  • Transistor State: In this mode, the PNP transistor is fully turned on, allowing the maximum possible collector current (IC) to flow, just like in the NPN saturation mode.
  • Purpose: Saturation mode is used when you want the PNP transistor to act as a closed switch, allowing maximum current flow between collector and emitter. It’s less common in amplifier applications but can be used in switching circuits.

In summary, the mode of operation of a PNP transistor is determined by the biasing conditions applied to its terminals, particularly the base-emitter junction. Proper biasing is essential to ensure the PNP transistor functions as an amplifier or a switch, as required by the specific circuit or application.

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