Transistors

Transistors

Definition and Basics

  • A transistor is a semiconductor device that has three terminals and two P-N junctions.

  • The junctions resemble those found in diodes, but the operation of a transistor is much more complex.

  • Understanding the interactions between the transistor's terminals requires knowledge of quantum physics, which extends beyond basic Systems Engineering.

Types of Transistors

  • There are two primary types of transistors:

    • PNP Transistor: N-type material is sandwiched between two P-type materials.

    • NPN Transistor: P-type material is sandwiched between two N-type materials.

Operational Modes

Switch or Amplifier

  • A transistor can function in two modes:

    • As a Switch: It simply turns current on or off.

    • As an Amplifier: It varies current flow.

Terminal Descriptions

  • Base:

    • The central layer of material in the transistor.

    • Voltages applied to the base control the current flow through the collector and emitter.

  • Collector:

    • The outer layer that collects electrons emitted by the emitter.

    • Represented by a diagonal line in circuit symbols.

  • Emitter:

    • The outer layer that emits electrons when a proper voltage signal is applied to the base.

    • Represented by an arrow in circuit diagrams indicating the direction of current flow.

Transistor as a Switch

NPN Transistor as a Low Side Switch

  • When the voltage at the base exceeds the threshold (commonly 0.6V), the transistor enters saturation, resembling a short circuit between collector and emitter.

  • If the base voltage is less than 0.6V, the transistor is in cutoff mode, acting as an open circuit between collector and emitter.

PNP Transistor as a High Side Switch

  • The emitter is connected to a high voltage while the collector is connected to the load, which is grounded.

  • To turn the load "on," the base voltage must be low (opposite behavior to the NPN switch).

Amplification Using Transistors

  • Transistors can amplify current, which allows them to process small output currents from integrated circuits (ICs) to control larger loads like lamps and relays.

  • Resistors are commonly included to convert the changing current to a changing voltage, thus also allowing for voltage amplification.

Darlington Pair

  • A Darlington pair consists of two connected transistors; the output current of the first is amplified by the second.

  • The overall current gain (β) is the product of the two individual gains:

    • eta{total} = eta1 imes eta_2

  • Offers a very high current gain (e.g., 10,000), with only a minimal base current needed to activate the switch.

  • To activate the Darlington pair, a minimum of 0.7V across both base-emitter junctions (in series) is needed, resulting in a required total of 1.4V.

Testing for Damage

  • Transistors can be damaged by excessive heat during soldering or by incorrect circuit usage.

  • Testing a potentially damaged transistor involves taking six readings:

    • The Base-Emitter (BE) junction should behave as a diode and conduct in one direction only.

    • The Base-Collector (BC) junction should also behave as a diode and conduct in one direction only.

    • The Collector-Emitter (CE) junction should not conduct in either direction.

Phototransistors

  • A phototransistor converts light energy into electric energy.

  • Capable only of producing current due to its bipolar semiconductor makeup, it focuses on energy passed through it.

  • Activation occurs through exposure to photons.

Applications of Phototransistors

  • Phototransistors have a broad range of applications:

    • Used in devices that sense light, such as smoke detectors and infrared receivers.

    • Utilized in CD players and various industrial applications, including astronomy, night vision, and laser range finding.