Learnabout Electronics

- Bipolar Junction Transistors (BJTs)

Module 3.4

BJT Animation

What you´ll learn in Module 3.4
  • After studying this section, you should be able to:
  • • Describe the basic operation of a silicon planar transistor.
  • • Understand the effect of varying the base/emitter and base/collector potentials.
  • • Describe the action of charge carriers in the transistor.


Using the Animation

Start the animation by either clicking Vce ON, which applies a positive voltage to the collector of the transistor, or use your mouse to drag the Vbe slider handle up to increase the voltage applied to the base. Applying more than about 0.6 volts will make the transistor base-emitter junction conduct.

When the transistor conducts, electrons are attracted from the emitter by holes appearing in the base. A few electrons will combine with holes to form base current, which is indicated by a few free electrons flowing around the base−emitter circuit.

With Vce turned on, the majority of free electrons in the base area are attracted across the collector−base depletion layer to form the large current flowing in the collector−emitter circuit.

As a small base current, caused by forward biasing the base−emitter junction has caused a large collector current to flow, the transistor is acting as a current amplifier.

bjt animation

Fig 3.4.1 How a Bipolar Junction Transistor Works

Experiment with different base voltages, and turn the collector voltage on and off. The "What´s Happening" box will describe the actions.

To identify the different parts of the transistor circuit click SHOW on the labels switch.

Non-Animated version of "How a BJT Works"

Download Full Screen Animation.

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