Transistors are solid state electronic switches.  Solid state means that there are no moving parts, and the switching action is regulated by the arrangement of the semiconductor materials.

A transistor has three terminals:

  • Collector – takes current form the top rail (wire connected to the positive of the power supply).
  •  Emitter – delivers current to the load.
  • Base – this is connected to the sensor part of the circuit.  The increase in voltage at the base turns the transistor on.

  Click on the picture to find out how a transistor works.

Answer Question 1

Here is a typical circuit involving a transistor.  It is a light operated switch that uses a relay to turn on a mains-powered bulb.

  1. When the LDR is in the dark, its resistance is high.
  2. The LDR is part of a potential divider.
  3. Therefore there is a large voltage across the LDR.
  4. There is a voltage drop across the base of the transistor so that the base is at 0.7 V.
  5. The resistor R3 limits the current to the base.
  6. The transistor is turned on.
  7. It can allow a big enough current to flow to turn the relay on.
  8. The reverse biased diode D1 is there to protect the transistor from high voltage spikes that can occur when the relay turns off.  These could wreck the transistor.


The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a Voltage controlled device.    This means that a voltage at the gate means that a current flows from the drain to the source.

There are three terminals:

  • gate – connected to the input device.
  • drain – connected to the positive, since electrons drain away to the positive.
  • source – the source of the electrons

We can use MOSFETs where we have a source of voltage that can provide very little current.  This circuit is a touch sensor:

The general characteristics for a MOSFET are:

  • The input resistance is very high, about 10 12 W.
  • The output resistance is about the same as a bipolar transistor.  The actual value depends on the type.  For a signal MOSFET it would be in the range 10 to 50 kW, while in a power MOSFET it would be somewhat lower.

Here the MOSFET is used as a switch.  There are advantages and disadvantages when compared to the bipolar transistor as a switch:

Advantages of a MOSFET Disadvantages
Switching time is about 10 times faster than a bipolar transistor Higher resistance than a bipolar transistor
Very much smaller switching current Can be destroyed by high voltages, especially static electricity
Less affected by temperature