The use of semi-conductors in place of mechanical switches is what makes a circuit “electronic,” because they enable electrical signals to be switched at extremely high speeds, which is not possible with mechanical circuits. There are many different semi-conductor.
DIODE:
Like a one-way valve for electrical current, this device enables only electrical current to pass through it in one direction–extremely useful by itself, but also the basis for all solid state electronics.
LIGHT EMITTING DIODE (LED):
This type of diode emits a small amount of light when electrical current passes through it.
LIGHT DEPENDENT RESISTOR (LDR):
This type of semi-conductor has a changing resistance, depending on the amount of light present.
BIPOLAR JUNCTION TRANSISTOR (BJT):
This is a current-driven electronic switch used for its fast switching properties.
METAL-OXIDE SEMICONDUCTOR FIELD-EFFECT TRANSISTOR (MOSET):
This is a voltage-driven electronic switch used for its fast switching properties, low resistance, and capability to be operated in a parallel circuit. These are the basis for most power amplifier circuits.
These devices all have multiple layers of positively and negatively charged silicon attached to a chip with conductive metal leads exposed for soldering into the circuit. Some transistors and MOSFETS have built-in diodes to protect them from reverse voltages and Back-EMF.