Electronic Properties of Materials: Solid State Devices

A diode is essentially a pn junction formed by making a contact between a p-type and an n-type semiconductor. The beauty of a diode lies in the junction between the p-type and n-type semiconductors, which allows the current to flow more easily in one direction than the other. The current in the other direction is limited by a leakage current, which is usually very small in magnitude. Thus, the junction acts as a rectifier.

An LED (light emitting diode) is a pn junction diode made from a direct bandgap semiconductor in which the n-type semiconductor is more heavily doped than the p-type semiconductor. In an LED, the electrons and holes recombine to emit photons. This process is called as radiative recombination. LEDs emit photons only when "forward biased", and this is the reason why you connect the long leg of the LED to the positive (cathode) of the battery and the short leg to the negative (anode) of the battery.

A silicon solar cell is also a pn junction in which the n-type semiconductor is more heavily doped than the p-type and is very narrow in width. The n-region is exposed to the sunlight (or the n-region is illuminated), which generates electron-hole pairs that give rise to the photocurrent.

A transistor is a three-terminal solid state device in which the current flowing between two terminals can be manipulated by varying the voltage between the third and one of the other two terminals. Transistors have the ability to provide current and voltage gains, which can be used to amplify weak signals (for example, a common base amplifier can be used for voltage amplification). Thus, transistors are used as switches. There are two major types of transistors:
  • bipolar junction transistor (BJT)
  • field effect transistor (FET)