Semiconductors are substances that have their electrical conductivities lying between that of good conductors and insulators. Semiconductors, without any impurity, are called intrinsic semiconductors. Germanium and silicon are the most commonly used intrinsic semiconductors. Both Ge (atomic number 32) and silicon (atomic number 14) belong to the fourth group of the periodic table, and they are tetravalent.

At temperatures near absolute zero, pure Ge and Si behave like perfect insulators. But their conductivities increase with increase in the temperature. For Ge, the binding energy of an electron in the covalent bond is 0.7 eV. If this energy is supplied in the form of heat, some of the bonds are broken, and the electrons are set free.

At ordinary temperatures, some of the electrons are set free from the atoms of Ge or Si crystal, and they wander in the crystal. The absence of an electron at a previously occupied place implies a positive charge at that place. A “hole” is said to be created at the place where the electron is set free. A (vacant) hole is equivalent to positive charge and it has a tendency to accept an electron.

When an electron jumps to a hole, a new hole is produced at the place where the electron was previously. The motion of electrons in one direction is equivalent to the motion of holes in the opposite direction. Thus, in intrinsic semiconductors, holes and electrons are produced simultaneously, and both of them act as charge carriers.

There are two types of extrinsic semiconductors: n-type and p-type.

n-type semiconductor: Elements such as arsenic (As), antimony (Sb) and phosphorus (P) are pentavalent, while Ge and Si are tetravalent. If a small amount of antimony is added to the Ge or Si crystal, as an impurity, then out of its five valent electrons, four will form covalent bonds with neighboring Ge atoms. But the fifth electron of antimony becomes almost free to move in the crystal.

If a potential voltage is applied to the doped Ge-crystal, the free electrons in doped Ge will move towards the positive terminal, and the conductivity increases. Since the negatively charged free electrons increase the conductivity of doped Ge crystal, it is called an n-type semiconductor.

p-type semiconductor: If a trivalent impurity like indium, aluminium or boron (having three valence electrons) is added in a very small proportion to tetravalent Ge or Si, then three covalent bonds are formed with three Ge atoms. But the fourth valence electron of Ge cannot form a covalent bond with indium because no electron is left for pairing.

The absence or deficiency of an electron is called a hole. Each hole is regarded as a region of positive charge at that point. As the conductivity of Ge doped with indium is due to holes, it is called a p-type semiconductor.

Thus, n-type and p-type are the two types of semiconductors, and their uses are explained as follows: A p-type semiconductor and an n-type semiconductor are joined together, and the common interface is called a p-n junction diode.

A p-n junction diode is used as a rectifier in electronic circuits. A transistor is a three-terminal semiconductor device, which is made by sandwiching a thin slice of n-type material between two bigger pieces of p-type material, or a thin slice of p-type semiconductor between two bigger pieces of n-type semiconductor. Thus, there are two types of transistors: p-n-p and n-p-n. A transistor is used as an amplifier in electronic circuits.

A comparison between a semiconductor diode and a vacuum would give a more vivid glimpse about the advantages of semiconductors.

• Unlike vacuum diodes, there are no filaments in semiconductor devices. Hence, no heating is required to emit electrons in a semiconductor.
• Semiconductor devices can be operated immediately after switching on the circuit device.
• Unlike vacuum diodes, no humming sound is produced by semiconductors at the time of operation.
• Compared to vacuum tubes, semiconductor devices always need a low operating voltage.
• Because semiconductors are small in size, the circuits involving them are also very compact.
• Unlike vacuum tubes, semiconductors are shock-proof. Moreover, they are smaller in size and occupy less space and consume less power.
• Compared to vacuum tubes, semiconductors are extremely sensitive to temperature and radiation.
• Semiconductors are cheaper than vacuum diodes and have an unlimited shelf life.
• Semiconductor devices do not need a vacuum for operation. 

In summary, the advantages of semiconductor devices far outweigh those of vacuum tubes. With the advent of semiconductor material, it became possible to develop small electronic devices that were more sophisticated, durable and compatible.

The most common semiconductor device is the transistor, which is used to manufacture logic gates and digital circuits. The applications of semiconductor devices also extend to analog circuits, which are used in oscillators and amplifiers.

Semiconductor devices are also used in integrated circuits, which operate at a very high voltage and current. The applications of semiconductor devices are also seen in daily life. For example, high-speed computer chips are made from semiconductors. Telephones, medical equipment and robotics also make use of semiconductor materials.