Semiconductor Electronics/Semiconductor/Doping

Doping is the process of adding impurities to intrinsic semiconductors to alter their properties. Normally Trivalent and Pentavalent elements are used to dope Silicon and Germanium. When an intrinsic semiconductor is doped with Trivalent impurity it becomes a P-Type semiconductor. The P stands for Positive, which means the semiconductor is rich in holes or Positive charged ions. When we dope intrinsic material with Pentavalent impurities we get N-Type semiconductor, where N stands for Negative. N-type semiconductors have Negative charged ions or in other words have excess electrons in it.

How Doping Works




Atoms follow a rule called Octet Rule. According to Octet-rule atoms are stable when there are eight electrons in their valence shell. If not, atoms readily accept or share neighboring atoms to achieve eight electrons in their valence shell. In the silicon lattice, each silicon atom is surrounded by four silicon atoms. Each silicon atom shares one of its electron in the valence shell to its neighboring silicon atom to satisfy the octet-rule. A schematic diagram of an intrinsic semiconductor is shown in image right (Figure : Intrinsic Silicon Lattice).

When we pop in a pentavalent element into the lattice. As you can see the image (Figure : N-type), we have doped the silicon lattice with Phosphorous, a pentavalent element. Now pentavalent element has five electrons, so it shares an electron with each of the four neighbouring silicon atoms, hence four electrons are tied up with the silicon atoms in the lattice. This leaves an extra electron. This excess electron is free to move and is responsible for conduction. Hence N-type (Negative Type) intrinsic semiconductor (silicon in this case) is made by doping the semiconductor with pentavalent element.

To create a P-type semiconductor, all we must do is to pop in a trivalent element into the lattice. A trivalent element has three electrons in its valence shell. It shares three electrons with three neighboring silicon atoms in the lattice, the fourth silicon atom demands an electron but the trivalent atom has no more electron to share. This creates a void in lattice which we call it has hole. Since the electron is deficient, the hole readily accepts an electron, this makes it a P-type (Positive type) extrinsic semiconductor.

As you can see in the image above (Figure : P-type), we have popped in boron (trivalent element) in silicon lattice. This has created a hole making the semiconductor a P-type material.

The case is no different with Germanium. It behaves the same as silicon; however, some properties differ which makes germanium based devices better in certain applications, while silicon based devices are used in other applications.