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By the end of this section, you will be able to:
  • Describe what occurs when n- and p-type materials are joined together using the concept of diffusion and drift current (zero applied voltage)
  • Explain the response of a p-n junction to a forward and reverse bias voltage
  • Describe the function of a transistor in an electric circuit
  • Use the concept of a p-n junction to explain its applications in audio amplifiers and computers

Semiconductors have many applications in modern electronics. We describe some basic semiconductor devices in this section. A great advantage of using semiconductors for circuit elements is the fact that many thousands or millions of semiconductor devices can be combined on the same tiny piece of silicon and connected by conducting paths. The resulting structure is called an integrated circuit (ic), and ic chips are the basis of many modern devices, from computers and smartphones to the internet and global communications networks.

Diodes

Perhaps the simplest device that can be created with a semiconductor is a diode. A diode is a circuit element that allows electric current to flow in only one direction, like a one-way valve (see Model of Conduction in Metals ). A diode is created by joining a p -type semiconductor to an n -type semiconductor ( [link] ). The junction between these materials is called a p-n junction    . A comparison of the energy bands of a silicon-based diode is shown in [link] (b). The positions of the valence and conduction bands are the same, but the impurity levels are quite different. When a p-n junction is formed, electrons from the conduction band of the n -type material diffuse to the p -side, where they combine with holes in the valence band. This migration of charge leaves positive ionized donor ions on the n -side and negative ionized acceptor ions on the p -side, producing a narrow double layer of charge at the p - n junction called the depletion layer    . The electric field associated with the depletion layer prevents further diffusion. The potential energy for electrons across the p-n junction is given by [link] .

Figure a shows two blocks place side by side, in contact. The left one is labeled p and the right one is labeled n. Figure b shows a valence band at the bottom and a conduction band at the top. There are holes within the valance band on the left, labeled holes at the top of the valence band. There are electrons above the conduction line on the right, labeled electrons at the bottom of the conduction band. Impurity bands are shown above the holes and below the electrons.
(a) Representation of a p-n junction. (b) A comparison of the energy bands of p -type and n -type silicon prior to equilibrium.
Figure a shows two blocks place side by side, in contact. The left one is labeled p and the right one is labeled n. Minus signs are shown in the p block near the side in contact. Plus signs are shown in the n block near the side in contact. Figure b shows a valence band at the bottom and a conduction line at the top. The valence band is higher on the left side almost reaching the central line between the two bands. There are holes with the valence band at the top, on the left. The conduction line is lower on the right, almost reaching the central line between the two bands. There are electrons just above the line, on the right. The displacement of the bands is labeled eV subscript 0, potential difference prevents diffusion of electrons from n side to p side.
At equilibrium, (a) excess charge resides near the interface and the net current is zero, and (b) the potential energy difference for electrons (in light blue) prevents further diffusion of electrons into the p -side.

The behavior of a semiconductor diode can now be understood. If the positive side of the battery is connected to the n -type material, the depletion layer is widened, and the potential energy difference across the p-n junction is increased. Few or none of the electrons (holes) have enough energy to climb the potential barrier, and current is significantly reduced. This is called the reverse bias configuration    . On the other hand, if the positive side of a battery is connected to the p -type material, the depletion layer is narrowed, the potential energy difference across the p-n junction is reduced, and electrons (holes) flow easily. This is called the forward bias configuration    of the diode. In sum, the diode allows current to flow freely in one direction but prevents current flow in the opposite direction. In this sense, the semiconductor diode is a one-way valve.

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Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
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