The Basics of Semiconductor Physics as the Foundation of Electronics

Metals have a large number of weakly bound electrons in what is called their conduction band. When an electric field is applied to a metal such as copper, the electrons migrate freely producing a current through the metal. Because of the ease by which large currents can flow in metals, they are called conductors. In contrast, other materials have atoms with valence atoms that are tightly bound, and when you apply an electric field, the electrons do not move easily. These materials are called insulators and do not normally sustain large electric currents. We have another class of materials i.e. elements in group IV of the periodic table, that have properties somewhere between conductors and insulators. They are called Semiconductors. Semiconductors such as Silicon and Germanium have current-carrying characteristics that depend on the temperature or the amount of light falling on them.

When a voltage is applied across a semiconductor, some of the valence electrons easily jump to the conductance band and then move in the electric field to produce a current, although comparatively smaller than that which would be produced in a conductor.

In a semiconductor crystal, a valance electron can jump to the conduction band, and its absence in the valance band is called a hole. A valance electron from a nearby atom can move to the hole, leaving another hole in its former place. This chain of events can continue resulting in a current that can be thought of as the movement of the holes in one direction or electrons in the other. The net effect is the same.

We can alter the properties of pure semiconductor crystals significantly by inserting small quantities of elements from group III or group V of the periodic table into the crystal lattice of the semiconductor. These elements are known as dopants and can be diffused or implanted into semiconductors. The process of adding impurities to a semiconductor is known as doping. The purpose of adding impurity is to increase either the number of free electrons or holes in the semiconductor crystal. A thin crystal of silicon (chip) can have a minute pattern of dopants deposited on and diffused into its surface resulting in devices that are the basis of all modern electronics.

A semiconductor in an extremely pure form is called an intrinsic semiconductor. When a suitable impurity is added to a pure semiconductor it becomes an extrinsic semiconductor. Depending upon the type of impurity added, extrinsic semiconductors are classified as:

• N-type semiconductor
• P-type semiconductor

N-type Semiconductor

When a small amount of pentavalent impurity is added to a pure semiconductor, it is known as n-type semiconductor. The addition of pentavalent impurity provides a large number of free electrons. Examples of pentavalent impurities are Arsenic and Antimony. The kinds of impurities that produces n-type semiconductor are known as donor impurities because they donate or provide free electrons to the semiconductor crystal.

Let’s consider what happens if dopants are embedded in the crystal lattice of Silicon or Germanium. Both of these substances have four valance electrons. Silicon has four valance electrons that form symmetrical electron bonds in the crystal lattice however, if Arsenic or Phosphorus from group V is added to the crystal lattice, one of the five valance electrons in each dopant atom remains free to move around. The resulting semiconductor is called n-type silicon type due to the electrons available in the crystal lattice as charge carriers. The same process happens with Germanium crystal say when a small amount of pentavalent impurity such as Arsenic is added to it as shown in the diagram below:

P-type Semiconductor

When a small amount of trivalent impurity is added to a pure semiconductor, is termed as a p-type semiconductor.

The addition of trivalent impurity provides a large number of holes in the semiconductor. Examples of trivalent impurities are Gallium, Boron and Indium. These impurities that produce p-type semiconductor are known as acceptor impurities because the holes created can accept the electrons.

Let’s consider a pure Germanium crystal. When a small amount of trivalent impurity like Gallium is added to Germanium crystal, there exist a large number of holes in the crystal. This is because, Gallium is trivalent; that is, its atom has three valance electrons. Each atom of Gallium fits into Germanium crystal but only three co-valent bonds can be formed. In the fourth co-valent bond, only Germanium atom contributes one valance electron while Gallium has no valance electron to contribute as its entire three valance electrons are already engaged in the co-valent bonds with the neighbouring Germanium atoms. Therefore, the fourth bond is incomplete, being short of one electron. This missing electron is called a hole.

Hence, for each Gallium atom added, one hole is created. A small amount of Gallium provides millions of holes. This same process occurs when Silicon is doped with Boron or Gallium from group III. A hole can jump from atom to atom, effectively producing a positive current, i.e. electrons moves to occupy the holes, and this effectively looks like holes moving. The resulting semiconductor is called p-type silicon due to the holes, which are effectively positive charge carriers.

You can also read: Microwave Tubes

pn Junction

When a p-type semiconductor is appropriately joined to n-type semiconductor, the contact surface is called pn junction.

The semiconductor usually contains one or more pn junctions. The pn junction is in fact the control element for semiconductor devices.

Properties of pn Junction

At the instant pn junction formation, the free electrons near the junction in the n region begin to diffuse across the junction into the p region where they combine with the holes near the junction. The result is that the n region loses the electrons as they diffuse into the junction. This creates a layer of positive charges (pentavalent ions) near the junction. As the electrons move across the junction, the p region loses holes as the electrons and holes combine. The result is a layer of negative charges (trivalent ions) near the junction.

The two layers of positive and negative charges form the depletion region/depletion layer as illustrated below:

The term depletion implies the region near the junction is depleted of charge carriers (free electrons and holes) due to diffusion across the junction. Once the pn junction is formed and depletion layer created, the diffusion of electrons stops i.e. the depletion region acts as a barrier to the further movement of free electrons across the junction.

The positive and negative charges set up an electric field shown by the arrow in Figure (d) above. The electric field is a barrier to the free electrons in the n-region. There exists a potential difference across the depletion layer and is called barrier potential (V_b). The barrier potential of a pn junction depends on several factors which are:

• Type of semiconductor
• Amount of doping
• Temperature

When an external dc voltage is applied to the pn junction in such a direction that it cancels the potential barrier, thus permitting current flow, it is called forward biasing.

Reverse biasing occurs when an external dc voltage applied to the pn junction in such a direction that the potential barrier is increased.

Bottom Line

The purpose of doping a semiconductor such as Silicon is to elevate and control the number of charge carriers in the semiconductor. In an n-type semiconductor, the charge carriers are electrons while in a p-type semiconductor, they are holes. The interaction between n-type and p-type semiconductor materials forms the basis for most semiconductor electronic devices.