14.3. INTRINSIC SEMICONDUCTOR

We shall take the most common case of Ge and Si whose lattice structure is shown in Fig. 14.3. These structures are called the diamond-like structures. Each atom is surrounded by four nearest neighbours.

In this section, we are looking at the fundamental structure of intrinsic semiconductors like Germanium (Ge) and Silicon (Si). Their atoms are arranged in a specific repeating pattern called the diamond structure. This means that each atom in the semiconductor is bonded to four other atoms in the vicinity, creating a strong and stable arrangement. Such structures are key to how these materials behave electrically.

Think of a diamond as a city where each house (atom) is connected to four other houses. If you take one house out, the others still remain intact, similar to how the atoms in the semiconductor are bonded to each other. This bonding is crucial for the flow of electricity.

We know that Si and Ge have four valence electrons. In its crystalline structure, every Si or Ge atom tends to share one of its four valence electrons with each of its four nearest neighbour atoms, and also to take a share of one electron from each such neighbour. These shared electron pairs are referred to as forming a covalent bond or simply a valence bond.

Silicon and Germanium have four electrons in their outermost shell, known as valence electrons. In a solid crystal, each atom shares its electrons with neighboring atoms, creating strong covalent bonds. This sharing allows the electrons to hold the atoms together tightly, which is what gives semiconductors their solid structure and enables control of electrical characteristics.

Imagine a group of friends sharing their snacks during a picnic. Each friend (atom) gives a little bit of their food (electron) to others to form a group (covalent bond). This teamwork holds the group together and keeps everyone connected.

As the temperature increases, more thermal energy becomes available to these electrons and some of these electrons may break away (becoming free electrons contributing to conduction). The thermal energy effectively ionises only a few atoms in the crystalline lattice and creates a vacancy in the bond.

When the temperature rises, some of the electrons in the covalent bonds gain enough energy to break free from their bonds, turning into free electrons that can move and conduct electricity. This process also creates vacancies or 'holes' where the electrons used to be, both of which contribute to electrical conduction in the semiconductor.

Think of a crowded room where people are cooperatively holding onto each other (covalent bonds). When someone turns on a heater (increasing temperature), a few people start moving around more freely. Those who leave create empty spots, just like free electrons and holes in the semiconductor lattice.

In intrinsic semiconductors, the number of free electrons, n_e is equal to the number of holes, n_h. That is n_e = n_h = n_i, where n_i is called intrinsic carrier concentration.

In intrinsic semiconductors, there is a balance between free electrons and holes. For every electron that escapes the covalent bond, a hole is created, leading to an intrinsic carrier concentration. This constant ratio is essential for understanding how semiconductors conduct electricity, especially at low temperatures.

Imagine a basket of apples (holes) where every time you take out an apple (a free electron), another apple appears in its place (a hole). This balance keeps the number of apples constant, similar to how electrons and holes balance themselves in intrinsic semiconductors.

Suppose there is a hole at site 1. The movement of holes can be visualised as shown in Fig. 14.5(b). An electron from the covalent bond at site 2 may jump to the vacant site 1 (hole). Thus, after such a jump, the hole is at site 2 and the site 1 has now an electron.

The movement of holes can be conceptualized as a dance where the participants (electrons) move from one spot to another. When an electron jumps to fill a hole, it creates a new hole where it once resided. This process affects the overall conductivity as it enables the movement of charge through the semiconductor.

Think of a game of musical chairs. When the music stops, one player moves to fill an empty chair (the hole) while creating a new empty chair for the previous sitter. This frees up space and keeps the game going, similar to how electrons move and create holes in a semiconductor.

It may be noted that apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination occurs in which the electrons recombine with the holes. At equilibrium, the rate of generation is equal to the rate of recombination of charge carriers.

In an intrinsic semiconductor, there is a dynamic balance between generating free electrons and holes and recombining those carriers. At thermal equilibrium, these two processes occur at the same rate, ensuring stable electrical characteristics.

Consider a water tank; water flows in (generation of charge carriers) and flows out (recombination) at the same rate, keeping the tank level constant. Similarly, in a semiconductor, the number of free electrons and holes remains stable over time.

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