INTRODUCTION TO SEMICONDUCTORS MATERIAL Chapter 1 (Week 2)

1.3 Covalent Bonding 1.Covalent bonding occurs when pairs of electrons are shared by non-metal atoms. 2.When atoms combine into molecules to form a solid material, they arrange themselves in a fixed pattern called a crystal – atoms within the crystal structure are held together by covalent bonds (atoms share valence electrons). 3.Atoms are electrically stable when their valence shells are complete or fully occupied. 4.An intrinsic crystal is one that has no impurities. EKT 102: Basic Electronic Engineering

1.3 Covalent Bonding (cont.) The number of covalent bonds is equal to eight minus the group number in the periodic table. Group number = number of electrons in the valence shell. EKT 102: Basic Electronic Engineering

1.3 Covalent Bonding (cont.) EKT 102: Basic Electronic Engineering Covalent bonding – holding atoms together by sharing valence electrons Semiconductor atoms bond together to form a solid material = crystal Sharing of valence electron produces the covalent bond To form Si crystal Figure 12: Illustration of covalent bonds in silicon crystal (c) Covalent bonds in a silicon crystal

1.4 Conduction in Semiconductor The electrons of an atom can exist only within prescribed energy bands. Each shell corresponds to a certain energy band and is separated from adjacent shells by band gaps - no electrons can exist. EKT 102: Basic Electronic Engineering Figure 13: Energy band diagram for an unexcited (no external energy) atom in a pure (intrinsic) Si crystal.

1.4 Conduction in Semiconductor (cont.)  When an electron jumps to the conduction band, a vacancy (called a hole) is left in the valence band within the crystal. Recombination occurs when a conduction-band electron loses energy and falls back into a hole in the valence band. EKT 102: Basic Electronic Engineering Figure 14: Creation of electron-hole pairs in a silicon crystal

1.4 Conduction in Semiconductor The number of free electrons (also known as conduction electrons) is equals to the number of holes in the valence band. EKT 102: Basic Electronic Engineering Figure 15: Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes.

1.4 Conduction in Semiconductor (cont.) Electron Current When a voltage is applied across a piece of intrinsic silicon, the thermally generated free electrons in the conduction band, which are free to move, are now easily attracted toward the positive end. The movement of free electrons in a semiconductive material is called electron current. EKT 102: Basic Electronic Engineering Figure 16: Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons.

1.4 Conduction in Semiconductor (cont.) Hole Current  Electron remaining in the valence band are still attached to the atom – not free to move like free electron.  However, valence electron can move into nearby hole – leaving another hole it comes from.  Thus, hole has moved from one place to another.  The movement of electrons in a valence band is called hole current. Figure 17: Hole current in intrinsic silicon. EKT 102: Basic Electronic Engineering

1.5 N-Type and P-Type Semiconductors Doping Semiconductive materials do not conduct current well because of the limited number of free electrons in the conduction band and holes in the valence band. Intrinsic semiconductive materials must be modified by increasing the free electrons and holes to increase its conductivity and make it useful for electronic devices – by adding impurities. Doping is the process of adding impurity atoms to intrinsic semiconductors improve its conductivity. Tw types of doping – N-Type and P-Type. EKT 102: Basic Electronic Engineering

1.5 N-Type and P-Type Semiconductors (cont.) EKT 102: Basic Electronic Engineering Figure 18: Pentavalent impurity atom in a silicon crystal structure. An antimony (Sb) impurity atom is shown in the center. The extra electron from the Sb atom becomes a free electron.

1.5 N-Type and P-Type Semiconductors (cont.) N-Type Semiconductor No. of conduction electrons can be controlled by the no. of impurity atoms. Since most of the current carriers are electrons, semiconductor doped with pentavalent atoms is an n-type semiconductor. The electrons are called the majority carriers, while the holes is minority carriers. EKT 102: Basic Electronic Engineering

1.5 N-Type and P-Type Semiconductors (cont.) EKT 102: Basic Electronic Engineering Figure 19: Trivalent impurity atom in a silicon crystal structure. A boron (B) impurity atom is shown in the center.