Solid State Devices – Part 1

 

INTRODUCTION

In this chapter, we will try to understand what does it means by solid state devices. A solid state device is basically an electronic device used in nowadays instruments and system. Solid state device is made up of semiconductor element that is either conductor or insulator. The semiconductor has useful characteristic that is suitable to work as an amplifier or rectifier.

 

ATOMIC STRUCTURE

Atomic structure is the first theory that we need to understand before any further discussion being made. Let us start from the top to the bottom. From the top; lets refer to the periodic table of the elements, there are more than hundred elements listed there. Every one of the elements is made up of atoms with its own structure. In atom structure, there are three basic particles that are protons, neutrons and electrons. Protons hold positive charge, neutrons hold neutral charge and electrons hold negative charge. Nucleus is the core of atom structure. In the nucleus, there is the place where proton and neutron are located. Electron however located around the nucleus on its orbit. There are a few orbits depending on elements. This orbit we called as shell. The outermost shell is called valence shell or valence band. The electrons in the valence shell are called valence electrons.

 

VALENCE BAND

Valence band is also known as valence shell that is the outermost shell of an atom. In valence band, it can contain one up to eight valence electrons. If the valence electrons equal to one, it is nearly perfect conductor whereas if the valence electrons equal to eight it is insulator. So, we know that of producing an insulator the valence electrons must be equal to eight. There is a case where the valence electrons equal to four. In this case it is a semiconductor. In term of conductivity of an element, it depends on the number of electrons located in its valence band. In fact, conductivity is proportional to one and inversely proportional to the number of electrons in valence band (number of valence electron). Therefore, the higher conductivity correspond to the lesser its valence electrons and vice versa. In order to know an element is in which type, therefore we have to refer to their atomic number. Then calculate how many shell it has and how many electrons in every shell. The formula to calculate maximum number of electrons in every shell is N=2n*2, where n is the shell number.

 

FREE ELECTRONS

Electrons orbit the nucleus of an atom at a certain distance depending on which shell it belongs to. The electrons which located at the shell that is closest to the nucleus are having lesser energy compared to the electrons located at the outermost shell. In details, more far away the electrons to the nucleus of an atom more energy it has. Every shell has their own energy level where it is getting higher and higher as it moves away from the nucleus. Based on that understanding, electrons in valence band have highest energy among all other electrons and hence whenever it gets a little more energy (plus the energy that it has) it is able to leave the valence band and become a free electron in conduction band.

 

CONDUCTION BAND

We knew that the outermost shell of an atom is the valence band. Conduction band is the band that is outside the valence band separated by energy gap in between them. In conduction band, electrons are having high energy that makes it very easy to be influenced by external force.

 

ENERGY LEVEL AND ENERGY GAP

Energy level and energy gap are two different things. Energy level is the level of energy at a certain shell whereas energy gap is the energy different between conduction band and valance band. As mentioned earlier that of energy level for the outermost shell is definitely higher than the inner shell. It is due to the attractive force, F. Energy gap in the other hand is the amount of energy required for valence electron to jump from its valence band to the conduction band.

 

CONDUCTOR, INSULATOR, SEMICONDUCTOR

Conductor, insulator and semiconductor are three different types of materials. Conductor material is a material that easily conducts electrical current. The atom for conductor material contains one to three valence electrons. Smaller the valence electron is better in their conductivity. The material that does not conduct electrical current in a normal condition is called insulator. Insulator has five to eight valence electrons. Almost like conductor but for insulator higher valence electron better it isolate current. The energy gap for insulator is also far distance that makes the valence electron (in valence band) difficult to travel across the energy gap. However, if the external force in not under normal condition (extremely high), possibility of the valence electron to travel beyond the energy gap is true. This will make the material conducting current but not as good as the conductor. Semiconductor is a material between conductor and insulator. It is not as good as a conductor and also not as good as an insulator. The atom for semiconductor material contains four valence electrons and it is able to conduct electrical current. Intrinsic semiconductor however is a pure semiconductor without any impurities added in.