Muhammad Saiful Islam, S.I.M. Salman, Mossabbir Ahmed, Shamsul Alam, Istiuk Mahmud Ayon
Department of EEE, Ahsanullah University Of Science And Technology, Dhaka, Bangladesh. Email: firstname.lastname@example.org
In this topic,we have been considering the properties of the semiconductor material. We calculated electron and hole concentrations in thermal equilibrium and determined the position of the Fermi level. We then considered the no equilibrium condition in which excess electrons and holes are present in the semiconductor. We now wish to consider the situation in which a p-type and an n-type semiconductor are brought into contact with one another to form a p-n junction.
An electronic device controls the movement of electrons .The study of electronic devices requires a basis understanding of the relationship between electrons and other components of an atom .The movement of electrons within a solid and the bonding forces between atoms can then be investigated. This leads to a knowledge of the differences between conductors, insulators , and semiconductors and to an understanding of p-type and n- type semiconductor material .Junctions of p-type and n-type material that are called p-n junction are basic to all but a very few semiconductor devices. Force act upon electrons that are adjacent to a p-n junction, and these forces are altered by the presence of an external bias voltage.
The construction of every discrete solid state electronic device or integrated circuit begins with a semiconductor material if the highest quality. Semiconductors are a special class of element having conductivity between that of a good conductor and that of an insulator. So the characteristics of a semiconductor are different from a conductor or insulator. Insulators have a wide
forbidden gap, semiconductors have a narrow forbidden gap and conductors have no forbidden gap at all. One more thing that, if we pass heat through conductor,it will increase resistance. But in case of semiconductor, its resistivity is decreased and conductivity is increased. Again if we mix a little amount of impure material with pure semiconductor materials, the conductivity of semiconductor will be increased. Now we need to learn about energy bands to know why a material becomes conductor, semiconductor or insulator.
2. Energy bands
The electrons of an isolated atom are acted upon only by the forces within that atom. However when atoms are brought closer together as in a solid, the electrons come under the influence of forces from other atoms. Under these circumstances the energy levels that may be occupied bt electrons merge into bonds of energy levels.
Figure 1:Energy bands
With any given material there are two distance energy bands in which electrons may exist: 1. The valance band
2. The conduction band
Separating these two bands is an energy gap in which no electrons can normally exist. The gap is termed as the forbidden gap. The valance band and forbidden band are shown diagrammatically below:
3. Conduction of solids
Conduction occurs in any given material when an applied voltage causes electrons within the material to move in a desired direction. This may be due to one or both of the two process 1. Electron motion
2. Hole transfer
In electron motion, free electrons in the conduction band are moved under the influence of the applied electric field. Since electrons have a negative charge they are repelled from the electric...