BSIC SEMICOCONDUCTOR CONCEPTS INTRINSIC SILICON: A crystal of pure or intrinsic silicon has a regular lattice structure. Where the atoms are held at their fixed positions by bonds, called Covalent bonds, formed by four valence electrons associated with four each silicon atom.

BSIC SEMICOCONDUCTOR CONCEPTS INTRINSIC SILICON: At sufficiently low temperature are covalent bond are intact and not( very few) free electrons are available to conduct electric current.

INTRINSIC SILICON: Thermal ionization results in free electrons and holes in equal numbers and hence equal concentrations. These free electrons and holes move randomly through silicon crystal structure, and in this process some electrons may fill some of the holes.

INTRINSIC SILICON: This process, called RECOMBINATION. It results in disappearance of free electrons and holes. The recombination rate is proportional to the number of free electrons and holes, which in turn is determined by ionization rate.

Drift The process whereby charged particles move under the influence of electric field.

Diffusion The process of flow of particles from a region of high concentration to a region of low concentration. Diffusion Current The current that results from the diffusion of charged particles.

Drift Current The current that results from the drift of charged particles. Drift Velocity The average velocity of charged particles in the presence of an electric field.

INTRINSIC SILICON The ionization is a strong function of temperature. In thermal equilibrium the recombination rate is equal to the ionization or thermal-generation rate.

DOPED SEMICONDUCTORS. Doped semiconductors are the materials in which carrier of one kind (electrons or holes) predominate. Doped silicon in which the majority of charge carriers are negatively charged electrons is called n type.

DOPED SEMICONDUCTORS. While silicon doped so that majority of charge carriers are positively charged holes is called p type. Doping of a silicon to turn it into p type or n type is achieved by a small number of impurity atoms.

DOPED SEMICONDUCTORS. For instance, introducing impurity atoms of a penta-valent element such as phosphorus results in n type silicon.

NO BIAS

THE DIFFUSION CURRENT ID Because the concentration of holes is high in the p region and low in the n region, holes diffuse across the junction from the p region to the n region.

THE DIFFUSION CURRENT ID Similarly, electrons diffuse the junction from n side to p side. These two current components add together to form the diffusion current ID .Whose direction is from p side to n side, as indicated in the figure.

THE DEPLETION REGION The electrons that diffuse across the region quickly recombine with recombine with some of the majority holes present in the p region and thus disappear from the scene.

THE DEPLETION REGION This results also in disappearance of some majority holes, causing some of the bound negative charge to be uncovered (i-e no longer neutralized by holes). Thus in the p material close to the junction that is depleted of free electrons.

THE DEPLETION REGION This p region will contain uncovered bound negative charge, as indicated in the figure. From the above it follows that a carrier depletion region will exist on both sides of the junction.

THE DEPLETION REGION With n side of this region positively charged and p side negatively charged. This carrier depletion region or simply, depletion region is also called the SPACE CHARGE REGION.

THE pn JUNCTION UNDER REVERSE BIAS CONDITION. The pn junction is excited by a constant current source I in the reverse direction. To avoid breakdown, I kept smaller than IS.

THE pn JUNCTION UNDER REVERSE BIAS CONDITION. Note that the depletion layer widens and the barrier voltage increases by VR volts, which appears between the terminals as reverse voltage.

THE pn JUNCTION UNDER REVERSE BIAS CONDITION. The current I will be carried by electrons flowing in the external circuit from the n material to p material (that is, in direction opposite to that of I).

THE pn JUNCTION UNDER REVERSE BIAS CONDITION This will cause electrons to leave the n material and holes to leave p material . Thus reverse current I will result in an increase in the width of, and the charge stored in the depletion layer. Which will increase the voltage across depletion region.

THE pn JUNCTION UNDER FORWARD BIAS CONDITION. The pn junction is excited by a constant current source supplying a current I in forward direction. The depletion layer narrows and barrier voltage

THE pn JUNCTION UNDER FORWARD BIAS CONDITION. decreases by V volts, which appears as an external voltage in the forward direction.

THE pn JUNCTION UNDER FORWARD BIAS CONDITION. This current causes majority carriers to be supplied to both sides of the junction by the external circuit. Holes to the p material and electrons to the n material.

THE pn JUNCTION UNDER FORWARD BIAS CONDITION. These majority carriers will neutralized some of the uncovered, causing less charge to be stored in the depletion region. Thus the depletion layer narrows and the depletion barrier voltage reduces.

THE pn JUNCTION UNDER FORWARD BIAS CONDITION. This reduction in voltage cause more electrons to move from n side to p side and more holes to move from p side to n side. So that diffusion currents increases until equilibrium is achieved.