B Tech coaching in Delhi for EDC

Tejas Engineers Academy has launched an initiative to provide online articles for B Tech students. Rahul is an experienced teacher who is providing B Tech coaching in Delhi from last 10 years. He has done M Tech in Power Electronics from NSIT. He offers B Tech coaching in Delhi on Tejas Engineers Academy for the subject of electronic devices and circuits. Following article is provided by Rahul.

Space charge at a junction

The interface separating the N and P regions is called the metallurgical junction. Let us consider a step junction in which the doping concentration is uniform in each region but there is an abrupt change in doping at the junction. Initially, at the metallurgical junction, there is a very large density gradient in both the electronic and hole concentrations. Majority carrier electrons in the N region will begin diffusing into the P region and majority carrier holes in the p region will begin diffusing into the n region. Fig represents the net positively and negatively charged regions for a semiconductor pn junction where there are not external connection, no applied voltage, no field etc. The net positive and negative charges in the n and p regions induce an electric field near the junction in the direction from the positive to the negative charge, or from the n region to the p region. Since the electrons and holes are both pushed out of the space charge region by the electric field, the region is depleted of any mobile charge and is called the depletion region. Density gradients still exist in the majority carrier concentrations at each edge of the space charge region. A density gradient may be considered as producing a diffusion force acting on the electrons and holes at the edges of the space charge region are indicated in the figure. The electric field in the space charge region develops another force on the electrons and holes which is in the opposite direction to the diffusion force for each type of particle. In thermal equilibrium the diffusion force and the E field force exactly balance each other.

Rahul is an expert teacher for B.Tech Tuition in Delhi. He has written a research paper on space charge at a junction. In following article Rahul sir discusses about the diode and its biasing.

PN Junction as a diode

The essential electrical characteristic of a pn junction is that it constitutes a diode which permits the easy flow of current in one direction but restrains the flow in the opposite direction. We consider now, qualitatively, how this diode action comes about.

Forward biasing

When an external field with p region connected to positive terminal and n region connected to negative terminal of the battery, is applied across the junction, as shown, the junction is said to be forward biased. In this circuit arrangement, the holes on the p side being positively charged particles are repelled from the positive bias terminal and driven towards the junction. Similarly, the electrons on the n side are repelled from the negative bias terminal and driven towards the junction. The result is that the depletion region is reduced in width, and the barrier potential is also reduced. If the applied bias voltage is increased from zero the barrier potential gets progressively smaller and smaller until it effectively disappears and charge carriers can easily flow across the junction. Electronics from the n side are then attracted across to the positive bias terminal on the p side and holes from the p side flow across to negative bias terminal on the ns side. Thus a majority carrier current flows. Since barrier potential is very small a small forward voltage is sufficient to eliminate the barrier completely. Once the barrier is eliminated by the application of forward voltage, junction resistance becomes almost zero and a low resistance path is established in the entire circuit. The current is called the forward current, flow sin the circuit.

There are very less Institutes for B.Tech Tuition in Delhi. Students find it very difficult to understand the working of various types of diodes by themselves. In this article we aim to explain the operations of diode in very easy language. This article will be best for the category of B.Tech coaching Classes in Delhi.

In brief it cannot be said that when the junction is forward biased barrier is reduced and a forward voltage 0.3 v in case of germanium and 0. V in case of silicon is induced. The junction offers low resistance, called the forward resistance Rf to the flow of current and current flows in the circuit due to establishment of low resistance path and the magnitude of current depends upon the magnitude of applied forward voltage.

The mechanism of current flow in a forward biased pn junction is summed up as follows

1. The electrons from the negative bias terminal continue to arrive into the n region while the free electrons in the n region move towards the pn junction.
2. The electrons travel through the n region as free electrons i.e. the current in n region is by free electrons.
3. These free electrons on reaching pn junction, combine with holes and become valence electrons. Since a hole is in the covalent bond and therefore, when a free electron combines with a hole, it becomes a valence electron.
4. The electrons travel through p region as valence electrons and current in this p region is therefore, by holes.
5. These valence electrons, on reaching the left end of the crystal, flow into the positive terminal of the battery.
6. Thus we see that the current in the n type region is due to movement of electrons whereas in the p type region it is carried by the holes. However, in the external circuit i.e. in connecting wires the current is carried by electrons.

 

There are very less coaching institutes offering B Tech coaching in Delhi for biasing of special purpose diodes.

Reverse applied bias

If an external bias voltage is applied with positive terminal to n side and negative terminal to the p side of a pn junction. In this arrangement electrons from the n side are attracted to the positive bias terminal and holes from p side are attracted to the negative bias terminal. Thus holes from the impurity atoms on the p side of the junction are attracted away from the junction, and the electrons are attracted from the atoms on the n side of the junction. Thus the depletion region is widened, and the barrier potential is increased by the magnitude of the applied bias. With the increased barrier potential, there is no possibility of majority carrier current flow across the junction. Thus the pn junction is in a non conductive state.

In brief it can be said that when the pn junction is reverse biased barrier potential is increased and width of depletion is also increased the junction offers high resistance, called the reverse resistance Rr to the flow of current owing to establishment of high resistance path, very small current called the leakage current or reverse current flows thorough the electric.

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