PN Junction Diode and its Characteristics
PN junction diode is one of the fundamental electronics elements. Here we dope one side of a semiconductor piece with an acceptor impurity and another side with a donor impurity. PN junction diode is a two-terminal electronics element. PN junction diode came into use from 1950. We can distinguish PN junctions either as step-graded or as a linear graded. In step graded the concentration of dopants both, in the N side and the P side are uniform up to the junction. But in a linearly graded junction, the doping concentration varies almost linearly with the distance from the junction. When we do not apply any voltage across the PN diode, free electrons will diffuse through the junction to P side and holes will diffuse through the junction to N side and they combine with each other.
Thus the acceptor atoms in the p-side near the junction edge and donor atoms in n-side near junction edge become negative and positive ions respectively. The existence of negative ions in the p-type side along the junction and positive ions in the n-type side along the junction edge creates an electric field. The electric filed opposes further diffusion of free electrons from n-type side and holes from p-type side of the PN junction diode. We call this region across the junction where the uncovered charges (ions) exist, as depletion region.
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If, we apply forward bias voltage to the p-n junction diode. That means if positive side of the battery is connected to the p-side, then the depletion regions width decreases and carriers (holes and free electrons) flow across the junction. If we apply reverse bias voltage to the diode, the depletion width increases and no charge can flow across the junction.
P-N Junction Diode CharacteristicsLet's a voltage V is applied across a p-n junction and total current I, flows through the junction. It is given as. Here, IS = reverse saturation current e = charge of electron ɳ = emission co-efficient KB = Boltzmann constant T = temperature
The graph below shows the current-voltage characteristic of a PN junction diode.When, V is positive the junction is forward biased, and when V is negative, the junction is reverse biased. When V is negative and less than VTH, the current is minimal. But when V exceeds VTH, the current suddenly becomes very high. The voltage VTH is known as the threshold or cut in voltage. For Silicon diode VTH = 0.6 V. At a reverse voltage corresponding to the point P, there is abrupt increment in reverse current. This portion of the characteristics is known as breakdown region.