Formation of depletion layer:
A p – n junction is formed by joining n-type and p-type semiconductor materials as shown in figure.
(a) Since the n-region has a high electron concentration and the p-region a high hole concentration, electrons diffuse from the n-side to the p-side. This causes diffusion current which exists due to the concentration difference of electrons. The electrons diffusing into the p-region may occupy holes in that region and make it negative.
The holes left behind by these electrons in the n-side are equivalent to the diffusion of holes from the p-side to the n-side. If the electrons and holes were not charged, this diffusion process would continue until the concentration of electrons and holes on the two sides were the same, as happens if two gasses come into contact with each other.
But, in a p – n junction, when the electrons and holes move to the other side of the junction, they leave behind exposed charges on dopant atom sites, which are fixed in the crystal lattice and are unable to move. On the n-side, positive ion cores are exposed and on the p-side, negative ion cores are exposed as shown in Figure
(b) An electric field E forms between the positive ion cores in the n – type material and negative ion cores in the p-type material. The electric field sweeps free carriers out of this region and hence it is called depletion region as it is depleted of free carriers. A barrier potential Vb due to the electric field E is formed at the junction as shown in Figure.
(c) As this diffusion of charge carriers from both sides continues, the negative ions form a layer of negative space charge region along the pside. Similarly, a positive space charge region is formed by positive ions on the n-side. The positive space charge region attracts electrons from p-side to n-side and the negative space charge region attracts holes from n-side to pside.
This moment of earners happen in this region due to the formed electric field and it constitutes a current called drift current. The diffusion current and drift current flow in the opposite direction and at one instant they both become equal. Thus, a p – n junction is formed.
V-I characteristics of a junction diode:
Forward characteristics:
It is the study of the variation in current through the diode with respect to the applied voltage across the diode when it is forward biased. An external resistance (R) is used to limit the flow of current through the diode. The voltage across the diode is varied by varying the biasing voltage across the dc power supply.
The forward bias voltage and the corresponding forward bias current are noted. A graph is plotted by taking the forward bias voltage (V) along the X – axis and the current (I) through the diode along the Y – axis. This graph is called the forward V-I characteristics of the p – n junction diode.
Three inferences can be brought out from the graph:
(i) At room temperature, a potential difference equal to the barrier potential is required before a reasonable forward current starts flowing
across the diode. This voltage is known as threshold voltage or cut-in voltage or knee voltage (V ). It is approximately 0.3 V for Germanium and 0.7 V for Silicon. The current flow is negligible when the applied voltage is less than the threshold voltage. Beyond the threshold voltage, increase in current is significant even for a small increase in voltage.
(ii) The graph clearly infers that the current flow is not linear and is exponential. Hence it does not obey Ohm’s law.
(iii) The forward resistance (r ) of the diode is the ratio of the small change in voltage (∆V)to the small change in current(∆I), rf = \(\frac{∆V}{∆I}\)
However, if the applied voltage is increased beyond a rated value, it will produce an extremely large current which may destroy the junction due to overheating. This is called as the breakdown of the diode and the voltage at which the diode breaks down is called the breakdown voltage. Thus, it is safe to operate a diode well within the threshold voltage and the breakdown voltage.
Reverse characteristics:
In the reverse bias, the p-region of the diode is connected to the negative terminal and nregion to the positive terminal of the dc power supply. A graph is drawn between the reverse bias voltage and the current across the junction, which is called the reverse characteristics of a p-n junction diode.
Under this bias, a very small current in μA, flows across the junction. This is due to the flow of the minority charge carriers called the leakage current or reverse saturation current. Besides, the current is almost independent of the voltage. The reverse bias voltage can be increased only up to the rated value otherwise the diode will enter into the breakdown region.
