In this topic, you study PN junction diode – Definition, Theory & Symbol.
The PN junction diode is one of the semiconductor devices with two different semiconductor materials, one with excess of holes (P-type) and other with excess of electrons (N-type) joined together. A PN junction diode is also known as a semiconductor or crystal diode. The term diode identifies a two electrode or two terminal device. The most important characteristic of a PN junction is its ability to allow the flow of current in only one direction.
In the opposite direction, it offers very high resistance. Semiconductor diodes find wide applications in electronics viz. power supplies, industrial electronics, radio and TV instrumentation etc.
PN Junction Diode Symbol
A PN junction diode consists of a PN junction formed either in Germanium or Siliconcrystal. The diode has two terminals namely anode and cathode. The anode refers to the p-type and cathode refers to the N-type. The circuit symbol for a diode is an arrow and bar shown in Fig. 7.1. The arrowhead indicates the conventional direction of current flow when the diode is forward biased (i.e. from the positive terminal through the device to the negative terminal).
Fig. 7.1 : Symbol Of a diode
Unbiased PN Junction Diode
In Fig. 7.2, P-type and N-type semiconductor materials are shown side by side represented by a junction.
Fig. 7.2 . At junction, electrons cross from the N-side tofill holes in the layer of P-side close to the junction
The small circles in the P-type material represent holes, which are the majority charge carriers in P-type. The dots in the N-type material represent the majority charge carrier free electrons inN-type. Normally, the holes are uniformly distributed throughout the volume of the P-type semiconductor and the electrons are uniformly distributed in the N-type. Holes and electrons are close together at the junction, so some free electrons from N-side are attracted across the junction to till the adjacent holes on the P-side. They are said to diffuse across the junction. The free electrons which cross the junction create negative ions on the P-type. This happens by giving some atoms one more electron than their total number of protons in P-type. Similarly electron leaves the positive ion (the atom which consist of less number of electrons) behind them on N-side.
Barrier Voltage
The N-type and P-type materials are both electrically neutral before the charge carriers diffuse across the junction. But when these occurs, diffuse i.e. electrons from N-type cross the junction, negative ions are created on the P-side, the portion of the P-side close to the junction a negative voltage.
Similarly, positive ions are created on the N-side, the position Of the N-side close to the junction acquires a positive voltage. The negative voltage on the P-side, repels other electrons crossing from the N-side.Also positive voltage on N-side repels holes to cross the junction. Thus the initial diffusion of charge carriers creates a barrier voltage at the junction, which has negative on P-side and positive on N-side. This voltage at the junction is called as barrier voltage. The transfer of charge carriers and creation of barrier voltage occurs when the PN junctions are formed during the manufacturing process. The magnitude of the barrier voltage at the PN junction depends on doping concentration, electronic charge and junction temperature. Barrier voltages at 250C are0.3 or 0.2 V for Germanium and 0.7 V for Silicon.The potential or barrier voltage which is generated at the junction opposes the flow of majority carriers i.e. holes in P-region and electrons in N-region cross the PN junction but allow the flow Of minority carriers across the junction. See Fig. 7.3.
Fig. 73: Barrier voltage at the junction allows the now of minority charge carriers and opposes the flow of majority carriers
Depletion Region
The movement of charge carriers across the junction creates a layer of immobile ions on each side. This layer is called depletion layer. The depletion region is shown in
Fig. 7.4 (a).Depletion region
(a) Equal doping densities (b) Unequal doping densities
Fig. 7.4 .