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Analog Electronics is a core subject Mr. Bhagatveer Singh. He has done B.Tech and M.Tech from Delhi and he is working as a regular faculty in Tejas Engineers Academy and is providing B Tech coaching in Delhi from last 8 years. Following article is provided by Mr. Bhagatveer Singh.
Structure of pn junction
Most semiconductor devices employ one or more PN junction. The PN junction is the control element for the performance of all semiconductor devices such as rectifiers, amplifiers, switching devices, linear and digital integrated circuits. The PN junction is produced by placing a layer of P type semiconductor next to the layer of N type semiconductor. The interface separating the N and P regions is referred to as the metallurgical junction.
The p type semiconductor block has mobile holes and the same number of fixed negative acceptor ions. Similarly the n type semiconductor block has mobile or free electrons and the same number of fixed donor positive ions. Normally the holes, which are the majority charge carriers in p type of material, are uniformly distributed thorough the volume of that material. Similarly the electrons, which are the majority charge carriers in n type of material, are uniformly distributed through the volume of that material. Each region is electrically neutral because each of them carriers equal positive and negative charges.
On the formation of PN junction some of the holes from p type material tend to diffuse across the boundary into n type material and some of the free electrons similarly diffuse into the p type material. This happens due to density gradient (as concentration of holes is higher on p side than that on n side and concentration of electrons is higher on n side than on the p side). This process is known as diffusion.
As a result of the displacement of the charges, an electric field appears across the junction. Equilibrium is established when the field becomes large enough to restrain the process of diffusion. The electric charges are confined to the neighbourhood of the junction and consists of immobile ions. We see that the free electrons crossing the junction create negative ions on the p side by giving some atoms one more electron than their total number of protons. The electrons also leave positive ions behind them on the n side. As negative ions are created on the p side of the junction, the p side acquires a negative potential. Similarly the positive ions are created on the n side and the n side acquires a positive potential. The negative potential on the p side prevents the migration of any more electrons from the n type material to the p type material. Similarly the positive potential on the n side prevents any further migration ofholes across the boundary. Thus, the initial diffusion of charge carriers creates a barrier potential at the junction.
The region around the junction is compleltely ionsed. As a result there are no free electrons on the n side, nor there holes on the p side. Since the region around the junction is depleted of mobile charges it is called the depletion region, the space charge region, or the transition region. The thickness of the depletion region, is of the order of 1 micron.
The variation of electric field constitutes a potential energy barrier against the further diffusion of holes across the barrier.
The necessity for the existence of a potential barrier called the contact, or diffusion potential is now considered further. Under open circuited condtions, the net hole current must be zero. If this statement were not true, the hole densit at one end of the semiconductor would contine to increase indefinitely with time, a situation, which is obviously physically impossible. Sie the concentration of holes in the p side is much greater than that in the n side, a very large diffusion current tends to flow across the junction from the p type material to the n type material. Hence an electric field must build up across the junction in such a direction that a drift current will tend to flow across the untion from the n side to the p side in order to counterbalance the diffusion current. This equilibrium condition of zero resultant hole current allows us to determine the height of the potential barrier in terms of the donor and acceptor concentrations. The magnitude of barrier potential V is of the order of few tenths of a volt.
Barrier voltage depnds on doping density, electronic charge and temperature. For a given junction, the first two factors are fixed, thus making barrier potential dependent on temperature. Increase in temperature creates more minority charge carriers leading to their increased drift across the junction. These extra minority charge carriers reduce the width of the depletion layer equivalent to reduction in potential barrier. It is found that for either silicon or germanium diodes, the barrier potential decreases for each Celsius degree rise.
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