This post discussed p-n junction definition, working of p-n junction diode,p-n junction forward bias, unbiased p-n junction, p-n junction formation, p-n junction diode symbol, p-n junction diode theory.
What is a p-n junction?
p-n junction is a single piece of a semiconductor material (either Si or Ge) with one portion doped with pentavalent impurity and the other portion doped with trivalent impurity .p-n junction (Figure 1). The boundary dividing the two halves or portions of such a semiconductor is called a junction and the arrangement is known as p-n junction.
How p-n junction is formed?
A small quantity of trivalent impurity say indium is fused to a thin wafer (i.e., very thin slice) of n-type germanium or silicon semiconductor. This process produces p-type germanium just below the surface of contact. This p-type semiconductor along with n-type semiconductor wafer forms a p-n junction.
Similarly, a p-n junction can be made by fusing a small quantity of pentavalent impurity into a p-type semiconductor, p-n junction can be obtained when p-type semiconductor is heated in phosphorus gas to result into diffused n-type layer on the semiconductor.
Depletion Layer in a p-n junction
What is a depletion layer or depletion region or space charge ? How is this layer or region formed ?
The region around the p-n junction having no mobile charge carriers is known as depletion region or depletion layer or space charge region.
The two important processes called diffusion and drift occur during the formation of p-n junction. The concentration of holes is higher on p-side than that on n-side of p-n junction and concentration of electrons is higher on n-side than that on p-side of the p-n junction. The holes diffuse through the junction from higher concentration region (p-region) to lower concert region (n-region). That is, holes diffuse through the junction from p-region to n- region and combine with electrons in the n-region and hence get neutralized. Similarly, electrons diffuse through the junction from n-region to p-region and combine with holes in p-region and hence get neutralized.
When holes diffuse through the junction, the p-region near the junction is left with negative ions (or acceptor ions) which remain fixed in their positions in the crystal lattice. On the other hand, when electrons diffuse through the junction, the n-region near the junction is left with positive ions (or donor or) fixed in their positions in the crystal lattice.
Hence, the negative ions or acceptor ions near the junction on p-side form negative charged region and positive ions or donor ions near the junction on n-side form positive charged region. This space charge region on both sides of the p-n junction taken together is called depletion layer or region because it has no mobile charges (Figure 4). The thickness of depletion layer is about (0-5 um.)
Junction Barrier or Potential Barrier
What is junction barrier or potential barrier ? How is junction barrier formed across a p-n junction ?
The potential difference due to negative immobile ions on p-side of the junction and positive immobile ions on the n-side of the junction is called potential barrier. Potential barrier prevents the movement of electrons from n region to p region and movement of holes from p region to n region through the junction.
The depletion layer contains positive and negative immobile ions on either side of the p-n junction. The positive and negative ions set up a potential difference across the p-n junction which opposes the further diffusion of electrons and holes through the junction. This potential difference is called potential barrier generally represented as Vb. The potential barrier is about 0-7 volt for silicon crystal and 0-38 volt for germanium crystal at room temperature.
An electric field (E=Vb/d. where d is the thickness of the depletion layer) is set up across the junction. This electric field is directed from positive charge to negative charge across the junction. The electric field exerts force on electrons in p-region to move towards n-side and also exerts force on holes in n-region to move towards p-side, Thus, a drift current begins to flow due to the drifting of holes and electrons across the junction. The flow of drift current is opposite to the flow of diffusion current set up due to the diffusion of electrons and holes through the junction (Figure 5). The drifting continues till drift current becomes equal to the diffusion current. In equilibrium state when drift current is equal to the diffusion current net current becomes zero. The potential barrier, of p-n junction is represented by figure.
Semiconductor diode or p-n junction diode
What is a p-n junction diode? Draw symbol to represent p-n junction diode.
Semiconductor diode consists of a p-n junction having metallic contacts at both the ends as shown in figure 7(A). A cell or a battery can be connected across the metallic contacts of the p-n junction diode. Symbolic representation of a p-n junction diode is given in figure 7(B):
The arrowhead represents p -type semiconductor and the vertical bar represents n-type semiconductor. The arrow represents the direction of conventional electric current through the diode.
Working of p-n junction diode under forward bias
Explain forward biasing in a p-n junction diode with the help of diagrams.
A p-n junction is said to be forward biased when the positive terminal of a cell or a battery is connected to p-side and the negative terminal of the cell or the battery is connected to the n-side of the junction diode.
A p-n junction diode when not connected to a cell or battery is shown in figure 8(A). The potential barrier of height Vb of unbiased p-n junction is also shown in the figure.
When a cell or a battery is connected across the p-n junction diode such that positive terminal is connected to p-region and negative terminal is connected to n-region (figure 8(b)]. a forward potential difference of V volt is applied across the diode. This potential difference reduces the potential barrier (Vb). The effective potential barrier reduces to (Vb – V) and the thíckness of the depletion layer also decreases (Figure 8(B)) The junction resistance becomes very low. The holes (majority carriers) in p-region and electrons(majority carriers) in n-region acquire sufficient energy to overcome the potential barrier across the junction. The crossed over electrons in p region and holes in n region are in fact minority carriers so this process of cross over is called minority carrier injection. Concentration of injected holes in n-side and that of injected electrons in p side increases a lot near the junction that ends. The difference in concentration with distance (i.e. concentration gradient) makes the injected holes and electrons diffuse to the ends of n-side and p side respectively.
The movement of holes and electrons constitute diffusion hole current (Ih) and diffusion electron current(Ie) respectively.
The sum of lh and Ie is the total current flowing through the junction diode i.e., I=Ie +Ih.
Working Of p-n junction diode under Reverse bias
Explain reverse biasing in a p-n junction diode with the help of diagrams.
A p-n junction is said to be reverse biased when the positive terminal of a cell or a battery is connected to the n-side and negative terminal is connected to the p-side of the p-n junction diode.
A p-n junction when not connected to a cell or battery is shown in figure 9(A). When a cell is connected in reverse mode to the p-n junction diode as shown in figure 9(B), a potential difference of V volt is applied across the diode. This potential difference adds to the potential barrier (Vb). The effective barrier potential increases to (Vb + V ) and also the thickness of the depletion layer increases (9B).The Junction resistance increases in reverse bias. The majority carriers in p-region and n-region respectively are attracted by the negative and positive terminals of the battery. Thus, both holes and electrons are drifted away from the junction. As a result of this, holes in the p-region and electrons in the region cannot cross through the junction. Therefore, the flow of current in the diode is almost stopped.
There is a small reverse saturation current (i.e., a current which cannot increase anymore) due to sweep of the minority carriers in p-region and n-region. This current is not affected by the increase in applied voltage but increases with the increase in temperature. This is because the minority carriers density responsible for reverse saturation current increases with increase in temperature.
If the reverse bias in increased to a high value, the covalent bonds near the junction break down and a large number of electron-hole pairs are liberated. Thus, the reverse current increases abruptly to a very high value. This phenomenon is called breakdown and this value of reverse voltage is called breakdown voltage (V).
Characteristics of p-n junction diode
Using circuit diagram and graphs, explain the V-I characteristics of a p-n junction diode in forward biasing and in reverse biasing
The variation of current with the applied voltage across the junction diode gives the characteristics of p-n junction diode.
Forward bias characteristic
The graph showing the variation of current with the variation of applied voltage, when diode is forward biased is known as forward bias characteristic of p-n junction diode. The circuit diagrams to study forward bias characteristic of p-n junction diode is shown in figure 10.
When the battery voltage is zero (1.e. switch S is open),diode does not due to junction barrier p0tential.In this case the diode current is zero, When switch S is closed, voltage is applied across the diode and the barrier potential starts decreasing. Thus, a small current begins to flow. The forward current increases slowly at first but as soon as the battery voltage is increased with the help of Rheostat R,the forward current increases rapidly. The battery voltage at which the forward current starts increasing rapidly is known as knee voltage (Vk) or threshold voltage or cut in voltage (figure 11). For germanium diode, the knee voltage is about 0.2 V, whereas for silicon diode it is about 07 V. When forward voltage is above the knee voltage, the junction diode behaves almost like a conductor.
Conclusions: (a) V-I graph for diode is not a straight line passing through the origin. Thus, diode does not strictly obey Ohm’s law, In other words, diode is a non-ohmic device,
(b) The resistance across the junction of a diode in large below the knee voltage .
(c)The resistence across the junction of a diode decreases above the knee voltage.
2.Reverse bias characteristic :
The graph showing the variation of current with the variation of applied voltage, when junction diode is reverse biased is known as reverse bias characteristic of the junction diode.
The circuit diagram to study reverse bias characteristic is shown in Figure 12. When P-n junction is reverse biased, the majority carriers in and a region are repelled away from the junction. There is small current due te the minority carriers.
This current attains its maximum or saturation value immediately and is independent of the applied reverse voltage, It depends on the temperature of the junction diode.
As the reverse voltage is increased to a certain value, called break down voltage (Vbr) large amount of covalent bonds in p and n-regions are broken. As a result of this, large electron-hole pairs are produced which diffuse through the junction and hence there is a sudden rise in the reverse current (figure 13).
Once break down voltage is reached any increase in the applied voltage leads to the high reverse current which may damage a simple junction diode.
Voltage ampere (i.e., V-I) characteristics : The forward and reverse bias characteristics of junction diode taken together are also known as voltage ampere characteristic (figure 14)
Static and Dynamic resistances of a Junction Diode
Define static resistance and dynamic resistance of a junction diode.
In a junction diode, current does not linearly follow applied voltage, so it does not strictly obey Ohm’s law. The resistance of a diode is the function of the operating current. Diode has two types of resistances i.e.
D.C. or Static resistance of the diode is defined as the ratio of the d.c. voltage across the diode to the direct current flowing through it.
A.C. or Dynamic resistance of the diode is defined as the ratio of the small change in voltage to the corresponding small change in current in the diode.
i.e., rd= ΔV/ΔI