p-n Junction

When a p-type semiconductor is suitably joined to n-type semiconductor, the contact surface is called p-n junction. Most semiconductors devices contain one or more p-n junctions.

Formation and properties of p-n junction :

Consider a crystal, one-half of which is doped with a p-type impurity and the other half with an n-type impurity. Initially, the p-type semiconductor has mobile holes and the same number of immobile negative ions carrying exactly the same total charge as the total positive charge represented by the holes. Similarly, the n-type semiconductor has mobile electrons: and the same number of fixed positive ions carrying same total charge as the total negative charge. on the mobile electrons. Hence, each region is initially neutral:

•Due to concentration gradient existing in two regions of the p-n junction, some of°the holes will diffuse across the boundary into the n-type region and electrons will diffuse across the boundary into p-type region as in the figure given. below. Due to this diffusion process of holes and electrons, the two sections of the junction diode no longer remain neutral.

into the p-section, it falls into the vacancy and completes the covalent bond. This process is known as the electron-hole recombination. As a result of diffusion of charge-carriers across the junction, the n-region of the junction will have its electrons neutralized by holes from the p-section, leaving only ionised donor atoms (positive charges) that are immobile. Similarly, the p-region of the junction will have ionised acceptor atoms (negative charges) that are bound and can not move.

Consequently, p-region acquires excess negative charge which repels any more electrons trying to migrate from the n-type to p-type semiconductor. Similarly, n-region acquires excess positive charge that prevents any further migration of holes across the boundary, i.e., the accumulation of charges of opposite polarities in the two sections of the junction produces an electric field between these regions and it appears as though a fictitious battery were connected across the junction with its negative terminal connected to the p-region and the positive terminal connected to the n-region. Due to this electric field, the further flow of electrons from the n-region to the p-region and that of holes from the p-region to the n-region is opposed. Owing to this, a potential barrier V is developed across the junction that opposes further diffusion of free charge carriers into opposite sections. In the vicinity of the junction, a region, known as the depletion region, is developed which has immobile charges and is devoid of free charges.

The magnitude of potential barrier is about 0.3 V for germanium junction diode and 0.7

V for silicon junction diode. However the widthl depletion region and magnitude of potential barrier depends on the semiconductor crysta and its doping concentration. Now taking the width of depletion region as 10-6 m electric baAof silicon p-n junction is found to be of the order of E =

10-6

× 105 Vm.

across the junction

Thus, it is seen that the formation of p-n junction results in a very strong electric field

Reverse bias junction

When the polarity of the applied voltage is reversed, the junction is said to be reverse biased. In’other words, a p-n junction is said to be reverse biased, if the positive terminal of the external battery is connected to n-side and the negative terminal of battery is connected to the p-side of the junction.

In this case, the holes in the p-side are attracted towards the negative electrode S while the free electrons are attracted towards the positive electrode T. As a result, the depletion

Consequently, the junction behaves as an insulator. However, due to the thermally generated electron-hole pairs within p-region as well as the n-region, a small current known. as reverse bias current or leakage current (few microamperes) still flows.

Some covalent bonds always break due to heat energy of crystal molecules. Electrons liberated during this process in the p-region vibrate to the left across the junction, whereas the holes generated in the n-region move to the right under the influence of electric field produced by the applied battery. Hence, the minority carriers maintain a small electron hole combination current known as the reverse current. If the reverse bias is made large, all the covalent bonds near the junction break and a large number of electron-hole pairs are liberated. Thus, the reverse current increases abruptly to a high value. The maximum reverse potential difference, which a junction can bear, is known as the reverse breakdown voltage or Zener voltage.

It can be seen that during reverse bias, the applied DC voltage aids the potential barrier.

As a result, diffusion of holes and electrons across the junction decreases. This makes the depletion region thick and thus the junction diode offers high resistance to the flow of current.

Characteristics of a p-n junction diode:

We shall now stuấy how the current flows during forward bias and reverse bias of a p-n junction diode. There are two types of characteristics of p-n junction diode — forward bias characteristics and reverse/bias characteristics.

Forward bias characteristics:

Characteristics depict graphical relation between the voltage applied to the junction and the current through the junction

The forward bias connection at a p-n junction is shown in figure (a). Forward bias characteristics depict the graphical relation between forward bias voltage applied to the junction and forward current through the junction. Voltameter V and ammeter mA measures the forward bias voltage and current through the diode respectively. On plotting these values, we obtain the forward bias characteristics as shown in figure (b).

In the beginning, when the applied forward voltage is low, practically no current flows through the junction diode, as the potential barrier opposes the applied voltage. Thus, a small forward current flows, as long as the applied forward voltage does not exceed the potential barrier. It is denoted by portion 0A as shown in figure (b). It is found that, beyond

forward voltage V = V known as knee voltage, the forward bias current increases almost

linearly. At knee voltage, the applied voltage overcomes the barrier potential.

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