p – n Junction - Electronics - science lessons for life


Friday, January 13, 2017

p – n Junction - Electronics

p – n Junction
By doping one side of an intrinsic semiconductor such as silicon or germanium with a group III element to form a p-type semiconductor and the other side with a group V element to form an n-type semiconductor, a p-n junction can be formed at the centre of the semiconductor. Such a junction shows an electrical behaviour that is different from normal conductors.

p – n junction
As shown in Figure (a) as soon as the p–n junction is formed, the free electrons in the n-region diffuse across the junction towards the p-region and the holes in the p-region diffuse towards the n-region. Due to this diffusion, electrons and holes recombine forming a region devoid of charges near the junction. This region is known as the depletion layer or depletion region. As shown in Figure (b), extra electrons have entered the p-side of the depletion region giving it a negative charge while extra holes have entered the n-side of the depletion region giving it a positive charge generating a voltage difference across the junction. This potential difference repels the charge carriers impeding the diffusion of charge carriers across the junction. Therefore this potential difference is known as a “potential barrier”. This potential barrier is represented in the above figure as a hypothetical battery.

The magnitude of the potential barrier in a p-n junction formed by Si is about 0.7 V while that formed by Ge is about 0.3 V.

Biasing a p-n Junction
Applying a potential difference across the p-n junction using an external electric source is known as biasing. Depending on the direction of the bias voltage across the junction, it behaves in one of two ways. Let us engage in below activity to demonstrate this.
Apparatus required : IN 4001 diode, A 2.5 V torch bulb, Two 1.5 V dry cell batteries, A switch, A circuit board, Connecting wires
Figure C

  • Connect the circuit on the circuit board (a project board/ bread board is better for this purpose) as shown above Figure C.
  • Turn on the switch and observe the bulb.
  • Next, disconnect only the battery and reconnect the battery as shown below Figure D.
  • Turn on the switch again. Observe the bulb.
Determine which of the above biasing methods allows a current to pass through the diode. You will observe that the bulb turns on only when the diode is connected as shown in Figure C. According to this, you can use the junction diode when you need the current to flow only in a desired direction in a circuit.

Reverse biased p-n junction
Let us consider what happens when a battery is connected to the junction with its negative terminal connected to the p-type semiconductor and its positive terminal connected to the n-type semiconductor.

Figure  D – Reverse biased p-n junction
In this case, the free electrons in the n-region are attracted towards the positive potential while the holes are attracted towards the negative potential, broadening the depletion layer. There is no carrier (charge) flow across the junction. Only the depletion region broadens depending on the potential difference of the external source. Since there is no charge flow, connecting the external potential in this manner is known as reverse bias. Figure D(a) and (b) shows how the depletion layer behaves when it is reverse biased.

Forward biased p-n junction
Forward biasing of a p-n junction
In this case the external potential source is connected with the positive potential connected to the p-region and the negative potential connected to the n-region. While the holes in the p-region are repelled by the positive potential towards the junction, electrons in the n-region are repelled by the negative potential towards the junction. The depletion region narrows down due to this and if the externally supplied potential difference exceeds the potential barrier across the junction, carriers flow across the junction. When a high potential than 0.7V is applied depletion layer disappear and a current is flowing through the p-n junction. Then a current flows across the junction. Therefore, connecting the external voltage in this manner is known as forward basing.

p-n Junction Diode
Now you know that a current flows across a p-n junction only if it is forward biased in the manner described above. A component consisting only of such a p-n junction is known as a junction diode. The arrangement of p and n semiconductors inside a junction diode is illustrated below Figure(a) and the symbol used for a diode is shown below  Figure(b). Terminal A is known as the anode and terminal K is known as the cathode. Electricity is conducted through the junction only when the anode A is connected to the positive terminal of an external voltage supply and the direction of
current through the junction is shown with an arrow head below Figure (c).
Junction diode
The general outward appearance of a junction diode is shown below Figure. It has a cylindrical shape and a black color. The white or silver colored ring shows the terminal of the cathode. There are large numbers of various different types of diodes and a number is printed on the cylinder in order to identify them. But it should be remembered that the external appearance of a diode can vary widely.

General outward appearance of a junction diode
source by internet and books

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