Formation of P-N Junction
The P-N Junction
Definition : When a P-type semiconductor is suitably joined to an N-typed semiconductor under suitable condition by using special technique to form a junction, the contact surface is known as a p-n junction.
The P-N junction is the basic building block on which the operation of all the semiconductor devices is dependent.
The behaviour of the P-N junction is developed based on the semiconductor properties described in the earlier posts in the semiconductor theory.
Diode means a device with two elements namely anode and cathode. Since a P-N junction itself is a two element devices, it becomes the most basic electronic device i.e. the diode.
1. The Open Circuited P-N Junction (Unbiased p-n Junction)
As shown in Fig., a P-type semiconductor and an N-type semiconductor are joined together with the help of a special fabrication technique to form a P-N junction.
Terminals are brought out for the external connection with P and N-type semiconductors. The p-side is called as anode and the n-side is called as cathode.
The "n" side consists of a large number of electrons and few thermally generated holes whereas the "p" side consists of a large number of holes and a few thermally generated electrons.
Thus the electrons are majority carriers and holes are minority carriers in the n-region whereas their roles are exactly opposite in the p-region.
The P-N junction forms the basic semiconductor device called diode. The understanding of characteristics and behaviour of a P-N junction proves to be useful to understand the operation of many semiconductor devices. Let us now discuss the behaviour of P-N junction.
1.1 Diffusion principle
At the junction, one side has a high concentration of holes whereas the other side has high concentration of electrons.
Due to this a concentration gradient is created across the junction, and the process of diffusion of the charge carrier, as shown in Fig. 2.11.2.
2. Formation of the Depletion Region : (Unbiased P-N Junction)
The behaviour of a P-N junction immediately after its formation is as follows. Note that no external voltage is applied between the terminals of the P-N junction, hence the P-N junction is said to be unbiased.
The free electrons from "n" side will diffuse into the p-side and recombine with the holes present there. Each electron diffusing into the "p" side will leave behind a positive immobile ion on the n-side as shown in Fig.
Fig. 3: Creation of positive and negative immobile ions
When an electron combines with a hole on the "p" side, an atom which accepts this electron, loses its electrically neutral status and becomes a negative immobile ion as shown in Fig. 2.11.3.
Thus immobile positive ions are formed on the n- side whereas immobile negative ions are formed on the p side of the junction.
Due to this recombination process, a large number of positive ions accumulate near the junction on the n-side and a large number of negative immobile ions will accumulate on the p-side near the junction as shown in Fig. 2.11.4.
The negatively charged ions on the p-side will start repelling the electrons, which attempt to diffuse into the p-side and after some time the diffusion will stop completely,
At this point, the junction is said to have attained an equilibrium. The P-N junction in the state of equilibrium is shown in Fig. 2.11.4.
Fig. 4: P-N junction with the depletion region
The shaded region on both sides of junction in Fig. contains only immobile ions and no free charge carriers such as electrons or holes. In other words this region is "depleted" of the free charge carriers.
Therefore this region is called as the "depletion region". This region is also known as the "space charge region".
- In the state of equilibrium, the depletion region gets widened to such an extent that electrons cannot cross the junction any more.
Note : In the open circuited P-N junction, the depletion region gets formed very quickly after the formation of the junction.
2.1 Width of the depletion region
We can define the width of the depletion region as the distance measured from one side to the other side of the depletion region.
Due to the presence of depletion region the electrons or holes can not diffuse to the other side. Hence it is said that the depletion region acts as a barrier.
Practically the width of depletion region is very small of the order of 0.5 to 1 micron where 1micron is equal to 1 x 10 ^-6 metres.
Thus the depletion region is very thin as compared to the widths of p and n regions.
3 Barrier potential or junction potential (Vj)
Due to the presence of immobile positive and negative ions on opposite sides of the junction, an electric field is created across the junction. This electric field is known as the "barrier potential" or "junction potential" or cut in voltage. It has fixed polarities as shown in Fig. 2.11.5.
The polarities of barrier potential are decided by the type of immobile ions present on the two sides of the junction. Thus the negative terminal of the barrier potential is on the p side and positive side is on the n-side as shown in Fig. 2.11.5.
This is called as barrier potential because it acts as ion a barrier to oppose the flow of electrons and holes across the junction.
The barrier potential represents the height of the barrier that is to be overcome for commencement of flow of electrons and holes.
Barrier potential is measured in volts. The barrier potential for Silicon is about 0.6 Volts whereas its value for the Germanium is 0.2 Volts. at 25°C.
Fig. 5: Barrier potential for different materials
3.1 Importance of barrier potential
Barrier potential is acting as a barrier (wall) which does not allow the electrons and holes to cross the junction
So if we want them to cross the junction, then an external voltage of appropriate polarity has to be applied in order to overcome the opposition of the barrier potential
Then the flow of electrons and holes across the junction can restart again.
Factors deciding the barrier potential value
Following are the factors which decide the value of the barrier potential :
1. Semiconductor material used ( Silicon or Germanium).
2. The intrinsic concentration of Si or Ge before doping.
3. The level of doping on p and n sides.
4. Temperature.
The complete unbiased p-n junction, immobile ions and the variation of barrier potential along the length is shown in the following figure.
Fig. 6: Open circuited P-N junction and variation of junctional potential
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