Earlier we have discussed the intrinsic and extrinsic types of semiconductor material. Semiconductor materials are used to develop many electronic devices. A p-n junction diode is one of those devices. In this article, we are going to discuss the formation, characteristics curves, depletion layer, potential barrier, bias circuit diagram, working principle and uses of a p-n junction diode.
Contents in this article:
- P-n junction diode
- p-n junction diode formation
- Potential Barrier of p-n junction
- Depletion layer of p-n junction
- Bias circuit of p-n junction diode
- Expression for diode current
- I-V curve of p-n junction diode
- Ideality factor (η) of diode
- Peak Inverse voltage (PIV) of diode
- Uses of p-n junction diode
A p-n junction diode is a semiconductor substrate with p-type and n-type doping at its two sides. Obviously, at p-side doping impurities are the Trivalent atoms and at n-side doping impurities are the pentavalent atoms.
There are four basic techniques of manufacturing a p-n junction diode—
- Grown Junction technique
- Alloy Junction technique
- Epitaxial technique
- Diffusion technique
Formation of p-n junction diode:
When two sides of a semiconductor substrate are doped with n-type and p-type respectively, the electrons from the n-side start to flow towards the p-side and the holes from the p-side start to flow towards the n-side, due to concentration gradients. At a region, n-type and p-type become in contact and produce a junction. This junction of p-type and n-type semiconductor is called p-n junction. Now, it has two electrodes – positive (p-side) and negative (n-side) and hence it is also called a diode.
Potential Barrier in p-n junction:
When p-type and n-type atoms meet at the junction, recombination between some of the electrons of the n-side and holes of the p-side takes place. As both n-type and p-type atoms are neutral, then n-type atoms and p-type atoms that have contributed to recombination will lose electrons and hole respectively.
So, n-type donor atoms will become positively charged ions and p-type acceptor atoms will become negatively charged ions after the recombination. This recombination takes place at a small width at either side of the junction and only the atoms nearer to the junction contribute to recombination. Now, the n-side near the junction gets positive potential and the p-side near the junction gets negative potential. This potential difference across the junction opposes electrons and holes to move further across the junction. Because electrons cannot move from higher potential to lower potential and holes cannot move from lower potential to higher potential. Therefore, this potential difference across the p-n junction is called “Potential Barrier”.
Depletion layer of p-n junction:
Depletion layer is the small region consisting only minority ions extended at either side of the junction. There are no majority carriers inside depletion region. In above diagram, W is the width of the depletion layer of p-n junction.
Symbol of p-n junction diode:
Biasing of p-n junction diode:
After formation of the p-n junction, a potential barrier arises across the junction which opposes further charge flow through the junction. At equilibrium, there will be no current crossing the junction. Now, to get current through the junction carriers need sufficient energy to overcome the potential barrier at the junction and this energy can be supplied by applying an external voltage across the p-n junction. The process of applying the external voltage across the p-n junction is known as biasing. A p-n junction can have two types of biasing – Forward bias and Reverse bias.
Forward Bias of p-n junction diode with circuit diagram:
If we connect the positive terminal of a battery with the p-side of the diode and negative terminal of the battery with the n-side of the diode, then the biasing is called Forward bias. Forward Bias (FB) allows the majority carriers to go forward i.e., when the diode is in forward bias, electrons will move from n-side to p-side and holes will move from the p-side to the n-side across the junction. In Forward Bias,
- Huge current flows across the junction
- Width of the depletion layer becomes smaller.
Reverse Bias of p-n junction diode with circuit diagram:
If we connect the positive terminal of a battery with the n-side of the diode and the negative terminal of the battery with p-side of the diode then the biasing will be called the Reverse Bias (RB). In Reverse bias, potential barrier increases, and majority carriers will experience more opposition to flow across the junction. However, the minority carriers will get the required energies to cross the junction as the reverse bias of p-n junction acts like the forward bias of minority carriers. Thus, in Reverse Bias,
- Very small current flows through the junction due to minority carriers.
- Width of the depletion layer increases.
Expression for diode current:
If V be the voltage across a p-n junction diode, then expression for the current through the junction of the diode is, I = I0 [exp(eV/ηKT) – 1 ] .
Here, T is the temperature and η is the ideality factor of diode.
Diode Current in forward bias, I = I0 exp(eV/ηKT)
Diode current in reverse bias is I = – I0 which is very small.
Working principle of p-n junction diode:
In forward bias condition, a high current flows across the p-n junction diode and in reverse bias condition a negligible current flows through the p-n junction. So, a p-n junction diode is active in forward bias condition and it is inactive or OFF in reverse bias condition. That means a p-n junction diode transfers current or any signal from input to its output only in forward bias condition. This is the main working theory of p-n junction diode. This principle can be verified by the characteristics curves of a p-n junction diode which are in the following sections.
p-n junction diode characteristics curve:
p-n junction diode has non-linear current-Voltage characteristics. It does not obey Ohm’s law and Kirchhoff’s law of current electricity. There are I-V curves of p-n junction diode at below for both in forward bias and reverse bias conditions. This is the characteristics of pn junction diode or the characteristic curve of pn junction diode.
Forward bias characteristics of p-n junction diode:
In the Forward bias, if the applied voltage exceeds the cut-in voltage (0.2 volt for Ge and 0.6 volt for Si) then current flows through the junction. The diode current increases exponentially to a very high value due to further increment of forward bias voltage. Above diagram shows the i-v curve of p-n junction diode in forward bias.
Reverse bias characteristics of p-n junction diode:
In the reverse bias condition, very small diode current flows due to minority carriers only and this amount of current is almost independent of the reverse bias voltage.
If we increase the reverse bias voltage further, the breakdown of the diode occurs at which reverse bias current becomes very high. Unfortunately, diodes cannot withstand at the breakdown region unless it is specially designed. Zener diode has a special design and some specifications such that it do not burn out even in the breakdown region.
I-V characteristics of Ideal Diode:
This is the I-V characteristic curve of ideal diode. Ideally, a p-n junction diode should have zero cut-in voltage. Also, it should have zero internal resistance in Forward bias and infinite internal resistance in Reverse bias. So, Diode current will be infinite even at zero forward bias voltage and there will be no diode current even in infinite reverse bias voltage.
Ideality Factor (η) of p-n junction diode:
Ideality factor (η) is a dimensionless parameter of a diode that determines the departure or deviation of diode characteristics from the ideal diode characteristics.
Idealility factor depends on the material, temperature and the physical construction of the diode. For Ge, η=1 and for Si, η=2.
Peak Inverse Voltage (PIV) of diode:
Peak inverse voltage is the maximum reverse bias voltage that can be applied across the diode before entering in breakdown region. So, after PIV, diode enters in breakdown region and it can burn out at a high temperature that arises due to the sudden rise of reverse bias current.
Uses of p-n junction diode:
A p-n junction diode has two major uses,
- As a switch
- As a rectifier to convert alternating current (AC) into direct current (DC).
Also, a p-n junction diode can be used as a circuit component in various electronic circuits.
So, in this article we learned about p-n junction diode circuit diagram, formation, characteristics curves and its uses. Beside this, we became to know how a depletion layer and potential barrier form in a p-n junction diode. This is all from this article. If you have any doubt on this topic you can ask me in the comment section.
- Intrinsic and Extrinsic type of Semiconductor Material
- Electronic circuit components and their uses (capacitor, Inductor, Diode, Transistor etc.)
- Use of Zener diode as voltage regulator