• In the previous section, we saw the basic structure of p-n junction diode. In this section we shall see the concept of diode biasing and how biasing is done.
  • There are two types of biasing namely forward biasing and reverse biasing. Here we discuss both types of biasing in detail.
  • In the section on forward biasing, we discuss the effects on depletion region due to forward biasing and also the effect of barrier potential.
  • In the section on reverse biasing, we discuss the effect of reverse biasing on the width of depletion region. We also discuss what is reverse saturation current and reverse breakdown voltage.
Biasing, in general, refers to the application of DC voltage across the terminals of device to establish certain operating condition for the device.Speaking of diode biasing in simple language, it simply means that external voltage is applied across the two terminals of the diode. Diode biasing can be done in two different ways

  • Forward biasing
  • Reverse biasing

Ofcourse the third condition is also possible when no bias is applied to the diode. That condition is simply called – no bias. We shall see in detail what is forward bias and reverse bias, and also the effect of biasing on the depletion region.

Forward biasing

When the positive terminal of the battery is connected to the p-type material and the negative terminal of the battery is connected to the n-type material, such a connection is called forward bias.

Diode in forward bias configuration. Width of depletion region decreases

Above figure shows the p-n junction diode in forward bias condition. The p region is connected to the positive terminal and n region is connected to the negative terminal of the DC voltage source. A resistor is also connected in series with the diode to make sure the current in the circuit does not rise above the maximum limit and damage the diode. When the diode is forward biased, the electric field in the depletion region and the external electric field due the DC voltage source are in opposite direction (This is shown in the above figure). This reduces the effective/net electric field in the depletion region. Recall that in the previous section on p-n junction diode, we had discussed that the flow of electrons and holes ceased due to the electric field. Since the net electric field is now reduced due to forward bias, electrons and holes now crosses the junction and constitutes a current. The direction of current is from p-region to n-region.

The flow of current can be explained as follows. The electrons and holes crosses the p-n junction as a result of reduced electric field in the depletion region. Electrons from the n-region crosses the junction and enters the p-region.Since the positive terminal of the battery is connected to the p-region, the electrons experiences an attractive force and moves to the positive terminal of the battery. Same discussion can also be applied to holes. Thus we can conclude that current flow takes place when the diode is forward biased.

Now we shall see the effect of applying forward bias to the diode. The topics we shall discuss are as follows

  • Effect on depletion region due to forward bias
  • Effect of barrier potential during forward bias

Effect on depletion region due to forward bias

When the diode is connected in forward bias, the electric field due to external voltage source and the electric field due to depletion region are in the opposite direction. This reduces the net electric field in the junction and the electrons can now pass from n-region to the p-region. As more electrons now flows into the depletion region, the number of positive ions is reduced. Same discussion can also be applied to holes. With the reduction in net electric field, the holes can now flow into the n-region. As the holes pass through the depletion region, the number of negative ions also decreases. Hence the the width of depletion region decreases due to reduction in the number of positive and negative ions. This is shown graphically in the figure below.

Effect of forward bias on depletion region

Effect of barrier potential during forward bias

Before preceding with this discussion, let us have a quick revision of what is  called as barrier potential (the concept is explained in detail here). When p-type and n-type materials are joined together, the electrons from n-type material starts to move towards p-type material and forms negative ions. Similarly the holes from the p-type material starts to move towards n-type material and forms positive ions. This separation of  positive and negative ions creates an electric field. When the electric field becomes sufficiently strong, it prevents further movement of electrons and holes. The potential resulting from such electric field act as barrier for further movement of electrons and holes. This potential is called barrier potential.

Now we come to the actual discussion- what is the effect of barrier potential when the diode is forward biased? When no bias is applied, the electrons cannot gain sufficient energy to overcome the potential barrier and move to the p-region. With the diode is forward biased, the electrons get enough energy from the voltage source to overcome the potential barrier and cross the junction. Similarly the holes get sufficient energy to overcome the barrier and cross the junction. The amount of energy required by the electrons to cross the junction is equal to the barrier potential (0.3 V for Ge and 0.7 V for Si). This simply means that when the diode is forward biased, the voltage drop across the diode is approximately 0.7 V (for Si). Actually, the amount of voltage drop is little above 0.7 V due to internal resistance of the material and contact resistance of the conducting material used to form the legs of diode.


 Reverse biasing

When the positive terminal of the battery is connected to n-type material and the negative terminal of the  battery is connected to p-type material, such a connection is called reverse bias.

PN junction diode in reverse bias configuration

Above figure shows the diode connected in reverse bias. You can clearly see that the negative terminal of the battery is connected to p-type material and the positive terminal of the battery is connected to n-type material. A resistor is also connected in series with the diode, although resistor is not required when the diode is reverse biased. When the diode is reverse biased, the electric field due to the battery and the electric field of the depletion region are in the same direction. This makes the electric field even stronger than that before reverse bias was applied. The electrons from the n-type material (majority carriers) now faces a stronger electric field and it becomes even more difficult for them to move towards the p-type material. Same discussion also applies to holes. The holes from the p-type material (majority carriers) now faces a stronger electric field and it becomes even more difficult to move from p-type to n-type material.Hence we conclude that there is no flow of current due to majority carriers when the diode is reverse biased.

We shall discuss following important points with regards to the reverse biased p-n junction.

  • Effect of reverse bias on the width of depletion region
  • Reverse saturation current
  • Reverse breakdown volatge

 Effect of reverse bias on the width of depletion region

Let us discuss how the width of depletion region changes when the reverse bias is applied. When the positive terminal of the battery is connected to the n-type semiconductor, the electrons from the n-type semiconductor are quickly drawn towards the positive terminal. (Refer the above figure). This reduces the number of majority carriers in n-type semiconductor. As the number of electrons reduces, additional positive ions are created. Similarly the holes from the p-type semiconductor are attracted towards the negative terminal of the battery. This reduces the number of holes in the p-type semiconductor and hence additional negative ions are created in the p-type material. Hence we conclude that as the number of positive and negative ions increases, the width of the depletion region increases. This is shown graphically in the figure below.

Effect of reverse bias on depletion region of PN junction diode. Width of depletion region increases.

Reverse saturation current

We saw in the earlier section that there is no flow of current due to majority carriers when the diode is reverse biased. However there is a very little flow of current (in nano ampere range for silicon diode) due to minority carriers that are produced in the crystal due to thermal energy. When the diode is reverse biased, the electrons from the p-type semiconductor are pushed towards the p-n junction by the negative terminal of the battery. Similarly the holes from the n-type semiconductor are pushed towards the junction by the positive terminal of the battery. This movement of electrons and holes constitutes a current called reverse saturation current.  The term saturation refers to the fact that it reaches its maximum value very quickly and does not change significantly with increase in reverse bias potential.

Reverse breakdown voltage

The magnitude of reverse current is of the order of nano-amperes for silicon devices. This current does not change significantly with the applied reverse bias potential. However, when the reverse bias is increased beyond a certain limit, the reverse current increases drastically. The voltage beyond which the reverse current increases drastically is called reverse breakdown voltage. The diode is said to undergo breakdown when the voltage is increased above breakdown voltage. Two mechanisms for diode breakdown are recognized- Avalanche breakdown and Zener breakdown. Both these mechanisms are discussed in detail in the section on avalanche and zener breakdown.