What is Biasing:
No electrons move through the pn junction at equilibrium. Generally the term bias refers to the use of a dc voltage to establish certain operating conditions for an electronic device. In relation to a diode, there are two bias conditions: forward and reverse biasing. Either of these bias conditions is established by connecting a sufficient dc voltage of the proper polarity across the pn junction.
There are two types of biasing generally used to operate the diode.
To bias a diode, we apply dc voltage across it. Forward bias is the condition that allows current through the pn junction as shown in below figure, a dc voltage source connected by conductive material ( contacts and wire ) across a diode in the direction to produce forward bias. This external bias voltage is designated as V (Bias). The resistor, R, limits the current to a value that will not damage the diode.
The negative side of V Bias is connected to the n region of the diode and the positive side to the p region . This is one requirement for forward bias. A second requirement is that the bias voltage, V Bias , must be greater than the barrier potential.
A Fundamental picture of what happens when a diode is forward biased is shown below.
Because like charges repel, the negative aide of the bias-voltage source “pushes” the free electrons, which are the majority carriers in the n region, toward the pn junction. This flow of free electrons is called electron current. The negative side of the source also provides a continuous flow of electrons through the external connection ( conductor ) and into the n region .
The bias voltage source imparts sufficient energy to the free electrons for them to overcome the barrier potential of the depletion region and move on through into the p region. Once in the p region , these conduction electrons have lost enough energy to immediately combine with holes in the valence band.
Now, electrons are in the valence band in the p region, simply because they have lost too much energy overcoming the barrier potential to remain in the conduction band. Since unlike charges attract, the positive side of the bias-voltage source attracts the valence electrons towards the left end of the p region. The holes in the p region provide the medium or pathway for these valence electrons to move through the p region. The electrons move from one hole to the next toward the left. The holes, which are the majority carriers in the p region, effectively ( not actually ) move to the right toward the junction. This effective flow of holes is called hole current .
As the electrons flow out of the p region through the external connection ( conductor ) and to the positive side of the bias – voltage source, they leave holes behind in the p region; at the same time , these electrons become conduction electrons in the metal conductor So, there is a continuous availability of holes effectively moving toward the pn junction with the continuous stream of electrons as they come across the junction into the p region.
The Effect of Forward Bias on the Depletion Region:
As more electrons flow into the depletion region , the number of positive ions is reduced. As more holes effectively flow into the depletion region on the other side of the pn junction the number of negative ions is reduced. This reduction in positive and negative ions during forward bias causes the depletion region to narrow.
The effect of the Barrier Potential During Forward Bias:
Recall the electric field between the positive and negative ions in the depletion region on either side of the junction creates an energy hill, that prevents free electrons from diffusing across the junction at equilibrium. This is know as barrier potential.
When forward bias is applied , the free electrons are provided with enough energy from the bis voltage source to overcome the barrier potential and effectively climb the energy hill , and cross the depletion region. The energy that the electrons require in order to pass through the depletion region is equal to the barrier potentials. In other words, the electrons give up an amount of energy equivalent to the barrier potential when they cross the depletion region. This energy loss results in a voltage drop across the pn junction equal to the barrier potential . An additional small voltage drop occurs across the p and n regions due to the internal resistance of the material . For doped semiconductive material, this resistance , call the Dynamic resistance , is very small and can usually be neglected.
Reverse Biasing is the condition that essentially prevents current through the diode, as we can see below in figure that a dc voltage source connected across a diode in the direction to produce reverse biasing. This external Bias voltage is designated as V BIAS just as it was for forward bias. Notice that the positive side of V BIAS is connected to the n region of the diode and the negative side is connected to the p region. Also note that the depletion region is shown much wider than in forward bias or equilibrium.