Forward and Reverse Biasing in Semiconductor Diodes

Types of Biasings used in Diodes:

In semiconductors, there are two types of biasing, one is called forward bias and other is called reverse bias. Forward bias eliminates the depletion region and causes a diode to pass current.  And reverse bias increase the size of depletion region and it blocks current. This diode biasing behavior changes with the change of materials. For example in case of silicon and germanium, bias voltages will be different for both. About biasing voltages you will learn in below explanation.  In below figures (a), you can see the forward and reverse bias representations.

Forward Bias, reverse bias, diode, diode biasing, Forward Biasing, Reverse Biasing


Here we will see that how this biasing process can be established in diodes, a diode is biased by placing some potential difference across diode terminals. This method can be seen in below figure (c) where you can see forward bias diode.

Because of the positive potential applied to the anode of diode and the negative potential applied to the cathode of diode, the depletion region disappears. Current flows from the negative terminal of the battery through the N region, across the non-existent depletion region, and through the P region to the positive terminal of the battery. It takes a specific value of voltage for a diode to begin conduction. Approximately .3 volts across a germanium diode or .7 volts across a silicon diode are necessary to provide forward bias and conduction to occur. A germanium diode requires a lower voltage due to its higher atomic number, which makes it more unstable. Silicon diode is used far more extensively than Reverse bias is accomplished by applying a positive potential to the cathode and a negative potential to the anode as shown in Figure (b). The positive potential on the cathode will attract electrons from the depletion region. At the same time, the negative potential on the anode will attract holes. The net result is that the depletion region will increase in size.

Forward Bias, reverse bias, diode, diode biasing, Forward Biasing, Reverse Biasing

Figure (b) and (c)

A forward biased diode will conduct, with only a small voltage drop over it. The voltage drop for a forward biased germanium diode is .3 volts, while .7 volts is normal for a silicon diode. We can say that a forward biased conducting diode is almost a short. A reversed biased diode will not conduct. Therefore, it can be considered an open circuit. We call a reversed biased diode cut off. Cut off refers to the current flow through the diode being blocked, or cut off.

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