Forward Bias

All About Biasing

In general, biasing is setting up a diode or any other devices alike to allow large amounts of current to flow through them in a specified direction. To understand the process of biasing efficiently, we usually consider the device as a Diode. A diode is a two-terminal semiconductor device. A diode can be connected to the source in several ways. One of those several methods of connecting the diode with the source is given by the Forward Bias definition. A junction diode acts as a closed switch when it is forward biased. It allows the current to flow through it. Any device for that matter is said to be forward-biased when the positive end of the device is connected to the anode of the source and the negative end of the device is connected to the cathode of the source (battery). The subsequent sections of this page give a detailed description of the characteristics of a p - n junction diode when it is forward biased. 

Basics of Forwarding Bias Diode

A terminal or a two-electrode device that is made up of semiconductors is called the p - n junction diode. It has a peculiar feature of allowing the current to flow in one direction and block the flow of current in the other direction. This kind of diode is formed by joining a p-type (trivalent impurity) semiconductor with an n-type (pentavalent impurity). The majority charge carriers in p-type semiconductors are holes and that in the case of n-type semiconductors is electrons.

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What is Forward Bias?

The application of an external voltage to the terminals of a p - n junction diode is called biasing. In the setup of a forward bias definition, the p - junction of the diode is connected to the positive terminal of the external voltage source and n - junction of the diode is connected to the negative terminal of the external voltage source.

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The applied voltage in the case of forward bias diode is opposite to that of the barrier potential of the junction. This results in the decreased effective potential barrier which in turn results in the decrease of junction width. This effect enables the majority of charge carriers to flow across the junction in more numbers. It is also important to consider that the amount of voltage required to eliminate the barrier completely is also considerably less. As the majority charge carriers are forced to move across the junction in a forward bias diode, the width of the depletion layer decreases at a rapid rate. The following sequence of events happens in a forward biased junction diode.

• A number of electrons and holes combine with each other after they cross the junction potential.

• In other words, every hole of the p- junction will combine with an electron from the n - junction. This results in the breakage of covalent bonds and the electrons liberated due to the break of covalent bonds move to the positive terminal of the source.

• A number of electron hole pairs are formed.

• The current carriers in the p-region are holes and the current carriers in the n-region are electrons.

Features of Forward Biased P - N Junction Diode

• A limiting resistance is always connected in series with the forward biased junction diode. This limiting resistance acts as an element that limits the flow of forward current into the diode. 

• The forward bias connection of a p - n junction diode results in the movement of majority charge carriers in p- region and the n- region towards the junction. Therefore, the region of immobile charges is reduced. This leads to the narrowing of the depletion layer.

• The forward voltage is referred to as Vd. In a forward biased p - n junction diode, the forward voltage and the electric field due to the space charge region oppose each other in terms of direction. Therefore, there is a negligibly small resultant electric field which is directed from n- junction to p- junction.

• There is a reduction of barrier height by the magnitude of forward voltage when the junction diode is forward biased.

Fun Facts

• A diode acts as a closed switch when it is forward biased and allows the current to flow through it. In the reverse bias condition, a diode acts as an open switch and does not allow current to flow through it.

• An LED is always forward biased. If it is connected in the reverse direction, it will not emit light and all the other LEDs leading this will not function in spite of being connected in the forward direction.