Forward Bias is a term commonly used in semiconductor electronics, particularly concerning diodes and transistors. A diode is considered to be in forward bias when its p-side is connected to the positive side of a power supply and its n-side to the negative side. In the case of an NPN transistor, the base-emitter junction is forward-biased, which means the base is positive relative to the emitter. Conversely, for a PNP transistor, the base is negative relative to the emitter.

In this article, we will discuss all Forward Biases including Forward Bias of PN junctions and transistors.

What Is Forward Biasing?

Forward bias refers to a condition in electronics where a positive voltage is applied to the p-type material (anode) and a negative voltage to the n-type material (cathode) of a semiconductor device like a diode. This setup reduces the width of the depletion region at the p-n junction, which is a region devoid of free carriers (electrons and holes) and acts as a barrier to current flow in the diode.

In forward bias, the applied voltage essentially narrows this depletion zone and lowers the barrier, allowing charge carriers to cross the junction. As a result, electrons can move from the n-type material to the p-type material, and holes can move in the opposite direction, allowing electric current to flow through the device.

Forward Bias Definition

When positive terminal of the battery is connected to p-region and the negative terminal of the battery is connected to n-region of the junction diode then it s said to be forward biasing.

In simple words, we can say forward bias is a condition in which the voltage applied to a semiconductor diode allows current to flow easily through the diode.

Effects of Forward Biasing

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.

Forward Biasing Characterstics

The characteristics of a diode under forward bias:

• Conductivity Increases

In forward bias, the positive terminal of the voltage source is connected to the p-type (positive) semiconductor material, and the negative terminal is connected to the n-type (negative) semiconductor material. This arrangement reduces the potential barrier at the junction, allowing charge carriers (electrons in the n-type material and holes in the p-type material) to move across the junction more easily.

• Reduction in Barrier Potential

The forward bias reduces the potential barrier at the p-n junction. This barrier is essentially the energy barrier that prevents the flow of majority carriers across the junction.As the forward voltage is increased, the barrier potential decreases.

• Current Flow

With the reduced barrier potential, charge carriers can overcome the potential barrier and move across the junction.Current starts to flow through the diode in the direction of the external voltage.

• Low Resistance

Under forward bias, the diode exhibits low resistance, allowing for efficient conduction of current.

• Forward Voltage Drop

The forward voltage drop (V_f) is the voltage at which the diode starts conducting significantly. This voltage varies depending on the type of diode (e.g., silicon diodes typically have a forward voltage drop of around 0.7 volts).

What is P-N Junction Diode?

A P-N junction diode is a two-electrode semiconductor where the electric current flows only in one direction. The device does not allow the electric current to flow in the opposite direction. If a P-N junction diode facilitates the flow of electric current when the applied voltage is present, it is a forward bias P-N junction diode.

Different types of semiconductor materials, such as silicon, gallium arsenide, and germanium, are used to construct P-N junction diodes.

Read More about P-N Junction Diode.

Forward Bias P-N Junction Diode

The p-n junction is said to be forward-biased when the p-type is connected to the positive terminal of the battery and the n-type to the negative terminal. The built-in electric field at the p-n junction and the applied electric field are in opposing directions when the p-n junction is forward biased.

When any type of P-N junction is in forward bias, a resistor Rs must be connected in series with the diode.The function of the limiting resistance is to limit the forward current into the diode.

Under forward bias the field of the space charge region and forward voltage Vd will oppose each other. Hence, the resultant electric field is very small, and it is experimentally found that the field is always directed from N to P.When the P-N junction is forward bias, the barrier height reduces by |V0|(magnitude of VD).

Circuit Diagram of Forward Bias P-N Junction

The following is the illustration for circuit diagram of forward bias P-N Junction:

IV Curve For Forward Bias P-N Junction Diode

In forward biasing, forward current flows in the circuit. When the forward voltage equals zero, the value of forward current is also 0 mA. There is an increase in the current in forward bias diodes with increase in forward voltage till the point known as knee voltage. Beyond this point, the forward current increases rapidly.

Learn more about: IV Characterstics of PN Junction Diode.

Different Cases of Forward Bias

There are three cases for

• Case 1: If V_D< V₀

Barrier voltage (V₀) is dominating. Hence, no majority carrier will be crossing the junction.

• Case 2: If V_D >V₀

In this case, since the forward voltage of the diode which means more majority carriers will be able to cross the junction, and the forward current increases exponentially with the forward voltage V_D. The diode is now in a conducting state, or the diode is in the ON state.

• Case 3: If V_D = V₀

The effect of the barrier is nullified, i.e., the barrier will no longer oppose any majority carriers crossing the junction. Both the majority and minority carriers will be able to cross the junction. Hence, the forward current is small, or it just starts to flow into the diode.

Forward Current Equation

The diode current equation shows the relationship between the current flowing through the diode as a function of applied voltage. The mathematical expression of the diode current is given below.

I=I_ο[e^qV/ηkT-1]

Where,

• I is current flowing through the diode,
• I_o is reverse saturation current,
• q is the charge of the electron,
• V is the voltage applied across the diode,
• η is the exponential ideality factor of the diode,
• K is the Boltzmann constant and K = 1.38 x 10-23 J/K, and
• T is the absolute temperature in Kelvin.

Forward and Reverse Bias

As we already discussed that, in forward biasing the external voltage is applied across the PN-junction diode. This voltage cancels the potential barrier and provides the low resistance path to the flow of current.

In reversed bias the negative region is connected to the positive terminal of the battery and the positive region is connected to the negative terminal. The reverse potential increases the strength of the potential barrier. The potential barrier resists the flow of charge carrier across the junction. It creates a high resistive path in which no current flows through the circuit.

Difference between Forward and Reverse Bias

The key differences between both forward and reverse bias are: