Forward biasing of a PN Junction refers to applying an external voltage across the diode that allows current to flow through it easily.
Biasing refers to an arrangement made in a diode or an electrical device to enable a more significant current flow in a specific direction. A diode or device can be connected to a source through different methods. One method is the forward bias, which works like a closed switch, allowing current to pass through it. A device is considered forward-biased when its anode is connected to the positive end and its cathode is connected to the negative end of the battery.
What is Forward Bias?
Forward biasing is a process where an external voltage is applied across the PN junction diode. During this process, the P-side of the diode is connected to the positive terminal, and the N-side is connected to the negative terminal of the battery.
When a voltage is applied to a PN junction in the opposite direction of the junction barrier potential, both the effective potential barrier and junction width decrease. This allows more majority carriers to flow across the junction with less voltage required to completely eliminate the barrier. Also, a forward-biased PN junction forces the majority of charge carriers to move across the junction, which decreases the width of the depletion layer.
- The number of electrons and holes combine when crossing the junction.
- When an electron from the N-side bonds with a hole in the P-side, the covalent bond breaks, and the electron moves toward the positive terminal.
- An electron-hole pair is formed.
- The current in the P region is due to holes.
- The current in N region due to electrons.
Before understanding the forward-biased state of a diode, let’s first understand what a PN junction diode is.
P-N Junction Diode
A P-N junction diode is a semiconductor device with two electrodes- anode and cathode-and it allows electric current to flow in only one direction. The diode only allows the electric current to flow in one direction and prevents it from flowing in the opposite direction. When a P-N junction diode allows the flow of electric current in the presence of applied voltage, it is referred to as a forward-biased P-N junction diode.
Silicon, gallium arsenide, and germanium are used to construct P-N junction diodes.
Properties of P-N Junction in Forward Bias
- A resistor Rs must be connected in series with the diode when a PN junction is in forward biasing. The series resistance limits the forward current to the rated current rating of the diode.
- When a P-N junction is forward-biased, majority carriers move towards the junction from both P and N regions. This movement reduces the depletion layer’s width by decreasing the region of immobile charges.
- When a diode is under forward bias, the electric field and the space charge region will oppose each other due to the forward voltage Vd. As a result, the electric field becomes very small. Experiments show that the electric field always flows from the N-type material to the P-type material under forward bias.
- When the P-N junction is forward-biased, the barrier height is reduced by the magnitude of the applied voltage.
P-N Junction Diode Under Forward Bias
When we apply the external voltage across a semiconductor diode, connecting the positive terminal of the source to the anode (p-side) and the negative terminal of the source to the cathode (n-side) of the diode, the diode is said to be forward-biased. This results in a decrease in the built-in potential of the diode, which reduces the width of the depletion region and the height of the barrier. The overall barrier voltage, in this case, is V0-VD, which is the difference between the built-in potential and the applied potential.
When the voltage is reduced to a small value, the barrier potential is also reduced to a minimal level. As a result, very little current flows through the junction. However, if the potential is increased significantly, the reduction in the barrier height will be more, allowing the passage of a greater number of carriers.
Different Cases of Forward Biasing of PN Junction
Case 1: If VD< V0 is applied.
The dominant factor is the barrier voltage (V0), meaning no majority carrier will cross the junction. As a result, the forward current is practically zero (although it can be between 10-12 to 10-15A), and the diode is in a forward-biased, non-conducting state. In other words, it is in an OFF state.
Case 2: If VD =V0 is applied
The barrier’s effect has been nullified, meaning it will no longer resist the majority carriers from crossing the junction. As a result, both the majority and minority carriers can cross the junction. This causes the small forward current to pass into the diode.
Case 3: If VD > V0
When the forward voltage of a diode, known as VD, is greater than the barrier voltage, Vo, more majority carriers can cross the junction, resulting in an exponential increase in the forward current with respect to the forward voltage VD. This leads to the diode being in a conducting or ON state. It is the state of the forward-biased diode.
Forward Current Equation of PN Junction Diode
The diode equation in forward bias is given below.
Where,
ID = Forward current flowing through the diode
IS = Saturation current
q = Charge of the electron (1.6 x 10-19 coulombs)
VD = Voltage applied across the diode
η =Ideality factor of the diode
K = Boltzmann constant (1.38 x 10-23 J/K)
T- Junction temperature in Kelvin
The PN junction produces a voltage called the thermal voltage(VT) due to temperature, represented by kT/q. The equation for diode current can be expressed using thermal voltage.
The value of thermal voltage VT is 26 millivolts when measured at room temperature. Let us assume a diode is an ideal diode. Therefore, the ideality factor(η) is 1. The diode equation is,
