Forward Bias

Biasing, in general, is defined as an arrangement made in the diode or an electrical device to allow a larger flow of current in a certain direction. Usually, a device or, more precisely, a diode can be connected to a source in different ways. One method is through the forward bias, which tends to work like a closed switch, allowing current to pass through it. Besides, a device is said to be forward-biased when the anode is fixed to the positive end and the cathode is connected to the negative end of the battery.

We will study more about forward bias in this lesson, including its formation, characteristics and P-N junction diode.

Table of Contents

• What Is Forward Bias?
• P-N Junction Diode
• Properties of P-N Junction in Forward Bias
• Different Cases of Forward Bias
• Forward Current Equation of PN Junction Diode
• Frequently Asked Questions – FAQs

What Is Forward Bias?

Forward bias or biasing is where the external voltage is delivered across the P-N junction diode. In a forward bias setup, the P-side of the diode is attached to the positive terminal, and N-side is fixed to the negative side of the battery.

Here, the applied voltage is opposite to the junction barrier potential. Due to this, the effective potential barrier and junction width decrease, which further results in more majority of carriers flowing across the junction. Moreover, the amount of voltage required is also less for the complete elimination of the barrier. The forward biased PN junction forces the majority of charge carriers to move across the junction. Due to this reason, there is a decrease in the width of the depletion layer.

• The number of holes and electrons are combined with each other once the junction is crossed.
• Each hole in the P side combines with an electron that is from the N side. Due to this reason, a covalent bond will break, and an electron generated from the covalent bond move towards the positive terminal.
• There is a formation of electron-hole pair.
• Holes carry current in the P region.
• Electrons carry current in the N region.

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.

Properties of P-N Junction in Forward Bias

• When any type of P-N junction is in forward bias, a resistor R_s must be connected in series with the diode.
• The function of the limiting resistance is to limit the forward current into the diode.
• When a P-N junction is forward biased, the majority carrier of the P and N region will be moving towards the junction, and this will reduce the region of immobile charges and, therefore, the width of the depletion layer is reduced.
• Under forward bias, the field, because of the space charge region and forward voltage V_d will be opposing 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 V_D).

Different Cases of Forward Bias

Case 1: If V_D< V₀ is applied.

Barrier voltage (V₀) is dominating. Hence, no majority carrier will be crossing the junction. Thus, the forward current is Zero (practically, forward current is 10^-12 to 10^-15A), and the diode is now forward biased and non-conducting, i.e., it is in an OFF state.

Also Read: Basic Logic Gates

Case 2: If V_D =V₀ is applied

The effect of the barrier is nullified, i.e., the barrier hereafter will not oppose any majority carriers in crossing the junction. Both the majority and minority carriers will be crossing the junction. Hence, the forward current is small, or it just passes into the diode.

Case 3: If V_D > V₀

Since the forward voltage of the diode, V_D is greater than V_o, more majority carriers will be crossing the junction, and the forward current exponentially increases with the forward voltage V_D. The diode is in conducting state, or we can say the diode is in the ON state.

Important considerations for solving numerical:

If Ge or Si is not specified in the question, take n = 1

· I_s Reverse saturation current, and it is in the range of 10^-10 to 10^-15 A.

· I_s is highly sensitive to temperature.

· I_s doubles for every 5⁰C rise in temperature, but we always apply the thumb rule that this current I_s also doubles for a 10⁰C rise in temperature.

· The forward current exponentially increases with the forward voltage across the diode (I_f = I_se^Vo/nV_t)

· When the PN junction is forward biased, holes are injected from P to N and e^– are injected from N to P.

· This majority carriers of p and n regions are entering a carrier or excess minority carrier.

· Forward current is injected into minority carrier current or excess minority carrier current.

In a forward bias p-n junction, the sequence of the events is as follows:

1. Injection

2. Diffusion

3. Recombination

· Forward current is a diffusion current because this current passes through the junction from a higher concentration to a lower concentration.

· Forward current flows from p-n and is in mA

· In a forward-biased p-n junction, the current up to the edge of the depletion layer is due to the drift of the majority carrier.

Also Read: Diodes

Considering a p⁺n junction operating under forward bias, the minority carrier concentration distribution is as follows:

As the minority carrier of p and n regions cross into the opposite regions, they become injected minority carriers. The injected minority carrier concentration will be maximum at the edge of the depletion layer on the opposite side, and then they diffuse into the region. Hence, forward current is diffusion current, and it is also a minority carrier current.

Forward Current Equation of PN Junction Diode

The diode equation is given as

I_D = I_S(e^qV_D/NkT – 1)

Here,

I_D = diode current in amps

I_S = Saturation current in amps (1 x 10^-12 amps)

e = Euler’s constant (∼ 2.718281828)

q = Charge of electron (1.6 x 10^-19 coulombs)

V_D = Voltage applied across the diode in volts

N = Emission coefficient ( between 1 and 2)

k = Boltzmann,s constant (1.38 x 10^-23)

T = Junction Temperature

The term kT/q is the voltage produced within the PN junction due to the temperature, and this temperature is called the thermal voltage. The value of kT/q is equal to 26 millivolts at room temperature. Let us assume N to be equal to 1. Then, the diode equation can be written as

I_D = I_S(e^V_D/0.026 – 1)