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 cathode is connected to the negative end of the battery.

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

Table of Content

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, 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. Forward biased PN junction forces the majority charges carries 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.
  • For each hole in P, junction combines with an electron that is from N junction. 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 carries a current in the P region.
  • Electrons carry a 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 diode.

Properties of P-N Junction in Forward Bias

  • When any type of P-N junction is in forward bias, a resistor Rs must be converted 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 P and N region will be moving toward 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 Vd 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 NàP
  • When P-N junction is forward bias, the barrier height reduces by |V0| ( magnitude of VD).

Different Cases

Case 1: If VD< V0 is applied.

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

Also Read: Basic Logic Gates

Case 2: If VD =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, forward current is small or fwd. current just passes into the diode.

Case 3: If VD > V0

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

Important Consideration for solving numerical:

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

· Is : Reverse saturation current and it is in the range of 10-10 to 10-15 A .

· Is is highly sensitive to temperature.

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

· The forward current exponentially increases with the forward voltage across the diode (If = IseVo/nVt)

· When PN junction is forward biased, holes are injected from Pà N and e are injected from NàP.

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

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

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

1. Injection

2. Diffusion

3. Recombination

· Forward current is a diffusion current because this current is passing through the junction from higher concentration to 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 drift of majority carrier.

Pn(0)à Injected hole concentration entering into the N side of injected minority carrier concentration crossing the edge of depletion layer on N-side.

Pnoà Thermal equilibrium minority carrier hole concentration in the N-side.

N(0)à Injected e concentration entering into the p-side or injected e concentration crossing the edge of the depletion layer on p-side.

Np0à Thermal equilibrium minority carrier e concentration on p-side.

Injected minority carrier concentration exponentially increases with the forward current of p-n junction diode is controlled by injected minority carrier concentration crossing the edge of the depletion layer.

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

Also Read: Diodes

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 also it is a minority carrier current.

Forward Current Equation of PN Junction Diode

The equation is given by

Is α ni2

But ni2 α T3

Hence Is is highly sensitive to the temperature

The reverse saturation current density,

(Js= Is/A) A/cm2

Forward current density à(Jf = If/A) A/cm2

Frequently Asked Questions

1. How do contact potential and the transition region width across a P-N junction vary with forward biased?

Answer:

Transition region width is reduced when P-N junction is forward biased and with an increase in the applied bias voltage the contact potential is reduced.

2. What kind of charges present on the opposite face of the junction?

Answer:

At P side of the junction, negative charges are present and at N side of the junction, positive charges are present.

3. A space region in P-N junction is called the depletion region. Give a reason?

Answer:

During the formation of P-N junction, the space around the junction is completely ionized. Due to this reason, there are no free holes in P region and no free electrons in N region. As the area around the junction is drained from mobile charges, it is called the depletion zone.

4. A depletion zone of P-N junction produces electricity. Give a reason?

Answer:

Segregating the densities of positive and negative space charges in p-n junction results in an electric field.

 

5. What is the main requirement to forward bias diode?

Answer:

The current is allowed to pass through the pn junction in forward bias mode. but one condition is external bias voltage negative side is connected to the n side of the diode and the external bias voltage positive side is connected to the p side of the diode.