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?
- P-N Junction Diode
- Properties of P-N Junction in Forward Bias
- Different Cases
- Forward Current Equation of PN Junction Diode
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 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 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.
Properties of P-N Junction in Forward Bias
- 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.
- When a P-N junction is forward biased the majority carrier of the 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 N to P
- When P-N junction is forward bias, the barrier height reduces by |V0| ( magnitude of VD).
Case 1: If VD< V0 is applied.
Barrier voltage (V0) is dominating. Hence no majority carrier will be crossing the junction. Hence, the 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.
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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 the 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 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 minority carrier current or excess minority carrier current.
In a forward bias p-n junction, the sequence of the events are as follows:
· 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.
Considering a p+n junction operating under forward bias, the minority carrier concentration distribution is as follows.
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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 diode equation is given as
ID = IS(eqVD/NkT – 1)
ID = diode current in amps
IS = 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)
VD = Voltage applied across 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
ID = IS(eVD/0.026 – 1)
Frequently Asked Questions
1. How do contact potential and the transition region width across a P-N junction vary with forward biased?
Transition region width is reduced when the 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?
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?
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 the P region and no free electrons in the 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?
5. What is the main requirement to forward bias diode?