HOW DIODE WORKS?
In the last post we have discussed about the construction and materials used in diodes formation.
In this post we will learn how a P-N junction diode works?
A P-N junction diode have the capability to conduct in only one direction. It prevents the flow of current in other direction. The conduction of diode is described by the biasing methods used..
WHAT IS BIASING?
The process by which diode is provided voltages to perform its basic function is called the biasing. For diodes there is two type of biasing techniques are used which are FORWARD bias and REVERSE bias.FORWARD BIAS:
I f the P side is connected to the positive supply and N side is connected with the negative supply of P-N junction the diode will became forward biased.Under no voltage or unbiased condition, the p-n junction diode does not allow the electric current. If the external forward voltage applied on the p-n junction diode is increased from zero to 0.1 volts, the depletion region slightly decreases. Hence, very small electric current flows in the p-n junction diode. However, this small electric current in the p-n junction diode is considered as negligible. Hence, they not used for any practical applications.
If the voltage applied on the p-n junction diode is further increased, then even more number of free electrons and holes are generated in the p-n junction diode. This large number of free electrons and holes further reduces the depletion region (positive and negative ions). Hence, the electric current in the p-n junction diode increases. Thus, the depletion region of a p-n junction diode decreases with increase in voltage. In other words, the electric current in the p-n junction diode increases with the increase in voltage.
Electron and hole current
Electron current
If the p-n junction diode is forward biased with approximately 0.7 volts for silicon diode or 0.3 volts for germanium diode, the p-n junction diode starts allowing the electric current. Under this condition, the negative terminal of the battery supplies large number of free electrons to the n-type semiconductor and attracts or accepts large number of holes from the p-type semiconductor. In other words, the large number of free electrons begins their journey at the negative terminal whereas the large number of holes finishes their journey at the negative terminal.
The free electrons, which begin their journey from the negative terminal, produce a large negative electric field. The direction of this negative electric field is apposite to the direction of positive electric field of depletion region (positive ions) near the p-n junction.
Due to the large number of free electrons at n-type semiconductor, they get repelled from each other and try to move from higher concentration region (n-type semiconductor) to a lower concentration region (p-type semiconductor). However, before crossing the depletion region, free electrons finds the positive ions and fills the holes. The free electrons, which fills the holes in positive ions becomes valence electrons. Thus, the free electrons are disappeared.
The positive ions, which gain the electrons, become neutral atoms. Thus, the depletion region (positive electric field) at n-type semiconductor near the p-n junction decreases until it disappears.
The remaining free electrons will cross the depletion region and then enters into the p-semiconductor. The free electrons, which cross the depletion region finds the large number of holes or vacancies in the p-type semiconductor and fills them with electrons. The free electrons which occupy the holes or vacancies will becomes valence electrons and then these electrons get attracted towards the positive terminal of battery or terminates at the positive terminal of battery. Thus, the negative charge carriers (free electrons) that are crossing the depletion region carry the electric current from one point to another point in the p-n junction diode.
Hole current
The positive terminal of the battery supplies large number of holes to the p-type semiconductor and attracts or accepts large number of free electrons from the n-type semiconductor. In other words, the large number of holes begins their journey at the positive terminal whereas the large number of free electrons finishes their journey at the positive terminal.
The holes, which begin their journey from the positive terminal, produce a large positive electric field at p-type semiconductor. The direction this positive electric field is opposite to the direction of negative electric field of depletion region (negative ions) near the p-n junction.
Due to the large number of positive charge carriers (holes) at p-type semiconductor, they get repelled from each other and try to move from higher concentration region (p-type semiconductor) to a lower concentration region (n-type semiconductor). However, before crossing the depletion region, some of the holes finds the negative ions and replaces the electrons position with holes. Thus, the holes are disappeared.
The negative ions, which lose the electrons, become neutral atoms. Thus, the depletion region or negative ions (negative electric field) at p-type semiconductor near the p-n junction decreases until it disappears.
The remaining holes will cross the depletion region and attracted to the negative terminal of battery or terminate at the negative terminal of battery. Thus, the positive charge carriers (holes) that are crossing the depletion region carry the electric current from one point to another point in the p-n junction diode.
Reverse biased p-n junction diode
The process by which, a P-N junction diode blocks the electric current in the presence of applied voltage is called reverse biased p-n junction diode.
In reverse biased p-n junction diode, the positive terminal of the battery is connected to the N-type semiconductor material and the negative terminal of the battery is connected to the P-type semiconductor material.
When the external voltage is applied to the p-n junction diode in such a way that, negative terminal is connected to the p-type semiconductor and positive terminal is connected to the n-type semiconductor, holes from the p-side are attracted towards the negative terminal whereas free electrons from the n-side are attracted towards the positive terminal.
In reverse biased p-n junction diode, the free electrons begin their journey at the negative terminal whereas holes begin their journey at the positive terminal. Free electrons, which begin their journey at the negative terminal, find large number of holes at the p-type semiconductor and fill them with electrons. The atom, which gains an extra electron, becomes a charged atom or negative ion or motionless charge. These negative ions at p-n junction (p-side) oppose the flow of free electrons from n-side.
On the other hand, holes or positive charges, which begin their journey at the positive terminal, find large of free electrons at the n-type semiconductor and replace the electrons position with holes. The atom, which loses an electron, becomes a charged atom or positive ion. These positive ions at p-n junction (n-side) oppose the flow of positive charge carriers (holes) from p-side.
If the reverse biased voltage applied on the p-n junction diode is further increased, then even more number of free electrons and holes are pulled away from the p-n junction. This increases the width of depletion region. Hence, the width of the depletion region increases with increase in voltage. The wide depletion region of the p-n junction diode completely blocks the majority charge carriers. Hence, majority charge carriers cannot carry the electric current.
However, p-n junction diode allows the minority charge carriers. The positive terminal of the battery pushes the holes (minority carriers) towards the p-type semiconductor. In the similar way, negative terminal of the battery pushes the free electrons (minority carriers) towards the n-type semiconductor.
The positive charge carriers (holes) which cross the p-n junction are attracted towards the negative terminal of the battery. On the other hand, the negative charge carriers (free electrons) which cross the p-n junction are attracted towards the positive terminal of the battery. Thus, the minority charge carriers carry the electric current in reverse biased p-n junction diode.
The electric current carried by the minority charge carriers is very small. Hence, minority carrier current is considered as negligible.
