P-N Junction Diodes, Physics tutorial

Introduction:

P-N junction makes the basis not just of rectifier diodes, however of many other electronic devices like LEDs, lasers, photodiodes and bipolar junction transistors and solar cells. A p-n junction aggregates recombination, generation, diffusion and drift effects into the single device. 

When a block of P-type semiconductor is positioned in contact by a block of N-type semiconductor in figure given below, then the result is of no value. It encompasses two conductive blocks in contact with one other, representing no unique properties. The problem is two separate and dissimilar crystal bodies. The number of electrons is balanced through the number of protons in both the blocks. Therefore, neither block consists of any total charge.

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Though, a single semiconductor crystal manufactured by P-type material at one end and N-type material at the other has certain exclusive properties. The P-type material consists of positive majority charge carriers, holes that are free to move about the crystal lattice. The N-type material consists of mobile negative majority carriers, electrons. Close to the junction, the N-type material electrons diffuse across the junction, joining with holes in P-type material.

P-N junctions are made by combining n-type and p-type semiconductor materials. As the n-type area or region consists of a high electron concentration and the p-type a high hole concentration, electrons diffuse from the n-type side to the p-type side. Likewise, holes flow through diffusion from the p-type side to the n-type side. When the electrons and holes were not charged, this diffusion procedure would continue till the concentration of electrons and holes on the two sides were similar, as occurs when two gasses come into contact with one other. Though, in a p-n junction, when the electrons and holes move to the other side of the junction, they leave behind exposed charges on dopants atom sites that are fixed in the crystal lattice and are not able to move. On n-type side, positive ion cores are exposed.

On the p-type side, negative ion cores are exposed. The electric field forms between the positive ion cores in the n-type material and negative ion cores in the p-type material. This area or region is termed as the depletion region as the electric field rapidly sweeps free carriers out, therefore the area or region is depleted of free carriers. A built in potential due to the electric field is made at the junction.

A p-n junction without external inputs shows equilibrium between carrier generation, recombination, diffusion and drift in the presence of the electric field in the depletion area or region. In spite of the presence of the electric field that makes an impediment to the diffusion of carriers across the electric field, a few carriers still cross the junction through diffusion. Most of the majority carriers that enter the depletion region move back in the direction of the region from which they originated. Though, statistically a few carriers will encompass a high velocity and travel in a sufficient net direction in such a way that they cross the junction. Once a majority carrier crosses the junction, it becomes a minority carrier. This will continue to diffuse away from the junction and can travel a distance on average equivalent to the diffusion length before it rejoins. The current caused through the diffusion of carriers across the junction is termed as the diffusion current.

Minority carriers that reach the edge of the diffusion area are swept across it through the electric field in the depletion region. This current is termed as the drift current. In equilibrium the drift current is limited by the number of minority carriers that are thermally produced in diffusion length of the junction.

In equilibrium, the total current from the device is zero. The electron drift current and the electron diffusion current precisely balance out (if they didn't there would be a total build-up of electrons on either one side or the other of the device). Likewise, the hole drift current and the hole diffusion current as well balance each other out.

P-N Junction and Junction Diodes:

The P-N junction has certain properties which have helpful applications in modern electronics. A p-doped semiconductor is relatively conductive. The similar is true of an n-doped semiconductor; however the junction among them can become depleted of charge carriers and therefore nonconductive, based on the relative voltages of the two semiconductor areas. By manipulating this non-conductive layer, p-n junctions are generally employed as diodes: circuit elements which let a flow of electricity in one direct ion however not in the other (opposite) direction. This property is described in terms of forward bias and reverse bias, where the term bias refers to the application of electric voltage to the p-n junction.

The forward-bias and the reverse-bias properties of the p-n junction mean that it can be employed as a diode. A p-n junction diode lets electric charges to flow in one direction, however not in the opposite direction; negative charges (that is, electrons) can simply flow via the junction from n to p however not from p to n and the reverse is true for holes. If the p-n junction is forward biased, electric charge flows freely due to reduced resistance of the p-n junction. If the p-n junction is reverse biased, though, the junction barrier (and thus resistance) becomes greater and charge flow is negligible.

Generally, p-n junctions are manufactured from the single crystal having different dopants concentrations diffused across it. Making a semiconductor from two separate pieces of material would introduce a grain boundary among the semiconductors that severely hinders its utility through scattering the electrons and holes. Though, in case of solar cells, polycrystalline silicon is often employed to decrease expense, in spite of the lower efficiency

The diode is one of the simplest semiconductor devices and it consists of the characteristic of passing current in one direction only. Though, dissimilar a resistor, a diode doesn't behave linearly with respect to the applied voltage as the diode consists of an exponential voltage or current relationship and thus can't be illustrated operationally through simply applying Ohm's law.

When an appropriate positive voltage (that is, forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes by the additional energy they need to cross the junction as the width of the depletion layer around the PN junction is reduced. By applying a negative voltage (reverse bias) outcome in the free charges being pulled away from the junction resulting in the depletion layer width being raised. This consists of the effect of rising or reducing the effective resistance of the junction itself allowing or blocking current flow via the diode.

Then the depletion layer broadens by a raise in the application of a reverse voltage and narrows by a raise in the application of a forward voltage. This is due to the differences in the electrical properties on the two sides of the PN junction resultant in physical changes taking place. One of the outcomes generates rectification as seen in the PN junction diodes static I-V (current-voltage) features.

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P-N junctions can't be employed as a rectifying device devoid of first biasing the junction through joining a voltage potential across it. Reverse Bias refers to the external voltage potential that rises the potential barrier while an external voltage that reduces the potential barrier is stated to act in the Forward Bias direct ion.

There are two operating areas and three possible biasing conditions for the standard Junction Diode: Zero Bias, Reverse Bias and Forward Bias.

Zero Bias:

If a diode is joined in a Zero Bias condition, no external potential energy is applied to the PN junction. Though when the diodes terminals are shorted altogether, a few holes (that is, majority carriers) in the P-type material having adequate energy to overcome the potential barrier will move across the junction against the barrier potential. This is termed as the Forward Current.

Similarly, holes produced in the N-type material (that is, minority carriers), find this condition favorable and move across the junction in the opposite direction. This is termed as the Reverse Current. This transfer of electrons and holes back and forth across the PN junction is termed as diffusion.

Zero Biased Junction Diode:

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The potential barrier which now exists depresses the diffusion of any more majority carriers across the junction. Though, the potential barrier helps minority carriers (that is, few free electrons in the P-region and few holes in the N-region) to drift across the junction. Then an Equilibrium or balance will be established if the majority carriers are equivalent and both moving in the opposite directions, in such a way that the total result is zero current flowing in the circuit. When this takes place the junction is stated to be in the state of Dynamic Equilibrium.

The minority carriers are continuously produced due to thermal energy so this state of equilibrium can be broken by increasing the temperature of the PN junction causing a raise in the generation of the minority carriers, thus resultant in an increase in leakage current however an electric current can't flow as no circuit has been joined to the PN junction.

Equilibrium:

In a p-n junction, devoid of an external applied voltage, an equilibrium condition is reached in which a potential difference is made across the junction. This potential difference is termed as built-in potential. After joining p-type and n-type semiconductors, electrons close to the p-n interface tend to diffuse into the p-region. Since electrons diffuse, they leave positively charged ions (donors) in the n-region. Likewise, holes close to the p-n interface begin to diffuse into the n-type region leaving fixed ions (acceptors) having negative charge. The regions close to the p-n interfaces lose their neutrality and become charged, making the space charge region or depletion layer.

Reverse Bias

If a diode is joined in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material. The positive voltage applied to the N-type material fascinates or attracts electrons in the direction of positive electrode and away from the junction, as the holes in the P-type end are as well attracted away from the junction in the direction of the negative electrode. The total result is that the depletion layer grows broader due to the lack of electrons and holes and presents a high impedance path, almost an insulator. The outcome is that a high potential barrier is made therefore preventing current from flowing via the semiconductor material.

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This condition shows a high resistance value to the PN junction and practically zero current flows via the junction diode by an increase in the bias voltage. Though, an extremely small leakage current does flow via the junction that can be evaluated in microamperes, if the reverse bias voltage applied to the diode is raised to an adequately high adequate value, it will cause the PN junction to overheat and fail due to the avalanche effect around the junction. This might cause the diode to become shorted and will outcome in the flow of maximum circuit current.

Zener Diode:

When the diode is reverse biased then only the leakage current of the intrinsic semiconductor flows. This current will just be as high as 1 µA for the most excessive conditions for silicon small signal diodes. This current doesn't rise appreciably by increasing reverse bias till the diode breaks down. At breakdown, the current rises greatly and the diode will be destroyed unless a high series resistance limits current. Generally a diode by a higher reverse voltage rating than any applied voltage is chosen to prevent this reverse breakdown. Silicon diodes are usually available by reverse break down ratings of 50, 100, 200, 400, 800 V and higher.

This is possible to fabricate diodes having a lower rating of a few volts for use as voltage standards and this effect is utilized to gain in Zener diode regulator circuits. Zener diodes encompass a certain - low - breakdown voltage. A typical value for breakdown voltage is for instance 5.6V. This signifies that the voltage at the cathode can never be more than 5.6V higher than the voltage at the anode, as the diode will break down - and thus conduct - when the voltage gets any higher. This efficiently controls the voltage over the diode.

Forward Bias:

If a diode is joined in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. When this external voltage becomes more than the value of the potential barrier, around 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overcome and current will begin to flow. This is due to reason that negative voltage pushes or repels electrons in the direction of the junction giving them the energy to cross over and join by the holes being pushed in the opposite direction towards the junction through the positive voltage. This outcome in a characteristics curve of zero current flowing up to this voltage point, termed as the knee on the static curves and then a high current flow via the diode having little raise in the external voltage.

The application of a forward biasing voltage on the junction diode outcomes in the depletion layer becoming extremely thin and narrow that shows a low impedance path via the junction thus allowing high currents to flow.

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This case shows the low resistance path via the PN junction allowing extremely large currents to flow via the diode with just a small raise in bias voltage. The real potential difference across the junction or diode is kept constant through the action of the depletion layer at around 0.3v for germanium and around 0.7v for silicon junction diodes. As the diode can conduct extremely high current above the knee point as it efficiently becomes a short circuit and exceeding the maximum forward current specification causes the device to disperse more power in the form of heat resultant in the quick failure.

Non-Rectifying Junctions:

It is familiar that not all the P-N junctions rectify. Schottky junction is a special case of a p-n junction, in which metal serves the role of the n-type semiconductor. The Shockley diode equation models the forward-bias operational features of a p-n junction outside the avalanche (that is, reverse-biased conducting) region.

Contact between the metal wires and the semiconductor material as well makes metal-semiconductor junctions termed as Schottky diodes. In a simplified ideal condition a semiconductor diode would never function, as it would be composed of some diodes joined back-to-front in series. However in practice, surface impurities in the part of semiconductor that touches the metal terminals will greatly decrease the width of those depletion layers to such an extent that the metal-

Semiconductor junctions don't act as diodes. These 'non rectifying junctions' act as ohmic contacts in spite of applied voltage polarity.

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