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EspañolP-N JUNCTION
A 'p-n junction' is formed by combining N-type and P-type semiconductors together in very close contact. The term ''junction'' refers to the region where the two types of semiconductor meet. It can be thought of as the border region between the P-type and N-type blocks as shown in the following diagram:
The p-n junction possesses some interesting properties which have useful applications in modern electronics. P-doped semiconductor is relatively conductive. The same is true of N-doped semiconductor, but the junction between them is a nonconductor. This nonconducting layer, called the depletion zone, occurs because the electrical charge carriers in doped n-type and p-type silicon (electrons and holes, respectively) attract and eliminate each other in a process called recombination. By manipulating this nonconductive layer, p-n junctions are commonly used as diodes: electrical switches that allow a flow of electricity in one direction but not in the other (opposite) direction. This property is explained in terms of the ''forward-bias'' and ''reverse-bias'' effects, where the term ''bias'' refers to an application of electric voltage to the p-n junction.
A common type of transistor, the bipolar junction transistor, consists of two p-n junctions in series, for example in the form n-p-n; no current can flow through it unless a separate small voltage is applied to the middle layer. The most common type of solar cell is basically a large p-n junction; the free carrier pairs created by light energy are separated by the junction and contribute to current.
The invention of the p-n junction is usually attributed to Russell Ohl, Bell Laboratories.
In a pn junction, without an external applied voltage, an equilibrium condition is reached in which a potential difference is formed across the junction. This potential difference is called built-in potential .
In an equilibrium PN junction, electrons near the PN interface tend to diffuse into the p region. As electron diffuses, they leave positively charged ions (donors) on the n region. Similarly holes near the PN interface begin to diffuse in the n-type region leaving fixed ions (acceptors) with negative charge. The regions nearby the PN interfaces loose their neutrality and became charged forming the space charge region or depletion layer (see ).
The electric field originated by the space charge regions opposes to the diffusion process both for electron and holes. There are two concurrent phenomena: the diffusion process that tends to generate more space charge, and the electric field generated by the space charge that tends to contrast the diffusion. When the equilibrium is reached, the carrier concentration profile is the one showed in with blue and red lines. In the figure are also showed the two concurrent phenomena that work over to reach the equilibrium.
The space charge region is a zone with a net charge provided by the fixed ions (donors or acceptors) that have been leaved ''uncovered'' by the majority carrier diffusion. When the equilibrium is reached the charge density is in good approximation rectangular. In fact, the region is completely depleted of majority carriers (leaving a charge density equal to the net doping level), and the edge between the space charge region and the neutral region is quite sharp (see ). The space charge region has the same charge on both sides of the PN interfaces, thus it extends more on the less doped side (the n side in figures A and B).
Forward-bias occurs when the ''P-type'' block is connected to the ''positive'' terminal of a battery and the ''N-type'' block is connected to the ''negative'' terminal, as shown below.
With this set-up, the 'holes' in the P-type region and the electrons in the N-type region are pushed towards the junction. This reduces the width of the depletion zone. The positive charge applied to the P-type block repels the holes, while the negative charge applied to the N-type block repels the electrons. As electrons and holes are pushed towards the junction, the distance between them decreases. This lowers the barrier in potential. With increasing bias voltage, eventually the nonconducting depletion zone becomes so thin that the charge carriers can tunnel across the barrier, and the electrical resistance falls to a low value. The electrons which pass the junction barrier enter the P-type region (moving leftwards from one hole to the next, with reference to the above diagram).
This makes an electric current possible. An electron starts flowing around from the negative terminal to the positive terminal of the battery. It starts at the negative terminal, moving towards the N-type block. Having reached the N-type region it enters the block and makes its way towards the p-n junction. The junction barrier can no longer keep the electron in the N-type region due to the forward-bias effect (in other words, the thin depletion zone produces very little electrical resistance against the flow of electrons). The electron will therefore cross the junction and move ahead into the P-type block. Once inside the P-type region, the electron, being thermally free (from bonding)—or mobile—will move through the rest of the crystal, making its way to the positive terminal of the power supply. Please note that the electron does not jump from one hole to the next in the p-region. This actually qualifies as electron-hole recombination which immobilises both hole and electron. The electron can move freely through the crystal without needing to jump into holes which is what happens when electrons do cross the depletion layer. This process will be repeated over and over again, producing a complete circuit path through the junction.
The Shockley diode equation models the operation of a p-n junction outside the avalanche region.
Connecting the ''P-type'' region to the ''negative'' terminal of the battery and the ''N-type'' region to the ''positive'' terminal, produces the reverse-bias effect. The connections are illustrated in the following diagram:
Because the P-type region is now connected to the negative terminal of the power supply, the 'holes' in the P-type region are pulled away from the junction, causing the width of the nonconducting depletion zone to increase. Similarly, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from the junction.
This effectively increases the potential barrier and greatly increases the electrical resistance against the flow of charge carriers. For this reason there will be minimal electric current across the junction.
At the middle of the junction of the p-n material, the depletion region widens with increasing reverse bias. The electric field grows as the reverse voltage increases. When the electric field increases beyond a critical level, the junction breaks down and current begins to flow, usually by either the Zener or avalanche breakdown processes. Both of these breakdown processes are non-destructive and reversible so long as current density does not exceed levels that could cause thermal damage.
The forward-bias and reverse-bias properties of the p-n junction imply that it can be used as a diode. A p-n junction diode allows electric charges to flow in one direction, but not in the opposite direction; negative charges (electrons) can easily flow through the junction from n to p but not from p to n and the reverse is true for holes. When the p-n junction is forward-biased, electric charge flows freely due to reduced resistance of the p-n junction. When the p-n junction is reverse-biased, however, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal.
In the above diagrams, contact between the metal wires and the semiconductor material also creates metal-semiconductor junctions called Schottky diodes. In a simplified ideal situation a semiconductor diode would never function, since it would be composed of several diodes connected back-to-front in series. But in practice, surface impurities within the part of the semiconductor which touches the metal terminals will greatly reduce the width of those depletion layers to such an extent that the metal-semiconductor junctions do not act as diodes. These "nonrectifying junctions" behave as ohmic contacts regardless of applied voltage polarity.
★ Olav Torheim, ''Elementary Physics of P-N Junctions'', 2007.
★ Semiconductor
★
★ Semiconductor device
★
★ N-type semiconductor
★
★ P-type semiconductor
★ Transistor
★
★ Field effect transistor
★
★ Bipolar junction transistor
★
★
★ p-n-p transistor
★
★
★ n-p-n transistor
★
★ CMOS
★
★ NMOS
★
★ PMOS
★
★ Transistor-transistor logic
★ Capacitance voltage profiling
A silicon p-n junction with no applied voltage.
The p-n junction possesses some interesting properties which have useful applications in modern electronics. P-doped semiconductor is relatively conductive. The same is true of N-doped semiconductor, but the junction between them is a nonconductor. This nonconducting layer, called the depletion zone, occurs because the electrical charge carriers in doped n-type and p-type silicon (electrons and holes, respectively) attract and eliminate each other in a process called recombination. By manipulating this nonconductive layer, p-n junctions are commonly used as diodes: electrical switches that allow a flow of electricity in one direction but not in the other (opposite) direction. This property is explained in terms of the ''forward-bias'' and ''reverse-bias'' effects, where the term ''bias'' refers to an application of electric voltage to the p-n junction.
A common type of transistor, the bipolar junction transistor, consists of two p-n junctions in series, for example in the form n-p-n; no current can flow through it unless a separate small voltage is applied to the middle layer. The most common type of solar cell is basically a large p-n junction; the free carrier pairs created by light energy are separated by the junction and contribute to current.
The invention of the p-n junction is usually attributed to Russell Ohl, Bell Laboratories.
| Contents |
| Equilibrium (zero bias) |
| Forward-bias |
| Reverse-bias |
| Summary |
| Non-rectifying junctions |
| References |
| See also |
Equilibrium (zero bias)
In a pn junction, without an external applied voltage, an equilibrium condition is reached in which a potential difference is formed across the junction. This potential difference is called built-in potential .
In an equilibrium PN junction, electrons near the PN interface tend to diffuse into the p region. As electron diffuses, they leave positively charged ions (donors) on the n region. Similarly holes near the PN interface begin to diffuse in the n-type region leaving fixed ions (acceptors) with negative charge. The regions nearby the PN interfaces loose their neutrality and became charged forming the space charge region or depletion layer (see ).
'Figure A.' A p-n junction in thermal equilibrium with zero bias voltage applied. Electrons and holes concentration are reported respectively with blue and red lines. Gray regions are charge neutral. Light red zone is positively charged. Light blue zone is negatively charged. On the bottom is showed the electric field, the electrostatic force on electrons and holes and the direction in which the diffusion tends to move electrons and holes.
The electric field originated by the space charge regions opposes to the diffusion process both for electron and holes. There are two concurrent phenomena: the diffusion process that tends to generate more space charge, and the electric field generated by the space charge that tends to contrast the diffusion. When the equilibrium is reached, the carrier concentration profile is the one showed in with blue and red lines. In the figure are also showed the two concurrent phenomena that work over to reach the equilibrium.
'Figure B.' A PN junction in thermal equilibrium with zero bias voltage applied. Under the junction, plots for the charge density, the electric field and the voltage are reported.
The space charge region is a zone with a net charge provided by the fixed ions (donors or acceptors) that have been leaved ''uncovered'' by the majority carrier diffusion. When the equilibrium is reached the charge density is in good approximation rectangular. In fact, the region is completely depleted of majority carriers (leaving a charge density equal to the net doping level), and the edge between the space charge region and the neutral region is quite sharp (see ). The space charge region has the same charge on both sides of the PN interfaces, thus it extends more on the less doped side (the n side in figures A and B).
Forward-bias
Forward-bias occurs when the ''P-type'' block is connected to the ''positive'' terminal of a battery and the ''N-type'' block is connected to the ''negative'' terminal, as shown below.
A silicon p-n junction in Forward-bias.
With this set-up, the 'holes' in the P-type region and the electrons in the N-type region are pushed towards the junction. This reduces the width of the depletion zone. The positive charge applied to the P-type block repels the holes, while the negative charge applied to the N-type block repels the electrons. As electrons and holes are pushed towards the junction, the distance between them decreases. This lowers the barrier in potential. With increasing bias voltage, eventually the nonconducting depletion zone becomes so thin that the charge carriers can tunnel across the barrier, and the electrical resistance falls to a low value. The electrons which pass the junction barrier enter the P-type region (moving leftwards from one hole to the next, with reference to the above diagram).
This makes an electric current possible. An electron starts flowing around from the negative terminal to the positive terminal of the battery. It starts at the negative terminal, moving towards the N-type block. Having reached the N-type region it enters the block and makes its way towards the p-n junction. The junction barrier can no longer keep the electron in the N-type region due to the forward-bias effect (in other words, the thin depletion zone produces very little electrical resistance against the flow of electrons). The electron will therefore cross the junction and move ahead into the P-type block. Once inside the P-type region, the electron, being thermally free (from bonding)—or mobile—will move through the rest of the crystal, making its way to the positive terminal of the power supply. Please note that the electron does not jump from one hole to the next in the p-region. This actually qualifies as electron-hole recombination which immobilises both hole and electron. The electron can move freely through the crystal without needing to jump into holes which is what happens when electrons do cross the depletion layer. This process will be repeated over and over again, producing a complete circuit path through the junction.
The Shockley diode equation models the operation of a p-n junction outside the avalanche region.
Reverse-bias
Connecting the ''P-type'' region to the ''negative'' terminal of the battery and the ''N-type'' region to the ''positive'' terminal, produces the reverse-bias effect. The connections are illustrated in the following diagram:
A silicon p-n junction in Reverse-bias.
Because the P-type region is now connected to the negative terminal of the power supply, the 'holes' in the P-type region are pulled away from the junction, causing the width of the nonconducting depletion zone to increase. Similarly, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from the junction.
This effectively increases the potential barrier and greatly increases the electrical resistance against the flow of charge carriers. For this reason there will be minimal electric current across the junction.
At the middle of the junction of the p-n material, the depletion region widens with increasing reverse bias. The electric field grows as the reverse voltage increases. When the electric field increases beyond a critical level, the junction breaks down and current begins to flow, usually by either the Zener or avalanche breakdown processes. Both of these breakdown processes are non-destructive and reversible so long as current density does not exceed levels that could cause thermal damage.
Summary
The forward-bias and reverse-bias properties of the p-n junction imply that it can be used as a diode. A p-n junction diode allows electric charges to flow in one direction, but not in the opposite direction; negative charges (electrons) can easily flow through the junction from n to p but not from p to n and the reverse is true for holes. When the p-n junction is forward-biased, electric charge flows freely due to reduced resistance of the p-n junction. When the p-n junction is reverse-biased, however, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal.
Non-rectifying junctions
In the above diagrams, contact between the metal wires and the semiconductor material also creates metal-semiconductor junctions called Schottky diodes. In a simplified ideal situation a semiconductor diode would never function, since it would be composed of several diodes connected back-to-front in series. But in practice, surface impurities within the part of the semiconductor which touches the metal terminals will greatly reduce the width of those depletion layers to such an extent that the metal-semiconductor junctions do not act as diodes. These "nonrectifying junctions" behave as ohmic contacts regardless of applied voltage polarity.
References
★ Olav Torheim, ''Elementary Physics of P-N Junctions'', 2007.
See also
★ Semiconductor
★
★ Semiconductor device
★
★ N-type semiconductor
★
★ P-type semiconductor
★ Transistor
★
★ Field effect transistor
★
★ Bipolar junction transistor
★
★
★ p-n-p transistor
★
★
★ n-p-n transistor
★
★ CMOS
★
★ NMOS
★
★ PMOS
★
★ Transistor-transistor logic
★ Capacitance voltage profiling
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