A p-n junction is formed by combining P-type and N-type semiconductors together in very close contact. A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order An N-type semiconductor (N for Negative) is obtained by carrying out a process of doping, that is by adding an impurity of valence -five elements to A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that Normally they are manufactured from a single crystal with different dopant concentrations diffused across it. Creating a semiconductor from two separate pieces of material introduces a grain boundary between them which would severely inhibit its utility by scattering the electrons and holes. A grain boundary is the interface between two grains in a polycrystalline material The term junction refers to the region where the two regions of the 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:

A silicon p-n junction with no applied voltage. Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14

The p-n junction possesses some interesting properties which have useful applications in modern electronics. A p-doped semiconductor is relatively conductive. Electrical conductivity or specific conductivity is a measure of a material's ability to conduct an Electric current. The same is true of an 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. In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region In Physics, a charge carrier denotes a free (mobile unbound particle carrying an Electric charge. In Semiconductor production doping is the process of intentionally introducing impurities into an extremely pure (also referred to as intrinsic) semiconductor to The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J An electron hole is the conceptual and mathematical Opposite of an Electron, useful in the study of Physics and Chemistry. In the Solid state physics of Semiconductors carrier generation and recombination are processes by which mobile Electrons and Electron holes 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. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes 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. Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical

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. In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals A bipolar (junction transistor ( BJT) is a type of Transistor. 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. A solar cell or photovoltaic cell is a device that converts Solar energy into Electricity by the photovoltaic effect.

The invention of the p-n junction is usually attributed to Russell Ohl, Bell Laboratories. Russell Ohl (1898 - 1987 was an American engineer who is generally recognized for Patenting the modern Solar cell (US Patent 2402662 "Light sensitive device" Bell Laboratories (also known as Bell Labs and formerly known as AT&T Bell Laboratories and Bell Telephone Laboratories) is the Research organization

Equilibrium (zero bias)

In a p-n 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 V_bi.

In an equilibrium PN junction, electrons near the PN interface tend to diffuse into the p region. As electrons diffuse, they leave positively charged ions (donors) on the n region. In Semiconductors physics the term donor is used to generically indicate a dopant atom that added to a semiconductor can form N-type regions Similarly holes near the PN interface begin to diffuse in the n-type region leaving fixed ions (acceptors) with negative charge. In Semiconductors physics the term acceptor is used to generically indicate a dopant atom that added to a semiconductor can form P-type regions The regions nearby the PN interfaces lose their neutrality and become charged, forming the space charge region or depletion layer (see figure A). In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region

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. The electric field is shown on the bottom, the electrostatic force on electrons and holes and the direction in which the diffusion tends to move electrons and holes.

The electric field created by the space charge region opposes the diffusion process for both electrons and holes. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can 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 counteract the diffusion. The carrier concentration profile at equilibrium is shown in figure A with blue and red lines. Also shown are the two counterbalancing phenomena that establish 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 left uncovered by majority carrier diffusion. In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region In Semiconductors physics the term donor is used to generically indicate a dopant atom that added to a semiconductor can form N-type regions In Semiconductors physics the term acceptor is used to generically indicate a dopant atom that added to a semiconductor can form P-type regions In Physics, a charge carrier denotes a free (mobile unbound particle carrying an Electric charge. When equilibrium is reached, the charge density is approximated by the displayed step function. 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 figure B). The space charge region has the same charge on both sides of the PN interfaces, thus it extends farther on the less doped side (the n side in figures A and B).

Forward-bias

Forward-bias occurs when the P-type semiconductor material is connected to the positive terminal of a battery and the N-type semiconductor material is connected to the negative terminal, as shown below.

A silicon p-n junction in Forward-bias.

With a battery connected this way, the holes in the P-type region and the electrons in the N-type region are pushed towards the junction. An electron hole is the conceptual and mathematical Opposite of an Electron, useful in the study of Physics and Chemistry. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J This reduces the width of the depletion zone. In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region The positive charge applied to the P-type material repels the holes, while the negative charge applied to the N-type material repels the electrons. As electrons and holes are pushed towards the junction, the distance between them decreases. This lowers the barrier in potential. At a point in space the electric potential is the Potential energy per unit of charge that is associated with a static (time-invariant Electric field With increasing forward-bias voltage, the depletion zone eventually becomes thin enough that the zone's electric field can't counteract charge carrier motion across the p-n junction, consequently reducing electrical resistance. The electrons which cross the p-n junction into the P-type material (or holes which cross into the N-type material) will diffuse in the near-neutral region. Therefore, the amount of minority diffusion in the near-neutral zones determines the amount of current that may flow through the diode.

Only majority carriers (electrons in N-type material or holes in P-type) can flow through a semiconductor for a macroscopic length. With this in mind, consider the flow of electrons across the junction. The forward bias causes a force on the electrons pushing them from the N side toward the P side. With forward bias, the depletion region is narrow enough that electrons can cross the junction and inject into the P-type material. However, they do not continue to flow through the P-type material indefinitely, because it is energetically favorable for them to recombine with holes. The average length an electron travels through the P-type material before recombining is called the diffusion length, and it is typically on the order of microns. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre ^[1]

Although the electrons penetrate only a short distance into the P-type material, the electric current continues uninterrupted, because holes (the majority carriers) begin to flow in the opposite direction. The total current (the sum of the electron and hole currents) is constant in space, because any variation would cause charge buildup over time (this is Kirchhoff's current law). For other laws named after Gustav Kirchhoff, see Kirchhoff's laws. The flow of holes from the P-type region into the N-type region is exactly analogous to the flow of electrons from N to P (electrons and holes swap roles and the signs of all currents and voltages are reversed).

Therefore, the macroscopic picture of the current flow through the diode involves electrons flowing through the N-type region toward the junction, holes flowing through the P-type region in the opposite direction toward the junction, and the two species of carriers constantly recombining in the vicinity of the junction. The electrons and holes travel in opposite directions, but they also have opposite charges, so the overall current is in the same direction on both sides of the diode, as required.

The Shockley diode equation models the forward-bias operational characteristics of a p-n junction outside the avalanche (reverse-biased conducting) region. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes

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 material is now connected to the negative terminal of the power supply, the 'holes' in the P-type material are pulled away from the junction, causing the width of the depletion zone to increase. An electron hole is the conceptual and mathematical Opposite of an Electron, useful in the study of Physics and Chemistry. Similarly, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from the junction. Therefore the depletion region widens, and does so increasingly with increasing reverse-bias voltage. In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region This increases the voltage barrier causing a high resistance to the flow of charge carriers thus allowing minimal electric current to cross the p-n junction.

The strength of the depletion zone electric field increases as the reverse-bias voltage increases. Once the electric field intensity increases beyond a critical level, the p-n junction depletion zone breaks-down and current begins to flow, usually by either the Zener or avalanche breakdown processes. A Zener diode is a type of Diode that permits current in the forward direction like a normal diode but also in the reverse direction if the voltage is larger Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials Both of these breakdown processes are non-destructive and are reversible, so long as the amount of current flowing does not reach levels that cause the semiconductor material to overheat and cause thermal damage.

Summary

The forward-bias and the reverse-bias properties of the p-n junction imply that it can be used as a diode. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes 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. The Schottky diode (named after German physicist Walter H Schottky; also known as hot carrier diode) is a Semiconductor Diode with 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. In fabricating Semiconductor devices contact between the metal wires and the semiconductor material automatically creates p-n junctions called Schottky diodes Such a semiconductor An ohmic contact is a region on a Semiconductor device that has been prepared so that the current-voltage (I-V curve of the device is linear and symmetric

References

1. ^ Hook, J. R. ; H. E. Hall (2001). Solid State Physics. John Wiley & Sons. ISBN 0-471-92805-4.  

• Olav Torheim, Elementary Physics of P-N Junctions, 2007.

See also

• Diode
• Diode modelling
• 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
• Deep-level transient spectroscopy
• Solar Cell