P-n junction 571755 224623904 2008-07-09T18:27:31Z Attilios 428795 /* References */ using reflist {{lowercase}} A '''p-n junction''' is formed by combining [[P-type semiconductor|P-type]] and [[N-type semiconductor|N-type]] [[semiconductors]] together in very close contact. 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. 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: [[Image:PN Junction Open Circuited.svg|thumb|250px|right|A [[silicon]] p-n junction with no applied voltage.]] The p-n junction possesses some interesting properties which have useful applications in modern electronics. A p-doped semiconductor is relatively [[Electrical conductivity|conductive]]. The same is true of an n-doped semiconductor, but the junction between them is a nonconductor. This nonconducting layer, called the [[Depletion region|depletion zone]], occurs because the electrical [[charge carrier]]s in [[doping (semiconductors)|doped]] n-type and p-type silicon ([[electrons]] and [[Electron hole|holes]], respectively) attract and eliminate each other in a process called [[recombination (physics)|recombination]]. By manipulating this nonconductive layer, p-n junctions are commonly used as [[diode]]s: circuit elements 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]]. == 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 <math>V_{\rm bi}</math>. 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 ([[donor (semiconductors)|donors]]) on the n region. Similarly holes near the PN interface begin to diffuse in the n-type region leaving fixed ions ([[acceptor (semiconductors)|acceptors]]) with negative charge. The regions nearby the PN interfaces lose their neutrality and become charged, forming the [[space charge region]] or [[depletion layer]] (see [[:Image:Pn-junction-equilibrium.png|figure A]]). [[Image:Pn-junction-equilibrium.png|550px|center|thumb|'''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. 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 [[:Image:Pn-junction-equilibrium.png|figure A]] with blue and red lines. Also shown are the two counterbalancing phenomena that establish equilibrium. [[Image:Pn-junction-equilibrium-graphs.png|550px|center|thumb|'''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 ([[donor (semiconductors)|donors]] or [[acceptor (semiconductors)|acceptors]]) that have been left ''uncovered'' by [[majority carrier]] diffusion. 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 [[:Image:Pn-junction-equilibrium.png|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. [[Image:PN Junction in Forward Bias.png|frame|right|A silicon p-n junction in Forward-bias.]] With a battery connected this way, the [[Electron hole|holes]] in the P-type region and the [[electrons]] in the N-type region are pushed towards the junction. This reduces the [[depletion width|width]] of the [[depletion zone]]. 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 [[electrical potential|potential]]. 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 [[micrometre|microns]].<ref>{{cite book |title=Solid State Physics |last=Hook |first=J. R. |coauthors=H. E. Hall |year=2001 |publisher=John Wiley & Sons |isbn=0-471-92805-4 }}</ref> 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 circuit laws#Kirchhoff's Current Law (KCL)|Kirchhoff's current law]]). 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 [[Diode#Shockley diode equation|Shockley diode equation]] models the forward-bias operational characteristics of a p-n junction outside the avalanche (reverse-biased conducting) region. == Reverse-bias ==<!-- This section is linked from [[Photodiode]] --> {{Mergefrom|Reverse-biased|date=September 2007}} 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: [[Image:PN Junction in Reverse Bias.png|frame|right|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 '[[Electron hole|holes]]' in the P-type material are pulled away from the junction, causing the width of the 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. Therefore the [[depletion region]] widens, and does so increasingly with increasing reverse-bias voltage. 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 breakdown|Zener]] or [[avalanche breakdown]] processes. 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]]. 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 diode]]s. 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 junction]]s" behave as [[ohmic contact]]s regardless of applied voltage polarity. ==References== {{reflist}} * Olav Torheim, ''[http://web.ift.uib.no/~torheim/pnsjikt.pdf Elementary Physics of P-N Junctions]'', 2007. ==See also== {{commonscat|PN-junction diagrams}} * [[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]] [[Category:Semiconductors]] [[be-x-old:P-n пераход]] [[bg:P-n преход]] [[ca:Junció PN]] [[cs:Přechod P-N]] [[de:P-n-Übergang]] [[eo:Junto (elektro)]] [[es:Unión PN]] [[et:Pn-siire]] [[ko:PN 접합]] [[id:P-n junction]] [[it:Giunzione p-n]] [[hu:P-n átmenet]] [[nl:Pn-overgang]] [[ja:Pn接合]] [[pl:Złącze p-n]] [[pt:Junção PN]] [[ro:Joncţiune p-n]] [[ru:P — n-переход]] [[sv:PN-övergång]] [[uk:P-n перехід]]