P-n junction

Template:DISPLAYTITLE:p-n junction A p-n junction is formed by combining P-type and 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:

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.

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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. 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 lose their neutrality and become charged, forming the space charge region or depletion layer (see figure A).

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 figure A with blue and red lines. Also shown are the two counterbalancing phenomena that establish equilibrium.

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. 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.

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. This reduces the 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 potential. With increasing forward-bias voltage, the depletion zone eventually becomes thin enough that the zone's electric field can't counteract majority charge carrier motion (physics) across the p-n junction, consequently reducing electrical resistance. The electrons (or holes) which cross the p-n junction into the P-type material (or into the N-type material for holes) 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.

Electrons flow through the circuit from the negative battery terminal to the positive terminal of the battery. They are emitted at the negative battery terminal, then move through the connection wire into the N-type material at which point they move towards the p-n junction. When the p-n junction electric barrier no longer keeps electrons contained in the N-type material (because the thin depletion zone produces little electrical resistance against the flow of electrons), the electrons cross the p-n junction moving into the P-type material. Once in the P-type material, the electrons, being thermally free (from bonding, i.e. mobile) will move through the rest of the P-type material and back to the positive terminal of the battery. Note that the electron does not jump from one hole to the next in the p-type material, that would entail an electron-hole recombination which immobilizes both hole and electron. Instead, the electrons move freely through the P-type material without needing to jump into (i.e. recombine with) holes. This process occurs for all electrons arriving at the N-type material, thus allowing an electrical current to flow through the circuit path that includes the p-n junction.

The Shockley diode equation models the forward-bias operational characteristics of a p-n junction outside the avalanche (foward-biased conducting) 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:

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. 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 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 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

• 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