P–n junction
A pn junction refers to a material transition in semiconductor crystals between regions with different doping. Areas in which the doping of the negative (N ) to positive (p ) changes, are found in many electrical devices in the semiconductor art. The peculiarity of the pn junction is the formation of a space charge region (also called depletion zone or barrier layer ) that allows the application of an external voltage current to flow in only one direction.
P -n junction in equilibrium
Doped semiconductors are uncharged in their ground state. There always exist the same number of free ( movable ) charge carriers such as fixed space charges of the ionized dopant atoms. Although the connection of two differently doped semiconductor materials is also neutral as a whole, however, this has a concentration gradient of the contained free mobile charge carriers result. Thus, the majority charge carriers will migrate by diffusion into the other semiconductor material, where their concentration is lower (concentration diffusion). That is, the electrons of the n- crystal struts in the P crystal where they recombine with holes. Holes diffuse in the p-type crystal and the n-side recombine with free electrons. Due to this diffusion and recombination now missing on either side of the carriers in the previously uncharged materials. Belonging to the absence of mobile charge carriers fixed with its dopant atoms no longer electrically compensated space charges cause an electric field, which exerts a force on the remaining free charge carriers. The drift motion caused thereby is opposite to the movement caused by diffusion and there is an equilibrium between the two.
The resulting electric field pushes the remaining free charge carriers back, so that both sides of the boundary between p- and n-crystal, a zone with no free charge carriers ( depletion zone ) is formed in which only remain the fixed space charges of the dopant atoms ( space charge region, RLZ ). The extent of this depletion region depends on the doping of the zone, and the intrinsic carrier density of the material. While high impurity density in the p- and n-type region, the space charge region is symmetrical. With unequal doping densities, the residual maturity continues to spread in the less heavily doped region.
Considering the energy band diagram of this arrangement, so have equalized by the diffusion process, the Fermi levels of the two crystals and it shows a curvature of the energy bands ( valence band and conduction band ) in the region of the pn junction. The above electrically neutral crystals have now obtained a space charge through the remaining, fixed charges, the negative and the n- crystal generates a positive pole in the P- crystal. The voltage thus created is the diffusion voltage (english built-in voltage, Ubi ) called. Also it depends on doping and material. A pn junction of silicon, the diffusion voltage of about 0.6-0.7 V is typical allocations for the carrier, the curvature of the energy bands of the wall a potential energy (e ... elementary charge ) represents the electrons and holes would have these wall face in order to reach into the respective other part. For this they need energy.
Pn junction electrical voltage applied
The power to overcome the diffusion voltage can be supplied in the form of electrical energy. This energy increases either the potential barrier or decreases it.
By application of an external voltage in the reverse direction ( N- crystal - the p- crystal), the electric field of the barrier layer is enhanced, and increases the extent of the space charge zone. Electrons and holes are drawn away from the barrier layer. There will only be a very low current, generated by the minority charge carriers ( reverse current ) other than the breakdown voltage is exceeded.
In polarity in the forward direction ( on p- crystal - on the n- crystal) of the potential wall is broken down. The electric field of the barrier layer will be completely neutralized at a certain applied voltage and it results in the externally applied electric field, a new electric field, which allows the charge transport through the entire crystal. New charge carriers flow from the external source to the barrier layer and recombine here continuously. With sufficient voltage is applied, a significant electric current flows.
Application
As shown above, directs the simple pn junction electric current in one direction very well, the other almost. Such an arrangement is called a diode ( semiconductor diode). An important application of the diode is thus the rectifier for converting alternating current into direct current. Special forms of the diode, the photodiode and the solar cell. In these, the opposite electrical charge of the space-charge region is used to separate generated electron-hole pairs. Photo diodes are therefore operated in the reverse direction. The impact of the resistance rises up, and the pn junction loses its influence on the electron-hole pairs.
Also, most other semiconductor devices include traditional-design one or more pn junctions to achieve their function, such as in the bipolar transistor, field effect transistor ( FET), MOS - FET, semiconductor detector, etc.
Calculation
The width of the space charge region, depending on the donor ( ND) and acceptor (NA ) is calculated at complete ionization of the dopant atoms according to Shockley
Wherein the permittivity of the vacuum, the relative permittivity, which is diffusion -setting voltage at the p-n contact, and the electron charge the voltage across the diode.
Metal - semiconductor junctions
If instead of two differently doped semiconductor, a metal contact with a p-or n -doped semiconductors, creating a metal-semiconductor junction. Depending on the materials and doping of the semiconductor is formed either a rectifying metal -semiconductor contact, also referred to as Schottky contact as it is applied to the Schottky diode. Or creating a contact with the linear transmission behavior, also referred to as an ohmic contact, which is essential for the wire bonding of semiconductor devices with metallic leads.