pn junction

A pn junction denotes a material junction in semiconductor crystals between regions with opposite doping . Areas in which the doping changes from negative (n) to positive (p) occur in many electrical components in semiconductor technology . The special feature of the pn junction is the formation of a space charge zone (also called depletion zone or barrier layer), which allows current to flow only in one direction when an external voltage is applied.

A pn junction acts like a flow valve , which is used, for example, in single-crystal semiconductor diodes and blocks (blocked state) or lets through (open state) applied current.

pn junction in equilibrium

Construction of a barrier layer (space charge zone, RLZ) in the asymmetrical pn junction (p-side is more heavily doped here). Based on the charge distribution in the top picture, the electric field and the electric potential at the transition are obtained by integrating the Poisson's equation . Due to the Gaussian integral theorem and the first Maxwell's equation , the electric field outside the space charge zone must be zero, since the total charge of the entire space charge zone must not violate electrical neutrality.

Doped semiconductors are uncharged in their ground state. There are always the same number of free (mobile) charge carriers as there are stationary space charges of the ionized doping atoms. Although the connection between two oppositely doped semiconductor materials is overall neutral, this results in a concentration gradient of the freely moving charge carriers contained therein. The majority charge carriers will migrate by diffusion into the other semiconductor material in which their concentration is lower (concentration diffusion ). That means: The electrons of the n-crystal strive into the p-crystal and recombine there with holes . Holes of the p-crystal diffuse to the n-side and recombine there with free electrons. Because of this diffusion and recombination, charge carriers are now missing on both sides in the previously uncharged materials. The stationary doping atoms belonging to the missing mobile charge carriers with their space charges that are no longer electrically compensated cause an electric field which exerts a force on the remaining free charge carriers. The drift movement caused by this is opposite to the movement caused by diffusion and an equilibrium is established between the two.

The resulting electric field pushes back the remaining free charge carriers, so that a zone without free charge carriers ( depletion zone ) is created on both sides of the boundary between p- and n-crystal , in which only the stationary space charges of the doping atoms remain ( space charge zone , RLZ). The extent of this depletion zone depends on the doping of the zone and the intrinsic charge carrier density of the material. With the same high doping density in the p and n regions, the space charge zone is symmetrical. If the doping densities are unequal, the RLZ spreads further into the less heavily doped area.

If one looks at the band model of this arrangement, the Fermi levels of the two crystals have equalized due to the diffusion process and there is a curvature of the energy bands ( valence band and conduction band ) in the area of ​​the pn junction. The previously electrically neutral crystals have now received a space charge due to the solid charges that remain, which creates a negative pole in the p-crystal and a positive pole in the n-crystal. The ensuing voltage is diffusion voltage (English built-in voltage , U _bi ) mentioned. It also depends on the doping and material. For a pn junction made of silicon , the diffusion voltage for typical doping is around 0.6 to 0.7 V. For the charge carriers, the curvature of the energy bands represents a potential wall of energy (e is the elementary charge ). The electrons and holes must have this Overcome wall to get to the other part. For that they need energy. ${\displaystyle \Phi _{\mathrm {D} }}$ ${\displaystyle E=\Phi _{\mathrm {D} }\cdot e}$

pn-junction with applied electrical voltage

The energy to overcome the diffusion voltage can be supplied in the form of electrical energy . This energy either increases or decreases the potential wall.

By applying an external voltage in the reverse direction (+ on the n-crystal, - on the p-crystal) the electric field of the barrier layer is strengthened and the expansion of the space charge zone is increased. Electrons and holes are drawn away from the barrier layer. Only a very small current flows, generated by minority charge carriers ( reverse current ), unless the breakdown voltage is exceeded.

If the polarity is in the forward direction (+ on the p-crystal, - on the n-crystal), the potential wall is broken down. The electric field of the barrier layer is completely neutralized from a certain applied voltage and a new electric field results with the externally applied electric field, which allows 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 applied, a significant electrical current flows .

application

As shown above, the simple pn junction conducts electricity very well in one direction and almost not in the other. Such an arrangement in the direction of current flow is called a diode (semiconductor diode). Special forms of the diode are, for example, the photodiode and the solar cell . In these, the opposite electrical charge of the space charge zone is used to separate generated electron-hole pairs . Photodiodes are therefore operated in the reverse direction. As a result, the effect of the resistance is canceled out and the pn junction loses its influence on the electron-hole pairs.

A direct application of the pn junction in the transverse direction to the current flow is the limitation of conductor tracks and their separation from one another. The always imperfect delimitation leads to the so-called leakage currents .

Most of the other semiconductor components also contain one or more pn junctions in the classic design to achieve their function, e.g. B. in bipolar transistor , field effect transistor (FET), MOS-FET , semiconductor detector , etc.

calculation

The width of the space charge zone, depending on the donor ( N _D ) and acceptor ( N _A ) is calculated upon complete ionization of the dopant atoms to Shockley to

${\displaystyle W(U)={\sqrt {{\frac {2\cdot \varepsilon _{\mathrm {r} }\cdot \varepsilon _{0}}{e}}\left({\frac {1}{N_{\mathrm {A} }}}+{\frac {1}{N_{\mathrm {D} }}}\right)(\Phi _{D}-U)}}}$,

where is the permittivity of the vacuum , the relative permittivity , the diffusion voltage established at the pn contact, the electron charge and the voltage across the diode. ${\displaystyle \varepsilon _{0}}$ ${\displaystyle \varepsilon _{\mathrm {r} }}$ ${\displaystyle \Phi _{\mathrm {D} }}$ ${\displaystyle e}$ ${\displaystyle U}$

Metal-semiconductor junctions

If, instead of two oppositely doped semiconductors, a metal is contacted with a p- or n-doped semiconductor, a metal-semiconductor transition occurs . With normal doping of the semiconductor, this is a rectifying metal-semiconductor contact. It is also known as the Schottky contact and is used in Schottky diodes . In the case of high doping or the formation of mixed crystals on the contact (e.g. Al-Si or WSi ₂ ), a contact with a linear transmission behavior occurs, which is referred to as an ohmic contact . This is used when wire bonding semiconductor components with metallic leads.

Web links

Commons : PN-junction diagrams  - collection of images, videos and audio files

• Philipp Laube: The pn junction. In: halbleiter.org. 2009, accessed September 21, 2009 . 
• PN transition from a scientific point of view , iwenzo.de
• 3D animations on the topic ( Memento from May 2, 2008 in the Internet Archive )
• Valve effect of the pn-layer at LEIFI with animations (school level)

Individual evidence

1. ↑ Rudolf Müller: Fundamentals of semiconductor electronics. 5th edition. Springer-Verlag, Berlin 1987. ISBN 3-540-18041-9 .
2. ^ Joachim Rudolf: Knaur's book of modern chemistry. Droemersche Verlagsanstalt, Munich 1971, p. 67, p. 74.
3. ↑ Stefan Goßner: Fundamentals of Electronics , 9th edition, Shaker-Verlag, Aachen 2016. ISBN 978-3-8265-8825-9 , Chapters 1 and 2.