Space charge zone

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A space charge zone  (RLZ), also known as a depletion zone or barrier layer , is an area in the transition between differently doped semiconductors in which space charges with excess and deficiency of charge carriers oppose each other, so that this zone appears charge-neutral to the outside in the case of equilibrium. Depending on the polarity of an externally applied electrical voltage , there are different configurations of electrical fields and thus good or only very weak electrical conductivity (it "blocks") in the area of ​​the depletion zone .

This physical effect is the basis for the rectifying function of the semiconductor component diode . In addition, space charge zones also play a fundamental role in other electronic components, e.g. B. in bipolar transistors or in junction field effect transistors .

Emergence

Above the pn junction before the diffusion process, below after the diffusion equalization in equilibrium and an electric field built up in the area of ​​the RLZ
Band diagram of a pn junction

When two differently doped semiconductor materials, an n- and a p-doped semiconductor, are brought into spatial contact, a pn junction is created . In the n-region is an excess of negatively charged electrons in front, in the p-type region, an excess of positively charged holes , also known as holes designated positively charged impurities in the semiconductor crystal.

The concentration gradient of charge carriers in the transition area between the n- and p-zone leads to a diffusion of charge carriers: electrons from the n-area migrate into the p-doped semiconductor, defect electrons diffuse into the n-doped semiconductor ( diffusion current ). The charge carriers recombine there with the other type of charge carrier. In total, there is an excess of negative space charge in the transition area in the p-semiconductor and an excess of positive space charge in the n-semiconductor; the space charge zone thus formed becomes depleted as a result of the recombination of free (mobile) charge carriers.

The electric field thus formed in the space charge zone counteracts further diffusion of charge carriers from the two zones ( anti-diffusion voltage ), since the field generates an opposite drift current . A case of equilibrium is created in which the diffusion current and drift current of charge carriers are in equilibrium, as shown in the figure on the right with the spatial distribution and the field profile. Viewed from the outside, the RLZ is field-free in equilibrium; there is no potential gradient that transports charge carriers across them.

Since diffusion processes are highly dependent on temperature, the size of the space charge zone changes as a result of temperature changes.

Behavior when applying an external voltage

If an electrical voltage is applied from the outside to the two semiconductor layers, this brings about a further electrical field in the semiconductor in addition to the field of the space charge zone in the case of equilibrium. Both fields overlap. Depending on the polarity of the external voltage, two main cases can be distinguished, which are decisive for the basic functions of electronic components such as diodes:

  1. In the blocking case (the p-semiconductor is subjected to a negative voltage compared to the n-semiconductor) the electric field strength increases in the area of ​​the space charge zone and leads to an increased drift current. The space charge zone increases in size until a new equilibrium is established. Since the density of free charge carriers in the space charge zone remains low, the electrical conductivity is low and limited to a small reverse current .
    If the external voltage is increased further, depending on the structure of the semiconductor, various breakdowns such as the Zener effect and, with greater field strengths, an avalanche breakdown occur . These breakdown effects can lead to the destruction of the semiconductor material in an uncontrolled manner or, as in the case of Zener diodes, be used in a targeted manner.
  2. In the case of passage (the p-semiconductor is subjected to a positive voltage compared to the n-semiconductor) the space charge zone is reduced, since the electric field triggered by the external voltage counteracts the electric field of the space charge zone. The drift current caused by the RLZ decreases and the diffusion current dominates. The density of free charge carriers in the transition zone increases sharply with the external voltage, the pn junction has good electrical conductivity. In this case, the mathematical description is given by the Shockley equation .

Metal-semiconductor contact

In addition to n- and p-doped semiconductors, space charge zones also form on metal-semiconductor contacts and can lead to rectifying behavior of these contacts, the Schottky contact , which is used in Schottky diodes . Due to the high number of free electrons in the metal, the space charge zone is almost limited to the corresponding semiconductor area.

literature

  • Robert F. Pierret: Semiconductor Device Fundamentals . 2nd Edition. Addison-Wesley, 1996, ISBN 978-0-201-54393-3 .
  • Holger Göbel: Introduction to semiconductor circuit technology . 2., arr. and exp. Edition. Springer, Berlin / Heidelberg 2006, ISBN 3-540-34029-7 .