Space charge

When space charge is called a spatially limited electrical charge distribution. It is caused by an excess of negative or positive charge carriers.

Space charge effects occur in many electronic devices (eg, electron tubes, semiconductor diodes, transistors ) in which they have significant influence on the electronic properties.

Also in electron and ion sources play space charge effects play an important role. Here, however, the electric fields associated with the space charges are often undesirable because they limit the achievable quality key beam properties, such as the intensity or energy resolution achievable.

Furthermore, space charges must be taken into account in the design of gas - and glow discharge.

In nature may arise in thunderclouds space charges, which are discharged in the form of flashes by the movement of water droplets and ice crystals.

Space charges in electron tubes

In electron tubes space charges are generated by thermionic ( Richardson Edison effect). To avoid unwanted interactions of the electrons produced by gas and to conserve the hot cathode tubes are operated in a vacuum.

The space charge effects occurring in a tube are shown in Figure 2 for the example of a simple tube diode. The light emitted from the hot cathode of the tube electrons are drawn to the anode. The electrons themselves generate electric fields and thereby distort the field distribution caused by the anode voltage significantly. This can go so far that at the source of the electrons ( the hot cathode ) arrives no field, as it is previously absorbed by the space charges. In this case, the anode current is not dependent on the number of emitted electrons from the cathode, but only by the anode voltage. This area of ​​the current-voltage characteristic is referred to as space-charge- limited current (see Figure 3).

Between cathode and anode results in a position-dependent density distribution independently adjusts so that the current density is the same everywhere. Thus, for example, a decrease in the current density in a particular area immediately to the fact that this additional space charge accumulation which shields the passage of the anode voltage on the front of it cargo, so that the current density falls until there as well, until a steady state has been reached.

The anode current and the current density can be described by the Langmuir- Langmuir'sche or Child'sche space charge law calculate:

The anode voltage, the vacuum dielectric constant, the elementary charge, the electron mass, the exposed anode surface and the cathode-anode spacing.

The equation is valid under the following ( only approximately valid) assumptions:

For large anode voltages can be achieved by increasing the anode voltage no additional anode current pull. This so-called saturation current is achieved when the anode voltage is so large that it can not be compensated by the space charge. In this case all electrons producing the cathode, removed by suction., The saturation current is thus larger, the more electrons which cathode emitted ( in Figure 3 is shown schematically by dashed three saturation curves for respectively different cathode temperatures).

Space charges in semiconductor devices

• The formation mechanisms and effects of space charge regions in semiconductor devices ( diodes, transistors ) are described in the main article pn junction.
• Similar effects also occur in semiconductor-metal transitions on ( Schottky diode).

• Electrodynamics
• Vacuum technology