IGBT Modules - Technologies, Driver and Application (Second Edition) - page 35

1.2 Diodes
The diode is the simplest component based on a pn-junction. It is an electronic
component with two electrodes whose electrical resistance depends significantly on the
polarity of the electrical voltage supplied and which have an asymmetrical, I/U
characteristic that depends very much on the direction of the current. Diodes therefore
serve as electrical valves, especially in the rectificationof alternating currents.
The electrode of the diode that is connected to the p-zone is called anode, and the
electrode connected to the n-zone is the cathode. Silicon (Si) is usually used as the
semiconductor material, along with germanium (Ge), gallium arsenide (GaAs), gallium
phosphide (GaP) and, increasingly over the last few years, silicon carbide (SiC
manufacturing diodes. However, only Si and SiC are relevant for asmaterials for power
The same valve effect as with a pn-junction can be achieved in a crystal by using a
metal semiconductor contact, as in the Schottky diode, which is also used in power
electronics. This type of diodewill be explained ingreater detail below.
A diode operating in forward direction (i.e. positive voltage applied at the anode contact)
shows a linear rise in forward current at low values of forward voltage U
because of its
ohmic bulk resistance. The operationally useful current flow that increases exponentially
with increasing forward voltage begins at 0.2V in low-power germanium diodes and at
0.7V in diodes made from silicon. If the diode is operated in reverse direction, the
reverse current is very low to start with and depends on the temperature. If the
maximum permissible reverse voltage, which depends on the structure and doping, is
exceeded, the blocking current increases sharply. If the current is not limited by
measureswithin the application the avalanche effect is triggered (chapte
, leading
to thermal breakthrough and eventually destroying the component.
There are rectifier diodes for rectifying alternating currents and switching diodes, which
act as switches in electrical circuits. Z-diodes and avalanche diodes usually serve to
stabilise the voltageor the current.
hows the layer design of a typical high power diode. The basis is the n
doped substrate, which also forms the cathode terminal. A weakly doped n
known as drift zone, is applied to the substrate in an epitaxial process (chapter
This region absorbs the blocking voltage U
. Its doping and gauge are therefore
directly related to the voltage range of the diode, as described in the previous chapters.
In high-voltage diodes, this layer can be quite substantial and, with its internal ohmic
resistance, cause a lot of forward loss. Diodes with this layer design are called PIN
diodes. P andN indicate the doping zones of the anodes and cathodes, while I indicates
an intrinsic, weakly doped zone between the p- andn
Depending on whether or not the electrical field of the applied blocking voltage
penetrates the weakly doped n
-region in normal operation, the diode is referred to as a
punch through (PT) or non-punch through (NPT) diode. In PT diodes, the electrical field
is taken up by the highly doped n
-zone, that is to say, it "punches through" the drift
Of the more than 170 variants of SiC, the polytypes 4H-SiC and 6H-SiC are encountered most frequently in
technological and commercial applications. Both types are amixture of the pure hexagonal polytypes 2H-SiC and
the pure cubic polytypes 3C-SiC. In each case, there are either one (4H-SiC) or two (6H-SiC) further cubic layers
between two hexagonal layers. The 4H-SiC ismore popular for electronic applications than the 6H-SiC because
of the higher, isotropic electronmobility.
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