IGBT Modules - Technologies, Driver and Application (Second Edition) - page 14

One diode structureD
One thyristor structureV
OneMOSFET structureT
One JFET structureT
between two adjacent IGBT cells
Fig. 1.2
Structures of an IGBT (showing two adjacent IGBT cells)
As each of these structures is relevant to understanding an IGBT, theywill be examined
individually in detail before summarising by noting the different types of IGBT designs.
Prior to this a brief digression into semiconductor physics is given to discuss relevant
aspects related to the contents of this book.
1.1.1 Intrinsicchargecarrier concentration
According to the Pauli Exclusion Principle, electrons of free atoms
can occupy only
very specific states, which are defined by their quantum numbers. For each of these
discrete states an associated energy can be given. If two or more atoms move very
close to each other, as in a crystal structure, the outermost electrons of one atom are
exposed to the electrical field of the other. This puts these electrons of the atoms into
certain energy states. Due to the multitude of atoms interacting with each other in a
(semiconductor) crystal, the adjacent discrete energy levels stack up, creating an
energy band structure. The width of an energy band depends on the strength of the
bonds between the electrons in its atoms. For example, strongly bonded electrons,
which interact onlyweaklywithadjacent atoms, form narrowenergy bands.
In traditional physics, two particles can never occupy the same state, but this is not the case at all in quantum
mechanics, where particles (e.g. electrons), can occupy the same state, as this is described by a probability
function. There is the restriction, however, that each particle that occupies the same state must differ from the
other particles in the same state by at least one quantum number. The spin (spin quantum number s), orbital
(main quantum number n and secondary quantum number l) and angularmomentum (magnetic quantum number
m) are all considered quantum numbers. This principle, in which two electrons must differ by at least one
quantum number, is called the Pauli Exclusion Principle, after the Austrian physicist Wolfgang Pauli (1900 -
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