Page 19 - Power Electronics Handbook

12 Power semiconductor devices
The second deposition technique is called sputtering. A bell jar is again
used, but now it is filled with an inert gas, such as argon. The
semiconductor, again face down in the top half of the jar, is connected to
the positive terminal of a high-voltage source and the material to be
deposited is placed at the bottom of the jar and connected to the negative
terminal. Under the effect of this voltage the argon is ionised and positive
ions bombard the cathode, causing it to sputter and emit material, which
settles on the semiconductor surface, i.e. the anode.
An alternative technique for metal deposition, which can also be used to
deposit other films such as polysilicon and silicon dioxide, is known as
chemical vapour deposition (Singer, 1995a). It is primarily used to produce
devices of very small geometry, as in integrated circuits.

1.3 Power rectifier principles
1.3.1 Physics of rectification
It is not intended in this brief introduction to delve into atomic physics.
Instead some of the basic concepts of semiconductors are introduced.
The structure of an atom is well known. It consists of a central nucleus
and revolving electrons in various orbits. The positive charge on the
nucleus is balanced by negative electronic charge. The atom is said to be
stable if its outer shell is full, the electrons for the first four shells, working
outwards from the nucleus, being 2,8,18,8 or 32. The number of electrons
in the outermost shell determines its group number. Silicon has 2, 8, 4
electrons, and is in group IV, and although unstable on its own, a crystal of
silicon forms stable covalent bonds between atoms. Each shares its four
electrons with neighbouring atoms and so has eight orbital electrons.
However, if energy is given to the material, say in the form of heat, an
electron can break away from its valency bond and cause conduction in the
material. This is called intrinsic conduction.
If an impurity of group V (donor) or group I11 (acceptor) is added to
silicon then there will be an abundance or shortage of electrons
respectively, causing increased conduction at a given temperature. This is
called extrinsic conduction. A shortage of electrons results in a p-type
doping, where holes are the majority carriers and electrons the minority
ones, whereas extra electrons results in an n-type material, in which the
carrier function is reversed.
Holes and electrons are constantly being created in a semiconductor.
They disappear due to recombination, a process which can be increased by
the presence of specially created trapping centres in the crystal. These hold
a carrier until an opposite polarity charge arrives for recombination.
A power rectifier is a two-layer device, similar in principle to a diode,
consisting of a p and an n layer formed within the same semiconductor
material. Figure 1.4 shows a simplified arrangement of the two layers. The
p layer has an abundance of holes (holes are the majority carrier) and the n
layer has electrons as the majority carriers. Electrons and holes form the
minority carriers in the p and n layers, respectively, as shown in Figure 1.4.
As the junction between the layers is approached, the concentration of p
and n decreases to match that in the other layer. Therefore there is a