Page 12 - Power Electronics Handbook
P. 12
Fabrication process 5
(iv) The maximum rate of rise of current which the device can withstand,
without being destroyed due to localised heating, and the maximum
rate of rise of voltage in the forward direction which it can withstand,
without turning on prematurely.
(v) The switching speed of the device, which influences its switching
losses and the maximum frequency at which it can be operated.
(vi) The rate at which a conducting device can recover its blocking
capability. This again influences its maximum operating frequency.
(vii) The maximum power dissipation which the device can withstand.
This characteristic is often linked to the maximum junction
temperature at which the device can operate and its thermal transfer
characteristic, that is, its ability to transfer heat to a heatsink.
(viii) The power gain of the device, which is the ratio of the controlled
power to the power needed in the control terminal. The higher this
gain, the lower the power dissipation in the control electronics of the
power semiconductor.
1.2 Fabrication process
The manufacturing processes for power semiconductors closely resemble
those used for other semiconductor devices. These consist of the following,
which are described further in this section:
(i) Preparation of a wafer of very pure silicon, which forms the base to
support the power semiconductor.
(ii) Oxide growth over selected areas of the semiconductor surface, to
protect the layers below it from contamination and to form a mask for
subsequent processing steps.
(iii) Growth of an epitaxial layer onto the silicon wafer, which forms a
controlled layer into which the various parts of the semiconductor
device can be formed.
(iv) Photolithography, used to control the areas where the p and n
components are formed.
(v) Diffusion, which is the most common method used to form the p and
n components of the semiconductor device.
(vi) Ion implantation, which is able to produce p and n areas to a high
precision.
(vii) Metal formation, which is used to interconnect the various p and n
parts of the semiconductor devices together, and to provide a base for
connection of the silicon to the package of the device.
1.2.1 cryst8l prcpprstion
In order to obtain high-quality semiconductor components it is important
to start with a semiconductor material which has a very low level of defects
in its crystal structure. Several techniques exist for growing bulk
semiconductor crystals, some of these being illustrated in Figure 1.1.
In the zone-levelling method a crucible, made from silica, holds the
impure silicon, and it is slowly moved along a quartz tube, containing an
inert gas. Zoned heating coils are placed along the length of the tube,