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,
   7   8   9   10   11   12   13   14   15   16   17