_________________________________________________________________ Semiconductor Diodes
CONSTRUCTION
1-58. Merely pressing together a section of P and N material is not sufficient to produce
a rectifying junction. To form a proper PN junction, the semiconductor should be in one
piece, but divided into a P-type impurity region and an N-type impurity region. This can be
done in different ways. One way is to mix P-type and N-type impurities into a single
crystal during the manufacturing process. Doing this causes a P-region to grow over part of
a semiconductor's length and an N-region to grow over the other part. This is called a
GROWN junction (see Figure 1-13, view (A)). Another way to produce a PN junction is to
melt one type of impurity into a semiconductor of the opposite type impurity. For example,
a pellet of acceptor impurity is placed on a wafer of N-type germanium and heated. Under
controlled temperature conditions, the acceptor impurity fuses into the wafer to form a P-
region within it (see Figure 1-13, view (B)). This type of junction is known as an ALLOY
or FUSED-ALLOY junction. This is also one of the most commonly used junctions.
Figure 1-13. Grown and Fused PN Junctions From Which Bars are Cut
1-59. Figure 1-14 shows a POINT-CONTACT type of construction. It consists of a fine
metal wire (called a cat whisker) that makes contact with a small area on the surface of a
N-type semiconductor (view (A)). The PN union is formed in this process by quickly
applying a high-surge current to the wire and the N-type semiconductor. The heat
generated by this current converts the material nearest the point of contact to a P-type
material (view (B)).
1-60. Still another process is to heat a section of semiconductor material to near melting
and then diffuse impurity atoms into a surface layer. Regardless of the process, the object
is to have a perfect bond everywhere along the union (interface) between the P and N
materials. Proper contact along the union is important because the union (junction or
interface) is the rectifying agent in the diode.
23 June 2005
TC 9-62
1-17
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