Page 33 - A Practical Guide from Design Planning to Manufacturing

The Evolution of the Microprocessor 9

Under Shockley’s direction, Morgan Sparks built the first working
junction transistor in 1949; by 1950, Sparks and Gordon Teal had vastly
improved their techniques. For the previous 2 years, Teal had been advo-
cating making semiconductor devices out of single crystals. The “crystals”
used at the time were really just amalgams of many crystals all with dif-
ferent orientations and many noncrystalline defects in between. Teal
built an apparatus to grow single crystals semiconductors without these
defects using the ideas of the Polish scientist J. Czochralski.
Using Czochralski’s method, a very small seed crystal is dipped into a
container of molten semiconductor and very slowly pulled out. As the
crystal is pulled out, atoms from the melt freeze onto its surface, grad-
ually growing the crystal. Each atom tends to freeze into proper place in
the crystal lattice producing a large defect-free crystal. Also important
is that impurities in the melt tend to stay in the melt. The crystal drawn
out is purer than the starting material. By repeatedly drawing crystals
and then melting them down to be drawn again, Teal achieved purity
levels vastly better than anything Shockley had worked with thus far.
In later years, the cylindrical ingots pulled from the melt would be sliced
into the round silicon wafers used today. Using these new techniques,
Sparks and Teal created the junction transistor Shockley had imagined.
Rather than two diodes side by side, Shockley imagined a sandwich
of three semiconductor layers alternating N-type, P-type, and N-type as
shown in Fig. 1-4. The emitter at the bottom injects electrons into the
base in the center. These electrons diffuse across the base to be captured
by the collector at the top. The voltage on the base controls the injec-
tion of electrons by the emitter. Because this device contains two P-N

+ +

− − − − − − − − −
− −
− −
N-type − − − − − − − −
collector − − − − − − − −
− −
− −
+ + + +
P-type + + + −
base + + − + − − + +
+
N-type − − − − − − − − −
− −
− −
emitter − − − − − − Current − − No
− −
− − −− flows − − − current
− −
− −
Figure 1-4 Junction transistor.