explanation is, “Although the metal contact for the base
is on the edge of the transistor, it’s electrically connected
through the N and N+ region to its active ring in between
the collector and emitter. A metal line is routed between
the collector and base, but is not part of the transistor.” A
thin layer of SiO2 insulates the metal line from the N material
The largest structures on the chip are the three high-current NPN transistors, magnified in Figure 13. The
structure is more classic looking, with interlocking fingers
for the emitter and base surrounded by a large three-sided
collector. In the schematic (Figure 8), the high current one
on the left is used to discharge the external timing capacitor
and the other two provide 200 milliamp source and sink
capabilities for the output.
Well ... that’s about it for the die.
My intent in this article was to briefly show you some
of the interesting structures in a bipolar 555. For more
detailed information, please see Ken’s blog where he covers
the current mirrors, differential pairs, and some of the
development history of the 555.
The GIGANTOR 555 Shield was a one-off affair,
designed to show that even a giant 555 board could be
somewhat useful. However, I doubt that the discrete board
will replace the current DIP or SOIC versions, but it does
illustrate just how complicated integrated circuitry can be.
Figure 13. Three high current (200 mA) NPN transistors
are used to discharge the external capacitor and drive
the 555’s output.
Figure 12. The PNP transistors are unlike the NPN units and
can be identified by their circular feature.
Figure 11. Integrated transistors are constructed of several
layers; the base of this NPN one is not in the center as you
Figure 10. This 100K L-shaped resistor uses an overlay
of N silicon to create a “pinching” effect to attain its high
62 SERVO 09/10.2018