http://zeptobars.ru/en/read/ULN2003-per-element-die-annotation
ULN2003 - practical example of «reading» microchip schematic
We often receive comments that while our microchip photos are beautiful
and interesting, it is completely unclear how integrated circuit
implements basic elements and form larger circuit. Of course it is
impossible to do a detailed review of an 1'000'000 transistor chip, so
we've found simpler example: ULN2003 - array of Darlington transistors.
Despite it's simplicity this microchip is still widely used and mass
manufactured. ULN2003 contains 21 resistors, 14 BJT transistors and 7
diodes. It is used to control relatively high load (up to 50V/0.5A) from
microcontroller pin. Canonical use case - controlling segments of large
7-segment LED displays.
ULN2003 die photo
Colors are more saturated, metal under contact pads is damaged by acid
and became brown.
Die size is 2.53x1.71 mm.
As we can see all 7 channels are identical, so we'll be analyzing only
1. Fortunately for us schematic is also available, so we can use it for
reference:
And now closeup of 1 channel with all elements annotated. I'll leave it
as an homework task to figure out mapping between schematic and layout.
But how individual transistor is made? it is known that cross-section of
planar BJT npn-transistor looks like this:
Thin base goes under emitter. While emitter and collector are both
n-type silicon, they are not equivalent. Doping concentration and
thickness are optimized for "current amplification" in 1 direction only.
Now we can take a look at individual transistor and try to figure out
how it is designed. P and N-type silicon has slightly different color.
This difference is quite small, but after cranking up saturation and
contrast almost to the maximum we can clearly distinguish between them.
Don't worry about dual emitters - they work just like 1 with combined area.
In order to avoid aluminum interconnect shorting wrong parts of the
transistor - it is covered by a layer of insulating transparent glass.
This glass has holes right above places where interconnect is supposed
to contact the transistor. You can clearly see this on the following
photo made with a lens that has narrower depth of focus: base contact is
out of focus as it's laying higher, on the transparent glass.
Collectors of both transistors in same channel - is effectively same
piece of silicon. This is ok because on the schematic collectors are
connected. Different channels are insulated by pn-junction between them
- it works like a pair of diodes not allowing current to flow to
neighbor channel. You can see these insulating "doped rectangles" on
higher-resolution photo
Now you can see that there is no magic smoke inside :-)
ULN2003 - practical example of «reading» microchip schematic
We often receive comments that while our microchip photos are beautiful
and interesting, it is completely unclear how integrated circuit
implements basic elements and form larger circuit. Of course it is
impossible to do a detailed review of an 1'000'000 transistor chip, so
we've found simpler example: ULN2003 - array of Darlington transistors.
Despite it's simplicity this microchip is still widely used and mass
manufactured. ULN2003 contains 21 resistors, 14 BJT transistors and 7
diodes. It is used to control relatively high load (up to 50V/0.5A) from
microcontroller pin. Canonical use case - controlling segments of large
7-segment LED displays.
ULN2003 die photo
Colors are more saturated, metal under contact pads is damaged by acid
and became brown.
Die size is 2.53x1.71 mm.
As we can see all 7 channels are identical, so we'll be analyzing only
1. Fortunately for us schematic is also available, so we can use it for
reference:
And now closeup of 1 channel with all elements annotated. I'll leave it
as an homework task to figure out mapping between schematic and layout.
But how individual transistor is made? it is known that cross-section of
planar BJT npn-transistor looks like this:
Thin base goes under emitter. While emitter and collector are both
n-type silicon, they are not equivalent. Doping concentration and
thickness are optimized for "current amplification" in 1 direction only.
Now we can take a look at individual transistor and try to figure out
how it is designed. P and N-type silicon has slightly different color.
This difference is quite small, but after cranking up saturation and
contrast almost to the maximum we can clearly distinguish between them.
Don't worry about dual emitters - they work just like 1 with combined area.
In order to avoid aluminum interconnect shorting wrong parts of the
transistor - it is covered by a layer of insulating transparent glass.
This glass has holes right above places where interconnect is supposed
to contact the transistor. You can clearly see this on the following
photo made with a lens that has narrower depth of focus: base contact is
out of focus as it's laying higher, on the transparent glass.
Collectors of both transistors in same channel - is effectively same
piece of silicon. This is ok because on the schematic collectors are
connected. Different channels are insulated by pn-junction between them
- it works like a pair of diodes not allowing current to flow to
neighbor channel. You can see these insulating "doped rectangles" on
higher-resolution photo
Now you can see that there is no magic smoke inside :-)
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