What’s inside the ULN2003 Darlington driver chip

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.
ULN2003 die photo
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.
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.
For more detail: What’s inside the ULN2003 Darlington driver chip

About The Author

Ibrar Ayyub

I am an experienced technical writer holding a Master's degree in computer science from BZU Multan, Pakistan University. With a background spanning various industries, particularly in home automation and engineering, I have honed my skills in crafting clear and concise content. Proficient in leveraging infographics and diagrams, I strive to simplify complex concepts for readers. My strength lies in thorough research and presenting information in a structured and logical format.

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