eli5: How are resistors, capacitors and coils placed on integrated circuits?
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When packing an integrated circuit it is common to solder the die to a thin PCB that you cover in epoxy. This thin PCB can also contain other circuit components. This is common for bypass capacitors, filter coils and other larger components. But it is also possible to create these components directly on the die in the Back End Of Line section, a thicker layer of copper and dielectric used to connect the semiconductors together. But the semiconducting layer often make a great resistor on its own where needed. However there are transistor circuits which can simulate resistors, capacitors and coils which tends to be cheaper to make in an IC.
They are built into the IC substrate just like the transistors are.
[In general](https://www.eeeguide.com/integrated-circuits/)
[Resistors](https://www.eeeguide.com/integrated-resistor/)
[Capacitors](https://www.eeeguide.com/parallel-plate-capacitor-in-ic-fabrication/)
[Inductors(coils)](https://miscircuitos.com/an-overview-of-on-chip-inductors-for-integrated-circuits-ic-types-pro-cons/) are possible but have significant drawbacks.
[And often they’re placed as SMD’s outside the IC](https://i.stack.imgur.com/tyX7O.jpg) like here on the backside of a CPU package if you can’t build one of the necessary specifications on the IC.
Here are [the examples with pictures](https://www.daenotes.com/electronics/devices-circuits/integrated-components-ic).
Capacitor is quite easy, given that the most widely used integrated transistor (MOSFET) has a capacitor as part of its design. You just use semiconductor as one plate, oxide as insulator, and a metal track as second plate. They also use diodes in reverse polarity (although it is no longer a capacitor, but a varicap)
Resistors is harder. In older times, they used a long track: 3-4 resistors could take 60% of the chip. Nowadays, resistors are made from very depleted semiconductor regions: similar to a base of bipolar transistor.
Inductors are the hardest: all you can do is to print [a spiral metal track](https://miscircuitos.com/an-overview-of-on-chip-inductors-for-integrated-circuits-ic-types-pro-cons/). This creates quite a low quality inductor. Creating an integrated ferromagnetic core is very difficult and expensive, so most integrated inductors have to live without.
You can build passive elements like resistors, capacitors, and inductors into the chip itself though the absolute accuracy is pretty bad without calibration and you will be pretty heavily constrained by the values you can achieve. Chips are made of several layers and you can control the order and thickness of the various layers to create different structures with different properties. On chip capacitors and inductors are measured in picofarads and nanohenries, respectively.
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