Why are the capacitors so big, while the transistors are tiny?
In: Technology
A capacitor is a non-conductive material sandwiched between two conductors. So you have three layers to start with. But the more capacitance you want, the more surface area you need. This surface area is increased by increasing the length and width of the sandwich. A small value capacitor might be no bigger than a fingernail. But a large value capacitor might have many feet of this sandwich all wrapped up into that container.
Generally, the higher the voltage a capacitor must handle, the thicker the sandwich must be. So a high voltage, but low capacitance capacitor can be the same size as a low voltage, but high capacitance capacitor.
This is in contrast to a transistor, which is just three microscopic dots on a chip of silicon or germanium. The closer these dots are to each other (without mixing together), the more efficient the transistor becomes (generally). But high voltage or current requirements also force us to make the dots bigger to handle it. Even so, these dots are still microscopic, and the packaging is large to handle the heat power transistors generate.
Tl;Dr: electrons are meant to cross a barrier in transistors. The smaller the barrier, the more efficient the transistor. Voltage is never meant to cross the barrier in a capacitor, so the barrier must be comparatively huge. High capacitance requires lots of surface area, and high voltages require thick barriers – each makes a capacitor very large compared to a transistor.
There’s a lot more that goes into the engineering of each, but I hope this is an adequate ELI5 version.
Capacitors run into physics limitations
If you need a certain amount of capacitance you can do the math and figure out the minimum volume that your capacitor could be. If it needs to be high voltage then the plates have to be further apart, and if you need high capacitance you need a lot of plate area, and if you want consistent performance you need a stable dielectric which is often not as good from a voltage or dielectric perspective so it makes your capacitor bigger.
There are a lot of applications where we want high value capacitors so we use large capacitors because that’s what we need
But we also have plenty of tiny tiny capacitors in computers. Every bit in RAM is a tiny capacitor that doesn’t need to be high capacity so we can make it quite tiny.
Transistors have a lot of performance characteristics improve as they get smaller, especially if you just want to run them fast and at low voltages like in CPUs.
They are very different kinds of devices.
All a transistor (in a computer) is doing is controlling the flow of a tiny current. This doesn’t need the transistor to be large, in fact it can be very small and still do its job brilliantly. However if we look at transistors that are designed to deal with bigger currents (inside a power supply for example), they are larger as they need more material to not be destroyed by the larger currents.
Capacitors are essentially two conductive sheets very close together. The capacitance is directly related to the electric field within the capacitor. To increase the strength of the electric field we can move the sheets closer, make the sheets bigger, or put a better dielectric (a material) between the two. If we have reached the limit of how close they can get, and already have the best material between them, then all we can do is make the sheets bigger. This means that we have to make the capacitor itself larger.
Capacitors’ capacitance is based on the surface of the plates inside. They need to be big compared to other electronics to have enough capacitance and to avoid leakage across the plates.
Transistors have some capacitance and making them smaller makes the capacitance smaller, which makes them faster. When you want capacitance, the the bigger you make it, the more capacitance you get.
Integrated circuits (computer chips) that do analog things use small capacitors frequently. They tend to use more chip area than transistors. The worst offender are ~~instructors~~ inductors, so you really try to make an integrated circuit design not need any of those.
A transistor is 3 pieces of specially treated silicon, placed in contact with each other. The size just determines how much energy it can safely handle (and how much it needs), and has little other impact on its function. An npn transistor is an npn transistor, whether it’s a 2n3904 with its own through-hole leads and plastic casing, or a tiny transistor etched onto a silicon wafer in a CPU.
Capacitors store electric charge between two not-quite-touching plates. The amount of charge a capacitor can store, and the voltage it can handle, is determined by the surface area of the plates and how far apart they are. A capacitor that can handle high voltages and hold lots of charge (such as a power supply filter capacitor) generally needs to have a large footprint. Small capacitors can be very small though – solid state drives use a capacitor to store each bit, and a 128GB SSD stores 1.024e12 bits of data, so it’s still possible to fit lots of capacitors into a relatively small area – they’re just maybe not as small as a transistor.
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