So, what is a capacitor?
Everyone knows the container. For example, water containers are called water containers, oil containers are called oil containers, and grain containers are called grain containers.
Therefore, a capacitor is a component used to store (store) electric charge.
To be able to store charge, positive and negative electrodes are usually required, and the mutual attraction between positive and negative charges is used to lock the charge in the capacitor. The structure of the positive and negative electrodes can be diverse: parallel plates, coaxial cylindrical barrels, concentric spherical surfaces, etc.
The size of the bucket is defined according to the amount of water, or it can be defined according to the ratio of the amount of water injected to the height of the water surface. Similarly, the size of the capacitor is equal to the ratio of the stored charge to the plate voltage, that is, C = Q/U. Just as the size of a bucket can be measured by the geometric size of a bucket, the size of a capacitor can also be expressed as C = ε•S/d, where S is the area of the plates and d is the distance between the plates.
The PN junction of the diode is equivalent to the two plates of the capacitor, so it has junction capacitance. Reverse voltage applied to the diode will change the thickness of the dissipation zone, that is, change the distance between the plates of the junction capacitance and change the size of the junction capacitance. This is the varactor diode, which can change the capacitance by changing the voltage. Varactor diodes play a key role in tuning circuits and voltage-controlled oscillators.
Capacitive sensing MEMS (Micro Motor System) uses mechanical compression or lengthening the distance between capacitor plates to change capacitance to sense. Most of the acceleration sensors used for step counting in mobile phones use this principle.