What Are Capacitors?

Capacitors are two-terminal passive electronic components that store energy in an electric field established between two conductors separated by a dielectric material. The stored charge is proportional to the applied voltage, with the proportionality constant defined as capacitance in farads. Capacitors are among the three fundamental passive circuit elements, alongside resistors and inductors, and they appear in virtually every electronic circuit as energy storage elements, signal filters, timing references, and coupling devices.

The performance of a capacitor depends on the geometry of its electrodes and on the electrical properties of the dielectric between them. The key dielectric parameters are permittivity, which determines capacitance per unit area, and dielectric loss, which governs how much energy is dissipated per cycle. The quality factor Q represents the ratio of stored energy to dissipated energy per radian; high-Q capacitors are essential in resonant circuits, oscillators, and precision filters where energy loss degrades performance.

Dielectric Materials and Capacitor Types

The choice of dielectric material largely determines a capacitor's frequency range, temperature stability, capacitance density, and maximum operating voltage. Aluminum electrolytic capacitors use an electrochemically grown oxide layer as their dielectric, achieving very high capacitance in a compact volume but limited to polarized DC applications with maximum voltages up to a few hundred volts. Tantalum electrolytic capacitors offer tighter tolerances and better frequency performance than aluminum types at smaller sizes, making them common in power supply decoupling on circuit boards. Ceramic capacitors, manufactured in multilayer stacks (MLCC), span the widest frequency range and are available in formulations from ultra-stable Class I (NP0/C0G) types with near-zero temperature coefficients, to high-permittivity Class II (X5R, X7R) types with higher capacitance density but some voltage and temperature dependence. Film capacitors, which use polyester, polypropylene, or polycarbonate as the dielectric, offer low loss, high voltage ratings, and excellent self-healing characteristics, making them the preferred choice in power electronics and precision analog circuits. The TDK Electronics technical magazine series on electrolytic capacitors provides design-oriented coverage of these material distinctions.

MOS Capacitors

The metal-oxide-semiconductor (MOS) capacitor is a planar structure formed by depositing a conducting gate over a thermally grown or deposited oxide on a semiconductor substrate. It is the fundamental building block of MOSFET transistors and the charge-coupled device (CCD). The MOS capacitor displays a characteristic voltage-dependent capacitance that reflects the accumulation, depletion, and inversion states of the semiconductor surface, information that is used diagnostically in process characterization through C-V measurements. In dynamic random-access memory (DRAM), trench and stack MOS capacitor structures store the charge bits; shrinking dimensions below 10 nm require high-k dielectrics such as hafnium oxide to maintain sufficient capacitance at reduced physical thickness. Research accessible through IEEE Xplore on switched-capacitor power electronics circuits shows how MOS-implemented capacitors are also central to switched-capacitor converters in CMOS technology.

Switched Capacitor Networks

Switched capacitor networks implement resistive and filter functions by periodically transferring charge between capacitors using transistor switches. Because the average current drawn during each switching cycle is proportional to capacitance and switching frequency, a switched capacitor behaves as an equivalent resistor whose value is set by a clock frequency rather than a physical resistor. This property enables highly accurate RC time constants and filter cutoff frequencies set by capacitor ratios and a crystal-controlled clock, with the precision inherent to ratio-matched components on an integrated circuit. Switched capacitor filters, integrators, and digital-to-analog converters are core building blocks of audio codecs, instrumentation amplifiers, and analog-to-digital converter front ends. The NIST precision electrical metrology program maintains primary capacitance standards used to calibrate the reference capacitors on which these networks depend.

Applications

Capacitors are used across a wide range of disciplines, including:

• Energy storage and smoothing in power supply filter stages
• Signal coupling and AC blocking in audio and radio frequency circuits
• Timing and oscillation in RC relaxation and crystal oscillator networks
• Energy storage in pulsed-power systems, motor starters, and defibrillators
• Charge storage in DRAM and CCD image sensor arrays
• Resonant frequency control in RF filters, impedance matching networks, and antenna tuning