An electrical component (or device) is an element that allows a flow of charges with a specific voltage-current relationship. It is provided with terminals, two or more, that connect leads or metallic pads, as access and exit points for the electric current, across which the voltage is applied. The simplest electrical component has only two terminals, so only one current and one voltage is available. The voltage-current relationship is known as a transfer function (or constitutive equation). Two terminals of the same component are also known as one-port, so the two-terminal component is a one-port device (Figure 4.1a).