Solid-state refers to electronic components, devices, and systems based entirely on the semiconductor. The expression was especially prevalent in the late 1950s and early 1960s, during the transition from vacuum tube technology to the semiconductor diode and transistor. More recently, the integrated circuit (IC), the light-emitting diode (LED), and the liquid-crystal display (LCD) have evolved as further examples of solid-state devices. In a solid-state component, the current is confined to solid elements and compounds engineered specifically to switch and amplify it. Current flows in two forms: as negatively charged electrons, and as positively charged electron deficiencies called holes. In some semiconductors, the current consists mostly of electrons; in other semiconductors, it consists mostly of holes. Both the electron and the hole are called charge carriers. An example of a non-solid-state component is a cathode-ray tube (CRT). In this device, electrons flow freely through a vacuum from an electron gun, through deflecting and focusing fields, and finally to a phosphorescent screen