The bipolar junction transistor is historically the first solid-state analog amplifier and digital switch, and formed the basis of integrated circuits (IC) in the 1970s. Starting in the early 1980s, the Metal–oxide–silicon field effect transistor had gradually taken over, particularly for main stream digital ICs. However, in the 1990s, the invention of silicon-germanium base heterojunction bipolar transistor (SiGe HBT) brought the bipolar transistor back into high-volume commercial production, mainly for the widespread wireless and wire line communications applications. SiGe HBTs are used to design radio-frequency (RF) ICs and systems for cell phones, wireless local area network, millimeter wave radios, and many more applications, due to its outstanding high-frequency performance and ability to integrate with CMOS for realizing digital, analog, and RF functions on the same chip. In real bipolar transistors the current voltage characteristics are more complex than those described by the Ebers-Moll equations.