Although most microelectronics products are now made of CMOS transistors, bipolar transistors remain important in microelectronics because of their inherently high speed for digital circuit applications and superior characteristics for analog circuit applications. There are two types of bipolar devices: the n-p-n type, which has a p-type base and n-type emitter and collector, and the p-n-p type, which has an n-type base and p-type emitter and collector. Commonly used bipolar devices are either lateral transistors, where the active device regions are arranged horizontally adjacent to one another and the active currents flow laterally, or vertical transistors, where the active device regions are arranged vertically one on top of another and the active currents flow vertically. Practically all high-speed bipolar transistors used in digital circuits are of the vertical n-p-n type.

For simplicity, only vertical n-p-n bipolar transistors will be considered explicitly here. The equations derived for vertical transistors apply to horizontal transistors as well, provided that the device parameter values are adjusted accordingly. Also, the equations for an n-p-n transistor can be extended to a p-n-p transistor simply by reversing the voltage and dopant polarities and using the appropriate device parameter values.