There are two types of bipolar transistors: NPN type and PNP type. NPN-type products have low withstand voltage to high withstand voltage. PNP-type products with withstand voltage of 400 V or less, and particularly those with withstand voltage of 200 V or less, are mainstream.
There is an amplification function to convert small signals to large signals. The ratio of collector current IC and base current IB (IC/IB) is called DC current gain, denoted as hFE.
When small current (IB) flows from base to emitter, current of IB x hFE flows from collector to emitter.
BJTs are current-driven devices driven by base current.
Operation of NPN transistor
Base current: Current from base to emitter
Collector current: Current from collector to emitter
Operation of PNP transistor
Base current: Current from emitter to base
Collector current: Current from emitter to collector