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Why are bipolar transistors called bipolar?

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NMOS transistorNegative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors. — transistor NMOS transistor
Negative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors.

Bipolar transistors are so named because their operation relies on the movement of both electrons and holes as charge carriers.123

In these devices, conduction involves two different polarities of charge carriers. Unlike field-effect transistors (FETs), which primarily rely on one type of carrier, a bipolar transistor uses both types to conduct current through its structure. Specifically, in a p-n-p or n-p-n configuration, the device functions by injecting carriers from the emitter into the base, where they must traverse the region to reach the collector. This dual-carrier process is fundamental to the device's ability to act as an amplifier or switch.314