Discrete Bipolar Junction Transistors (BJTs) are commonly used to construct analog signal amplification functions in audio, radio, and other applications. One of the first semiconductor devices to be mass produced, their characteristics are less favorable than those of other device types for applications involving high frequency switching and operation with high currents or voltages, but they remain a technology of choice for applications requiring analog signal reproduction with minimal added noise and distortion. BJTs come in two varieties - NPN and PNP - which refer to the sequence of semiconductor layers that make up the transistor. NPN transistors consist of a thin P-type semiconductor between two N-type materials, while PNP transistors have an N-type semiconductor between two P-types. This gives the two types opposite polarity operation. NPN transistors sink current, while PNP transistors source current.