The difference between PNP and NPN is the construction of the semiconducting material found inside the transistor. In a PNP transistor, the semiconducting material is made of three layers: a negative (N) layer between two positive (P) layers: Positive-Negative-Positive, or PNP. Likewise, an NPN transistor has a positive layer sandwiched between two negative layers: Negative-Positive-Negative, or NPN. Despite their different construction, both forms have positive and negative power leads and are connected to a device called the load, such as an attached indicator light, hub or IO-Link master, PLC, etc.