Transistor Pinout: NPN, PNP & BJT Explained

A transistor has no universal pin order. Identify the exact part number, manufacturer, package, and viewing direction before connecting it. PN2222A and P2N2222A are both NPN transistors in TO-92 packages, but their front-view pin orders are reversed.

The Central Semiconductor 2N2221A and 2N2222A datasheet shows the TO-18 leads arranged on a triangle rather than in a row, with lead 1 (Emitter) nearest the locating tab.

For more detail on the reversed TO-92 variants, use the 2N2222 pinout guide. If the device is an integrated circuit rather than a discrete transistor, compare its numbering with the TL072 pinout guide.

The diagrams show verified pin numbering for one TO-92 device and one TO-220 device. They do not define a pin order for every TO-92 or TO-220 transistor. Always match the body marking to the corresponding manufacturer drawing.

What Transistor Pin Labels Mean

A bipolar junction transistor (BJT) datasheet labels its three semiconductor terminals as base (B), collector (C), and emitter (E). These labels name the terminals, but they do not tell you which physical lead is first.

Pin numbers belong to the exact package drawing. The package view matters because a front view and a rear view mirror the outside leads. A metal tab, flat face, or locating tab is useful only when the datasheet uses that feature to define orientation.

How to Identify Transistor Pins

Start with the complete marking on the device body. Then find the manufacturer datasheet for that exact part and package option. A generic image search cannot establish the pinout because similar housings can contain devices with different lead orders.

BJT pin identification checklist

1. Read the full marking. Distinguish prefixes and suffixes, including PN2222A and P2N2222A.
2. Confirm the manufacturer. Different manufacturers can publish different package options under related names.
3. Match the package. Use the package name and mechanical outline in the same datasheet.
4. Use the stated view. Check whether the drawing shows the front, bottom, or another defined orientation.
5. Map numbers to names. Transfer each numbered lead to B, C, or E without reversing the view.
6. Check the footprint. Confirm whether the PCB drawing is viewed from the component side or the solder side before routing.

The table at the top demonstrates the result. PN2222A is E-B-C in the stated TO-92 front view, while P2N2222A is C-B-E in the same view. NPN polarity does not resolve that difference.

How to Connect a Transistor After Checking the Pinout

A pinout identifies terminals. It does not define a complete transistor circuit. The required connections, component values, supply polarity, and ratings depend on the exact device and schematic.

1. Verify the exact datasheet and package drawing.
2. Assign the schematic symbol terminals to the matching footprint pads.
3. Check the component-side and solder-side views before finalizing the board.
4. Compare the circuit voltage, current, power, and temperature limits with the exact datasheet.
5. Inspect the installed orientation before applying power.

For TIP31CG and TIP32CG in onsemi's TO-220 package, pin 2 and the metal tab are the collector. That detail applies to those listed parts and package drawing, not every three-lead power device.

Summary

Never infer a transistor pinout from NPN or PNP polarity, package shape, or a generic three-letter sequence. Use the exact body marking, manufacturer datasheet, package, and viewpoint together.

The verified examples show the risk clearly. PN2222A and P2N2222A reverse their outside leads in the same TO-92 front view. The 2N2222A TO-18 example uses a bottom view, while the TIP31CG and TIP32CG TO-220 examples use a front view.