A transistor pinout defines the physical mapping of the silicon die's internal regions (Emitter, Base, Collector for BJTs; Gate, Drain, Source for MOSFETs) to the external metal leads. Unlike mains wiring, which is strictly governed by regional color codes (NEC in the US, IEC 60446 in the EU), semiconductor pinouts are dictated by physical package standards and manufacturer datasheets. Assuming a universal pinout for a specific package shape is the fastest way to short-circuit a prototype board.
The Master Transistor Pinout Reference Table
The table below maps the most common through-hole and surface-mount transistor packages to their standard pinouts. Orientation Rule: For all TO-92 and TO-220 packages listed here, hold the component so the flat face (or the side with the printed text) is facing you, with the leads pointing downward. Pin 1 is on the left.
| Package Type | Regional Standard | Example Part | Pin 1 (Left) | Pin 2 (Center) | Pin 3 (Right) | Critical Notes |
|---|---|---|---|---|---|---|
| TO-92 | JEDEC (US) | 2N3904 / 2N2222 | Emitter | Base | Collector | Flat side facing you, pins down. |
| TO-92 | Pro Electron (EU) | BC547 / BC557 | Collector | Base | Emitter | Flat side facing you, pins down. Reversed E/C vs US parts. |
| TO-220 (BJT) | JEDEC (US) | TIP31C / TIP41C | Base | Collector | Emitter | Metal tab is internally tied to Pin 2 (Collector). |
| TO-220 (MOSFET) | JEDEC (US) | IRF520 / IRF3205 | Gate | Drain | Source | Metal tab is internally tied to Pin 2 (Drain). |
| SOT-23 (SMD) | Global | 2N7000 / MMBT3904 | Base/Gate | Emitter/Source | Collector/Drain | Top view, pins pointing away from you. |
Regional Semiconductor Standards and Package Variants
When identifying a transistor pinout, the alphanumeric prefix on the case tells you which regional naming standard was used to register the part. While these standards do not strictly mandate the pinout (the physical package does), they indicate the component's origin and help you locate the correct datasheet.
- JEDEC (US Standard): Parts typically start with 2N (e.g., 2N2222, 2N3904). The '2' indicates two PN junctions (a standard BJT). JEDEC registers the silicon die, not the package, meaning a 2N2222 can come in a TO-92, TO-18, or SOT-23 package. You must verify the package-specific pinout.
- Pro Electron (European Standard): Parts start with two letters followed by a number (e.g., BC547, BD139). The first letter denotes the material (B = Silicon, A = Germanium). The second letter denotes the application (C = low-power audio/RF, D = power audio). These are overwhelmingly found in TO-92 and TO-126 packages.
- JIS (Japanese Standard): Parts start with 2S (e.g., 2SC1815, 2SJ270). Often, the '2S' is omitted on the physical casing, so a part marked 'C1815' is actually a 2SC1815 NPN transistor.
Common Pinout Mistakes and Faded Marking Recovery
The 'Rows People Get Wrong' Trap
The most frequent error on the workbench is assuming all TO-92 transistors follow the Emitter-Base-Collector (EBC) layout. While the ubiquitous 2N3904 and 2N2222 use EBC, the equally common BC547 and BC557 use Collector-Base-Emitter (CBE). Always check the datasheet for the specific manufacturer's drawing; even within the same part number, different manufacturers occasionally use mirror-image pinouts for TO-92 packages.
Another major point of failure is the TO-220 Tab Trap. On almost all TO-220 BJTs and MOSFETs, the large metal mounting tab is internally connected to the center pin (Collector for BJTs, Drain for MOSFETs). If you mount a TO-220 to a grounded aluminum heatsink without using an electrically insulating mica or silicone pad and a nylon shoulder washer, you will instantly short your power rail to ground.
Safe Interpretation When Markings are Faded or Missing
If you inherit a bin of unmarked or faded transistors, do not guess the pinout. You can definitively identify the Base pin and determine if the BJT is NPN or PNP using a standard digital multimeter's Diode Test mode (the symbol with an arrow and a line).
- Find the Base: Test all three pins against each other in both directions. You are looking for one pin that shows a forward voltage drop (typically 0.5V to 0.7V) to the other two pins, while reading 'OL' (open loop) in the reverse direction. That common pin is the Base.
- Determine NPN vs. PNP: If the red probe was on the Base when you read the 0.6V drop, it is an NPN transistor. If the black probe was on the Base, it is a PNP transistor.
- Identify Emitter vs. Collector: Most modern multimeters have an hFE testing socket on the front panel. Plug the known Base into the correct B slot, and try the remaining two pins in the E and C slots. The configuration that yields a high hFE reading (typically 100-300) is the correct pinout. A reading of 0 or a very low number means E and C are swapped.
Transistor Pinout FAQ
How do I find a transistor pinout without a datasheet?
If the part number is legible, search for the manufacturer's specific datasheet rather than a generic pinout diagram. If the markings are entirely gone, use the multimeter diode-test method outlined above for BJTs. For MOSFETs, the Gate will read completely open ('OL') to both the Drain and Source in both directions, while the Drain-to-Source path will show a body diode drop (approx 0.4V - 0.6V) in one direction and 'OL' in the other.
Are all TO-92 transistor pinouts the same?
No. The TO-92 is merely a physical outline standard defined by JEDEC; it does not mandate the internal silicon die orientation. The 2N3904 is EBC, the BC547 is CBE, and the 2N7000 (a MOSFET in a TO-92 package) is Source-Gate-Drain. Always verify the pinout against the specific part number's datasheet.
Can I use a multimeter continuity test to check a transistor pinout?
Never use the continuity (beep) mode to test semiconductor junctions. Continuity mode applies a very low test voltage, often insufficient to forward-bias a silicon PN junction (which requires ~0.6V). You will get false 'open' readings. Always use the dedicated Diode Test mode, which supplies enough current to properly forward-bias the junction and display the voltage drop.
What happens if I wire a transistor pinout backward?
If you swap the Emitter and Collector on a BJT, the transistor enters 'reverse-active mode.' It will still conduct, but the current gain (hFE) will plummet to single digits, and the breakdown voltage drops significantly. If you swap the Base and Emitter, the base-emitter junction will likely experience Zener avalanche breakdown (usually around 6V to 7V), permanently degrading the transistor's gain and increasing leakage current, even if it doesn't visibly pop or burn.
