Transistor Symbols
Transistor symbols are graphical representations used in electronic schematics to identify the type of transistor and its terminals. Learning these symbols makes it much easier to understand circuit diagrams and identify how a transistor is connected and controlled.
What Is a Transistor Symbol?
A transistor symbol is a standardized graphical representation of a transistor used on a schematic diagram.
The symbol normally identifies the transistor's terminals and, depending on the transistor technology, indicates its polarity or operating principle.
The most common transistor symbols encountered in electronic circuits are:
- NPN transistor
- PNP transistor
- N-channel MOSFET
- P-channel MOSFET
- N-channel JFET
- P-channel JFET
- IGBT
- Darlington transistor
- Phototransistor
BJT Symbols
A bipolar junction transistor has three terminals: base, collector, and emitter.
The most important feature for identifying an NPN or PNP transistor on a schematic is the direction of the emitter arrow.
NPN Transistor Symbol
Collector
│
│
│
┌───┤
Base ─────────┤ │
└───┤
│
└───▶ Emitter
An NPN transistor has an arrow on the emitter pointing outward.
A useful memory aid is:
NPN = Not Pointing iN
The arrow points away from the base.
PNP Transistor Symbol
Collector
│
│
┌───┤
Base ─────────┤ │
└───┤
│
◀────┘
Emitter
A PNP transistor has an arrow on the emitter pointing toward the base.
The memory aid is:
PNP = Points iN
NPN vs PNP Symbols
| Feature | NPN | PNP |
|---|---|---|
| Terminals | Base, Collector, Emitter | Base, Collector, Emitter |
| Emitter arrow | Points outward | Points inward |
| Common use | Low-side switching and amplification | High-side switching and complementary circuits |
Understanding the BJT Arrow
The arrow is always located on the emitter of a conventional BJT symbol. It is not placed on the collector.
Its direction identifies the transistor polarity:
NPN → Arrow points OUT PNP → Arrow points IN
This is one of the quickest ways to identify an NPN or PNP transistor when reading a schematic.
MOSFET Symbols
A MOSFET has three main terminals:
- Gate
- Drain
- Source
MOSFET symbols differ from BJT symbols because the gate is electrically isolated from the channel.
The two main MOSFET polarities are:
- N-channel MOSFET
- P-channel MOSFET
N-Channel MOSFET Symbol
Drain
│
│
│
Gate ──────│ │
│ │
│ │
Gate ──────│ │
│
│
Source
An N-channel MOSFET is normally used when an n-type channel is required. It is extremely common in switching power supplies, motor controllers, load switches and digital circuits.
The exact MOSFET symbol may vary depending on the schematic convention and whether the body diode is explicitly shown.
P-Channel MOSFET Symbol
Source
│
│
│
Gate ──────│ │
│ │
│ │
Gate ──────│ │
│
│
Drain
P-channel MOSFETs are frequently used for high-side switching and complementary MOSFET circuits.
MOSFET Body Diode
Power MOSFETs contain an intrinsic body-diode path as a consequence of their semiconductor structure.
In many schematic symbols this diode is represented explicitly.
The body diode is particularly important when a MOSFET is used for power switching because it provides a current path in one direction when the MOSFET channel is not conducting.
Enhancement and Depletion MOSFET Symbols
MOSFETs can also be classified according to whether they are normally OFF or normally ON.
Enhancement-Mode MOSFET
An enhancement-mode MOSFET is normally OFF when the gate-to-source voltage is below its required threshold conditions.
It is the most common MOSFET type used in modern switching circuits.
Depletion-Mode MOSFET
A depletion-mode MOSFET is normally capable of conducting at zero gate-to-source voltage and requires an appropriate gate voltage to reduce or control the channel current.
Depletion-mode devices are less common than enhancement-mode MOSFETs.
JFET Symbols
A JFET has three terminals:
- Gate
- Drain
- Source
Unlike a MOSFET, the gate of a JFET forms a PN junction with the channel.
The arrow on a JFET symbol indicates the direction of the gate junction.
N-Channel JFET
Drain
│
│
│
│
Gate ────────▶ │
│
│
│
Source
For an N-channel JFET, the gate arrow points toward the channel.
A useful memory aid is:
N-channel → arrow iN
P-Channel JFET
Drain
│
│
│
│
Gate ────────◀ │
│
│
│
Source
For a P-channel JFET, the gate arrow points away from the channel.
JFET vs MOSFET Symbols
| Feature | JFET | MOSFET |
|---|---|---|
| Gate structure | PN junction | Insulated gate |
| Gate arrow | Used to identify polarity | Symbol convention differs |
| Terminals | Gate, Drain, Source | Gate, Drain, Source |
| Typical input impedance | High | Very high |
IGBT Symbol
An IGBT has three main terminals:
- Gate
- Collector
- Emitter
Its symbol resembles a MOSFET in the gate-control portion while using collector and emitter terminals associated with its bipolar conduction structure.
Collector
│
│
┌──┤
Gate ─────────┤ │
└──┤
│
│
Emitter
An IGBT is commonly used in high-power switching circuits such as inverters, motor drives and industrial power converters.
Darlington Transistor Symbol
A Darlington transistor contains two BJTs connected internally as a high-gain pair.
The schematic can be represented using two transistor symbols connected together.
C
│
┌──┴──┐
│ BJT │
B ────────┤ │
└──┬──┘
│
┌──┴──┐
│ BJT │
└──┬──┘
│
E
Many commercially available Darlington transistors are packaged as a single three-terminal device.
Phototransistor Symbol
A phototransistor is represented by a transistor symbol with arrows indicating incident light.
↘ ↘
Light
↓
┌────
│ BJT
└────
│
```
The light arrows distinguish the phototransistor from an ordinary transistor
symbol.
Phototransistors are commonly used for light detection and optical sensing.
Transistor Symbols and Terminal Identification
Being able to identify the terminals from a schematic is essential when repairing or designing circuits.
| Device | Terminals |
|---|---|
| NPN BJT | Base, Collector, Emitter |
| PNP BJT | Base, Collector, Emitter |
| N-channel MOSFET | Gate, Drain, Source |
| P-channel MOSFET | Gate, Drain, Source |
| N-channel JFET | Gate, Drain, Source |
| P-channel JFET | Gate, Drain, Source |
| IGBT | Gate, Collector, Emitter |
| Phototransistor | Base, Collector, Emitter |
Why Correct Symbol Identification Matters
Different transistor types can have completely different electrical characteristics even when they look physically similar.
Correctly identifying the symbol helps determine:
- How the transistor is controlled
- Which terminal is the input or control terminal
- Which terminals carry the main current
- The transistor polarity
- The expected direction of current
- The appropriate biasing method
This is particularly important when troubleshooting circuits from schematic diagrams.
Common Symbol Identification Mistakes
- Confusing NPN and PNP transistor symbols
- Forgetting that the BJT arrow is on the emitter
- Confusing a JFET with a MOSFET
- Assuming all three-terminal devices have the same pin arrangement
- Ignoring the body diode shown on a power MOSFET
- Confusing an IGBT with a MOSFET
- Assuming the physical package determines the transistor type
Symbols vs Physical Pinout
A schematic symbol shows the electrical function of a transistor, but it does not necessarily indicate the physical order of the pins on the actual component.
For example, two TO-220 transistors can have completely different pin assignments even though their packages look almost identical.
Always consult the manufacturer's datasheet before connecting a physical transistor.
Key Points
- BJTs use base, collector and emitter terminals.
- NPN and PNP BJTs are distinguished by the emitter-arrow direction.
- NPN BJT arrows point outward.
- PNP BJT arrows point inward.
- MOSFETs use gate, drain and source terminals.
- JFET symbols use a gate arrow to identify the gate-junction polarity.
- IGBTs use gate, collector and emitter terminals.
- Darlington transistors contain two transistor stages in one high-gain arrangement.
- Phototransistors are represented with light arrows.
- A schematic symbol does not necessarily indicate the physical pin order.
- Always check the datasheet before wiring a transistor.