A Field Effect Transistor (FET) symbol on a schematic dictates the voltage polarity, current direction, and physical package pinout you need on the bench. Governed primarily by IEEE 315 (US) and IEC 60617 (International) standards, the exact geometry of the gate, channel, and bulk connections tells you whether you are dealing with an N-channel enhancement MOSFET like the 2N7000 or a P-channel JFET like the J175. Misinterpreting these symbols—especially confusing P-channel for N-channel in high-current DC systems—can cause a shoot-through short circuit, destroying traces or creating a fire hazard. Here is the definitive reference.
The Complete FET Symbol Reference Table
The table below maps the standard structural symbols to their physical counterparts. This assumes standard silicon discrete devices; wide-bandgap (SiC/GaN) FETs use the same logical symbols but have vastly different threshold voltages.
| FET Type | Key Symbol Features (IEEE 315) | IEC 60617 Variant | Common Bench Parts | Practical Meaning & Use Case |
|---|---|---|---|---|
| N-Channel Enhancement MOSFET | Broken channel line. Gate separated by insulation gap. Source arrow points inward toward channel. | Often drawn as a structural symbol, but may use a box with 'G, D, S' labels and an inward arrow. | 2N7000, IRF540N, IRLZ44N | Normally OFF. Requires positive Vgs to turn on. The standard for low-side switching and logic-level driving. |
| P-Channel Enhancement MOSFET | Broken channel line. Gate separated by insulation gap. Source arrow points outward from channel. | Box symbol with outward arrow. Sometimes includes an inversion bubble on the gate. | IRF9540, BS250, Si2301 | Normally OFF. Requires negative Vgs (gate lower than source) to turn on. Used for high-side power switching. |
| N-Channel Depletion MOSFET | Solid channel line. Gate separated by insulation gap. Source arrow points inward. | Rarely used in modern IEC schematics; usually defaults to structural drawing. | BSS139, LND150 | Normally ON. Requires negative Vgs to turn off. Used in constant-current sources and high-voltage startup circuits. |
| P-Channel Depletion MOSFET | Solid channel line. Gate separated by insulation gap. Source arrow points outward. | Structural drawing with solid channel. | LND715 | Normally ON. Requires positive Vgs to turn off. Extremely rare in discrete form; mostly seen in specialized analog ICs. |
| N-Channel JFET | Solid channel line. Gate arrow touches the channel directly (no insulation gap). Arrow points inward. | Structural symbol identical to IEEE, or a box labeled JFET with inward gate arrow. | J310, 2N5457, MPF102 | Normally ON. Gate forms a PN junction. Used in RF amplifiers, audio preamps, and analog switches. |
| P-Channel JFET | Solid channel line. Gate arrow touches channel directly. Arrow points outward. | Structural symbol with outward gate arrow. | J175, 2N5460 | Normally ON. Requires positive Vgs to pinch off the channel. Used in complementary audio input stages. |
Rows People Get Wrong (and Reading Faded Schematics)
Schematics in legacy gear, especially 1980s audio equipment and early switching power supplies, are often faded, poorly copied, or drawn with non-standard idiosyncrasies. Here is how to safely interpret ambiguous symbols when the visual markings fail you.
Mistake 1: Confusing Depletion MOSFETs with JFETs
Both N-channel depletion MOSFETs and N-channel JFETs feature a solid channel line, indicating they are normally-on devices. The only visual difference is the gate insulation gap. If the schematic is faded and you cannot tell if the gate line touches the channel, do not guess based on the symbol alone.
The Bench Fix: Use your multimeter in diode-test mode. Measure between the Gate and Source pins on the physical component. A JFET gate is a direct PN junction; you will read a standard silicon diode drop (0.4V to 0.7V). A depletion MOSFET gate is insulated by silicon dioxide; your meter will read "OL" (open loop/infinite impedance) in both directions. According to ON Semiconductor's standard FET datasheets, this gate impedance distinction is the definitive physical test.
Mistake 2: The Missing Body Diode
In many older or simplified schematics, the intrinsic body diode of a power MOSFET is omitted from the symbol to reduce visual clutter. Never assume a MOSFET blocks reverse current just because the diode isn't drawn. In 99% of discrete 3-pin power MOSFETs (like the TO-220 packaged IRF3205), the body diode is physically built into the silicon die. If you are designing a motor H-bridge or a synchronous buck converter, you must account for this diode's reverse recovery time (trr) and forward voltage drop, even if the schematic symbol looks like a plain switch.
Mistake 3: Enhancement Mode Assumption on Faded Channel Lines
If the channel line is so faded you cannot tell if it is broken (enhancement) or solid (depletion), look at the gate bias network. If the gate features a 10kΩ to 100kΩ pull-down resistor to ground (for an N-channel), the designer intends for the device to be normally OFF. This virtually guarantees it is an enhancement-mode MOSFET. Depletion-mode FETs require a negative bias rail or a source-resistor self-biasing network to remain off at idle.
Regional and Standard Variants (IEEE vs. IEC)
The way a FET symbol is drawn depends heavily on where the schematic was drafted and the era of the design.
- IEEE 315 / ANSI Y32.2 (North America): This is the "structural" standard detailed in the table above. It relies on the physical geometry of the device (the channel line, the insulation gap, the arrow direction) to convey meaning. It is universally used in US-based university textbooks and legacy American schematics.
- IEC 60617 (International / Europe): While IEC permits the structural symbols, it heavily favors "box" symbols for complex integrated circuits. For discrete FETs, you will often see a simple rectangle with the letters G, D, and S inside, accompanied by an arrow on the source lead to indicate N or P channel. The All About Circuits semiconductor guide notes that modern European CAD tools (like Altium or KiCad default libraries) often blend these, using structural symbols for discrete parts and box symbols for ICs.
- Legacy UK BS 3939: Withdrawn in the late 1990s in favor of IEC 60617, but you will still encounter it if you are repairing vintage British audio gear (e.g., old Quad or Naim amplifiers). BS 3939 JFET symbols sometimes omitted the channel line entirely, drawing only the gate arrow and the drain/source connections. If you see a "floating" gate arrow pointing at a blank space between D and S, treat it as a JFET.
Frequently Asked Questions
What does the circle on a FET symbol mean?
The circle is an inversion bubble, borrowed from digital logic. When placed on the gate of a MOSFET, it indicates that the device is active-low. In power electronics, this is most commonly seen on P-channel enhancement MOSFETs used as high-side switches. Because a P-channel MOSFET turns on when the gate voltage is lower than the source (a logic LOW signal relative to the source rail), the bubble visually reminds the designer that a microcontroller GPIO pulling the gate to ground will turn the load ON, not OFF.
How do I tell N-channel from P-channel if the arrow is missing or faded?
If the source arrow is completely illegible, ignore the symbol and read the circuit topology. Look at the power rails. If the Drain connects directly to the positive VCC rail and the Source connects to the load, it is a P-channel high-side switch. If the Source is tied to ground (or a current-sense shunt to ground) and the Drain connects to the load, it is an N-channel low-side switch. As detailed in Texas Instruments' Power MOSFET Basics, N-channel devices are vastly preferred for low-side switching due to their lower on-resistance (Rds_on) and higher electron mobility, making topology a highly reliable clue.
Why does my schematic show a 4-pin MOSFET symbol but the physical part has 3 pins?
The 4-pin symbol includes the Bulk or Body (B) connection, which represents the silicon substrate. In IC design, the body is often tied to the most negative (or positive) rail in the system to prevent latch-up, independent of the source. However, in 99% of discrete 3-pin MOSFETs (like the TO-220 or SOT-23 packages), the manufacturer internally shorts the bulk to the source pin during packaging. If you are doing board-level repair or building a discrete DIY project, treat the 4-pin symbol as a 3-pin physical reality: the Body and Source are the same node. Only worry about the 4th pin if you are designing custom silicon or working with specialized RF MOSFETs where independent body biasing is used to tune threshold voltages.
