N-Channel JFET Symbol
Definition: The N-Channel JFET symbol represents a junction field-effect transistor whose N-type conductive channel between Drain and Source is narrowed by reverse-biasing the P-type Gate junction, drawn per IEC 60617 and ANSI Y32.2 / IEEE 315 as a vertical channel line with the gate arrow pointing IN toward the channel.
Also known as: N-channel JFET, junction field-effect transistor, N-JFET, junction FET, depletion-mode FET, JFET transistor, 2N5457.
What the N-Channel JFET symbol means
The N-Channel JFET symbol denotes a three-terminal, voltage-controlled semiconductor device (Gate, Drain, Source) in which current flows through an N-type silicon channel from Drain to Source. The gate forms a PN junction with the channel; applying a negative gate-source voltage (VGS) reverse-biases that junction, widening the depletion region and squeezing the channel until, at the pinch-off voltage VGS(off), drain current stops entirely. Because the controlling junction is reverse-biased, essentially no gate current flows, giving the JFET an extremely high input impedance of 10^9 ohms or more.
Unlike a MOSFET, a JFET is inherently a depletion-mode device: it is fully ON at VGS = 0 V, conducting its maximum drain current IDSS, and is progressively turned OFF as the gate is driven negative with respect to the source. The gate junction must never be forward-biased more than about 0.5 V or gate current will flow through the PN junction. In schematics the symbol appears wherever a low-noise, high-input-impedance amplifying or switching element is needed, and is distinguished from a BJT by its gate arrow meeting a straight channel bar rather than an emitter arrow on an angled lead.
How to identify the N-Channel JFET symbol
The JFET symbol is a vertical line (the channel) with the Drain lead leaving the top, the Source lead leaving the bottom, and the Gate lead meeting the channel from the side with an arrowhead. For the N-channel version the gate arrow points IN toward the channel (the arrow always points from P-type material to N-type, and here the gate is P-type against an N channel). This is the single reliable way to distinguish it from the P-channel JFET, whose arrow points outward.
IEC 60617 and ANSI Y32.2 / IEEE 315 draw the JFET essentially identically — channel bar, two straight D/S leads, arrowed gate lead — with the IEC version sometimes adding an envelope circle for a discrete packaged device. It differs from a MOSFET symbol in having a continuous (unbroken) channel line with no separate insulated-gate bar and no body/substrate arrow in the middle: the JFET gate arrow touches the channel directly because the gate is a junction, not an insulated electrode.
Function in a circuit
In operation, drain current ID flows through the N channel and is set by the gate-source voltage according to the square-law relation ID = IDSS × (1 − VGS/VGS(off))². At VGS = 0 the device conducts IDSS; as VGS goes more negative the depletion regions from the gate junction pinch the channel and ID falls, reaching zero at VGS(off) (typically −0.5 V to −8 V). In the ohmic region (low VDS) the JFET behaves as a voltage-controlled resistor; beyond pinch-off VDS it acts as a voltage-controlled current source with transconductance gm of typically 1–6 mS.
Because the gate draws essentially zero DC current, the JFET makes an excellent high-impedance buffer and low-noise amplifier front end. A common self-bias arrangement places a resistor in the source lead so the voltage drop across it holds the gate (grounded through a large resistor) negative with respect to the source — no negative supply needed.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 (database symbol set, formerly IEC 60617-05 for semiconductors) defines the JFET as a channel bar with drain and source leads and an arrowed gate lead, arrow toward the channel for N-channel. The IEC letter designator on schematics is T or Q per IEC 81346-2 conventions. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 clause for field-effect transistors defines the same channel-bar-plus-arrowed-gate construction, commonly drawn inside an envelope circle for discrete devices. The reference designator is Q followed by a number (Q1, Q2). |
| Key difference | IEC and ANSI JFET symbols are functionally identical; the main stylistic differences are the optional envelope circle (more common in ANSI/North American schematics) and whether the gate lead joins the channel opposite the source (offset-gate style) or at the channel centre. The arrow direction rule — in for N-channel, out for P-channel — is the same in both standards. |
Terminals / pins
| Pin | Name |
|---|---|
| gate | Gate |
| drain | Drain |
| source | Source |
Typical values
Typical small-signal N-channel JFETs: IDSS 1–20 mA (2N5457: 1–5 mA; J201: 0.2–1 mA; 2N3819 / BF245: 2–20 mA), VGS(off) −0.5 V to −8 V, transconductance gm 1–6 mS (1000–6000 µmho), maximum VDS 25–40 V, gate leakage current under 1 nA at 25 °C, input resistance 10^9–10^12 ohms, and noise figures low enough for microphone preamps and electrometer front ends. Power dissipation for TO-92 parts is typically 310–625 mW.
Where the N-Channel JFET symbol is used
- Low-noise audio preamplifier input stages, including condenser microphone impedance converters and guitar effects pedals
- RF amplifiers, oscillators, and mixers in receivers up to VHF/UHF where low noise and square-law mixing are wanted
- Constant-current sources built from a single JFET with gate tied to source (current-regulator diodes are exactly this)
- Analog switches and choppers exploiting the ohmic region as a voltage-controlled resistor
- High-input-impedance buffer stages for oscilloscope probes, pH meters, and electrometer instrumentation
- AGC and muting circuits where the channel resistance is varied smoothly by a control voltage
Example
In a guitar-pedal input buffer schematic, an N-Channel JFET (2N5457) has its Gate pin fed from the input jack through a 1 MΩ resistor to ground, its Drain pin tied to the +9 V rail, and its Source pin loaded with a 3.3 kΩ resistor to ground forming a source follower; the self-bias across the source resistor holds VGS a volt or so negative, the stage presents about 1 MΩ of input impedance to the pickup, and the output is taken from the Source at slightly less than unity gain.
Key facts
- Gate arrow direction is the identifier: pointing IN toward the channel = N-channel JFET; pointing OUT = P-channel. The arrow always points from P-type to N-type material.
- The JFET is a depletion-mode device: fully ON at VGS = 0 V (conducting IDSS) and turned OFF by reverse gate bias, the opposite sense to an enhancement-mode MOSFET.
- Drain current follows the square law ID = IDSS × (1 − VGS/VGS(off))², giving smooth, low-distortion voltage control.
- The gate-channel PN junction must stay reverse-biased; forward-biasing it beyond about +0.5 V (N-channel) causes gate current to flow.
- Input impedance is extremely high (10^9–10^12 ohms) because only junction leakage current flows into the gate — typically under 1 nA.
- In the ohmic region (small VDS) a JFET behaves as a voltage-controlled resistor, the basis of JFET analog switches and AGC circuits.
- IDSS and VGS(off) vary widely between individual devices of the same part number (e.g. 2N5457 IDSS spans 1–5 mA), so bias circuits must tolerate the spread.
- Schematic reference designator is Q (ANSI/IEEE 315) or T/Q (IEC); common part numbers include 2N5457, 2N3819, J201, BF245, and MPF102.
Frequently asked questions
How do I tell an N-channel JFET symbol from a P-channel JFET symbol?
Look at the gate arrowhead. On an N-channel JFET the arrow points IN toward the channel bar; on a P-channel JFET it points OUT away from the channel. The rule is that the arrow points from P-type material to N-type material — the gate of an N-channel device is P-type, so the arrow aims into the N channel.
What is the difference between a JFET symbol and a MOSFET symbol?
A JFET symbol shows the arrowed gate lead touching a continuous channel line directly, because the gate is a PN junction in contact with the channel. A MOSFET symbol shows the gate as a separate bar insulated from the channel, usually with a broken (enhancement) or solid (depletion) channel line and a substrate arrow in the middle. If the arrow is on the gate lead itself, it is a JFET; if the arrow is on the body/substrate connection, it is a MOSFET.
Why is a JFET called a depletion-mode device?
Because its channel conducts fully with zero gate voltage and is turned off by depleting it of carriers. Reverse-biasing the gate-channel junction widens the depletion region, narrowing the conductive channel until drain current stops at the pinch-off voltage VGS(off). A JFET cannot be operated in enhancement mode, since forward-biasing the gate junction would make gate current flow.
What do IDSS and VGS(off) mean on a JFET datasheet?
IDSS is the saturation drain current with the gate shorted to the source (VGS = 0 V) — the maximum useful drain current of the device. VGS(off), also called the pinch-off voltage VP, is the gate-source voltage at which drain current is reduced essentially to zero (negative for an N-channel JFET, typically −0.5 V to −8 V). Together they define the square-law transfer curve ID = IDSS × (1 − VGS/VGS(off))².
Why are JFETs used in audio preamps instead of BJTs?
Three reasons: the JFET gate draws essentially no current, so it presents a very high input impedance (megohms) that does not load high-impedance sources like guitar pickups and condenser capsules; its voltage noise at audio frequencies is low with no shot noise from base current; and its square-law transfer characteristic produces predominantly soft, low-order distortion that is often considered musically pleasant.
Can a JFET be used as a switch?
Yes. In the ohmic region a JFET acts as a resistor of a few tens to hundreds of ohms when ON (VGS = 0) and as an open circuit of gigohms when driven past VGS(off). JFET analog switches and choppers exploit this, though the ON resistance is higher and less constant than a MOSFET's, so MOSFETs dominate modern analog-switch ICs.
Related symbols
- Darlington Pair symbol
- IGBT symbol
- N-Channel MOSFET symbol
- NPN Transistor (BJT) symbol
- Op-Amp symbol
- P-Channel MOSFET symbol
Place the N-Channel JFET symbol on a wiring diagram or schematic in the free online circuit diagram maker — no download required.