The two primary types of FET transistor you will encounter on the workbench are the Junction Field Effect Transistor (JFET) and the Metal-Oxide-Semiconductor FET (MOSFET). While both are voltage-controlled devices that use an electric field to modulate current flow, their internal construction dictates entirely different use cases. MOSFETs dominate power switching, motor control, and digital logic due to their high input impedance and fast switching speeds. JFETs, conversely, excel in low-noise analog amplification, RF front-ends, and simple constant-current sources.
This guide cuts through the semiconductor physics to give you the practical data you need: how to read their symbols, how to bias them correctly, how to test them with a digital multimeter (DMM), and which exact part numbers to keep in your component bins.
The Main Types of FET Transistor and Their Symbols
Before wiring a circuit, you must be able to identify the device type and pinout from its schematic symbol. All FETs share three primary terminals: the Gate (G), Drain (D), and Source (S). The Gate controls the current flowing from Drain to Source.
JFET Symbols and Pinouts
The JFET symbol features a solid vertical line representing the semiconductor channel, with the Drain and Source attached to opposite ends. The Gate is represented by an arrow touching the channel directly, indicating a physical PN junction.
- N-Channel JFET: The Gate arrow points inward toward the channel. It requires a negative Gate-to-Source voltage (Vgs) to restrict current flow.
- P-Channel JFET: The Gate arrow points outward away from the channel. It requires a positive Vgs to restrict current.
MOSFET Symbols and Pinouts
The MOSFET symbol replaces the direct Gate connection with a distinct gap between the Gate terminal and the channel line, representing the silicon dioxide insulating layer. This insulation is what gives MOSFETs their near-infinite DC input impedance.
- Enhancement MOSFET (Normally OFF): The channel line is dashed. It requires a threshold voltage (Vgs(th)) to turn on. This is the most common type used in digital and power switching.
- Depletion MOSFET (Normally ON): The channel line is solid. It conducts with 0V on the gate and requires a voltage to turn it off.
Operation Regions and Typical Ratings
Unlike Bipolar Junction Transistors (BJTs) that are strictly current-controlled, FETs operate in distinct regions based on the voltages applied to the Gate and Drain. Understanding these regions is mandatory for selecting the right device.
| Region | Condition (N-Channel Enhancement) | Behavior | Typical Use Case | Example: IRF520 (Power MOSFET) | Example: J201 (Audio JFET) |
|---|---|---|---|---|---|
| Cutoff | Vgs < Vgs(th) | No channel formed; Id ≈ 0 | Switching (OFF state) | Vgs < 2.0V | Vgs < Vgs(off) (e.g., < -2V) |
| Linear (Ohmic) | Vgs > Vgs(th) AND Vds < (Vgs - Vgs(th)) | Acts as a voltage-controlled resistor | Analog switches, variable resistors | Vgs = 10V, Vds < 6V | Rarely used in linear audio amps |
| Saturation (Active) | Vgs > Vgs(th) AND Vds > (Vgs - Vgs(th)) | Current Id is constant, controlled only by Vgs | Amplification, constant current sources | Vgs = 5V, Vds = 12V (Id ≈ 15A) | Vgs = -1V, Vds = 6V (Id ≈ 1mA) |
For a deeper dive into the semiconductor physics governing these regions, the All About Circuits semiconductor textbook provides an excellent foundational breakdown of channel formation and pinch-off.
How to Select and Bias a FET for Your Circuit
Selecting between the types of FET transistor depends entirely on your application. If you are switching a 12V solenoid from a 5V microcontroller, you need a logic-level N-channel Enhancement MOSFET. If you are building a high-impedance buffer for an electric guitar pickup, you need an N-channel JFET.
Application Circuit: JFET Common-Source Audio Preamp
JFETs are notoriously tricky to bias because their parameters (like Idss and Vgs(off)) vary wildly even within the same manufacturing batch. The most robust way to bias a JFET for linear amplification is using a self-biasing source resistor configuration.
Circuit Specifications:
- Transistor: J201 (N-Channel JFET)
- Supply (Vdd): 12V DC
- Drain Resistor (Rd): 2.2 kΩ
- Source Resistor (Rs): 470 Ω
- Gate Resistor (Rg): 1 MΩ (ties Gate to Ground, provides high input impedance)
- Coupling Capacitors (Cin, Cout): 1 µF (film or ceramic)
Biasing Math:
Assume the J201 settles at a quiescent drain current (Id) of 2 mA. The voltage drop across the Source resistor (Rs) is Vs = Id × Rs = 2mA × 470Ω = 0.94V. Because the Gate is tied to ground via Rg, Vg = 0V. Therefore, the Gate-to-Source voltage is Vgs = Vg - Vs = 0V - 0.94V = -0.94V. This negative bias automatically pushes the JFET into the saturation (active) region without requiring a negative power supply. The Drain voltage will sit at Vd = 12V - (2mA × 2.2kΩ) = 7.6V, giving a healthy Vds of ~6.6V for symmetrical AC signal swing.
Failure Modes and Multimeter Testing
FETs fail in predictable ways. MOSFETs usually die from gate oxide punch-through caused by Electrostatic Discharge (ESD) or exceeding the maximum Vgs (typically ±20V). They also fail from thermal runaway if not properly heatsunk during linear operation, or avalanche breakdown if inductive kickback exceeds the Vds rating. JFETs are generally more robust against ESD but will fail if the Gate-Source PN junction is forward-biased beyond its current limits.
How to Test an N-Channel Power MOSFET with a DMM
You can verify the health of a power MOSFET using the Diode Test mode on a standard multimeter. Follow these numbered steps exactly:
- Discharge the Gate: With the MOSFET out of circuit, touch your finger across the Gate and Source pins, or use a 10kΩ resistor to short them. This bleeds off any trapped charge in the gate capacitance.
- Check the Body Diode: Set your DMM to Diode Test. Place the Red probe on the Source and the Black probe on the Drain. You should read a forward voltage drop of roughly 0.4V to 0.6V. Reverse the probes (Black on Source, Red on Drain); the meter should read 'OL' (Open Loop).
- Charge the Gate: Keep the Black probe on the Drain. Momentarily touch the Red probe to the Gate. This applies the DMM's internal ~3V to the gate, turning the MOSFET on.
- Verify Conduction: Move the Red probe back to the Source (Black remains on Drain). The meter should now read a very low voltage drop (near 0.00V or a short), indicating the channel is conducting.
- Discharge and Re-verify: Short the Gate to the Source again. Check Drain-to-Source with the Red probe on Source; it should revert to the 0.4V - 0.6V body diode reading. If it stays shorted, the gate oxide is punctured and the part is dead.
Safe Default Part Numbers for the Workbench
When prototyping, reaching for a known-good default part saves hours of datasheet hunting. Below are the standard-bearer FETs that cover 95% of hobbyist and bench applications.
| Part Number | Type | Vds (Max) | Id (Continuous) | Rds(on) / Vgs(off) | Vgs(th) | Best Application |
|---|---|---|---|---|---|---|
| 2N7000 | N-Ch MOSFET (TO-92) | 60V | 200 mA | 1.2 Ω @ 10V | 0.8V - 3.0V | Low-side switching for LEDs/relays |
| BSS138 | N-Ch MOSFET (SOT-23) | 50V | 220 mA | 1.6 Ω @ 4.5V | 0.5V - 1.5V | 3.3V to 5V I2C level shifting |
| IRLZ44N | N-Ch MOSFET (TO-220) | 55V | 47 A | 14 mΩ @ 5V | 1.0V - 2.0V | Arduino/ESP32 high-power motor control |
| IRFZ44N | N-Ch MOSFET (TO-220) | 55V | 49 A | 17.5 mΩ @ 10V | 2.0V - 4.0V | 12V/24V automotive and PSU switching |
| J201 | N-Ch JFET (TO-92) | 20V | 5 mA | Vgs(off): -0.5V to -4V | N/A | Audio preamps, mic buffers, RF |
Note: Always check the specific manufacturer's datasheet (e.g., from Vishay or ON Semiconductor) for thermal derating curves. A 47A rating on the IRLZ44N assumes an infinite heatsink and a case temperature of 25°C; in free air on a breadboard, it will overheat at roughly 2A to 3A.
Frequently Asked Questions
What is the difference between a BJT and the different types of FET transistor?
A Bipolar Junction Transistor (BJT) is a current-controlled device; you must supply continuous base current to maintain collector current, which results in lower input impedance and higher drive losses. The types of FET transistor (JFETs and MOSFETs) are voltage-controlled. Once the gate capacitance is charged, virtually zero DC current flows into the gate, making FETs vastly superior for battery-powered circuits, high-impedance sensor interfaces, and microcontroller GPIO driving.
Can I swap an N-channel JFET for an N-channel MOSFET in an amplifier?
Not without heavily redesigning the bias network. An enhancement MOSFET is normally OFF and requires a positive Vgs to turn on, usually achieved via a voltage divider on the gate. An N-channel JFET is normally ON and requires a negative Vgs to pinch off the channel, usually achieved via a source resistor (self-biasing). Dropping a 2N7000 into a circuit designed for a J201 will result in zero output because the MOSFET will remain in cutoff.
Why did my MOSFET explode when I connected it to an Arduino GPIO?
You likely used a standard-threshold MOSFET (like the IRF520 or IRFZ44N) instead of a logic-level MOSFET (like the IRLZ44N). Standard MOSFETs require 10V on the gate to fully enhance and achieve their rated Rds(on). An Arduino outputs 5V. At 5V, a standard MOSFET operates in the linear (ohmic) region, acting as a high-value resistor rather than a closed switch. The resulting I²R heat dissipation causes rapid thermal runaway and catastrophic failure. Always match the Vgs(th) and the Rds(on) test conditions in the datasheet to your microcontroller's logic voltage.
How do I protect the gate of a MOSFET from static damage?
The silicon dioxide layer separating the gate from the channel is incredibly thin and can be punctured by as little as 20V of static electricity. To protect it: always store MOSFETs in anti-static foam or shorting rings; add a 10kΩ pull-down resistor between the Gate and Source to bleed off stray charges; and place a 12V or 15V Zener diode across the Gate and Source to clamp voltage spikes in noisy industrial or automotive environments.
