The field-effect transistor (FET)
The field-effect transistor (FET) is a voltage-controlled semiconductor device commonly used in circuits such as amplifiers and switches. To determine its functionality, methods such as measuring pin resistance with a multimeter, checking transconductance, and testing voltage withstand capability can be used. Below are the specific steps and precautions:
*I. Preliminary Observation and Preparation**
1. **Visual Inspection**
- Check if the pins are oxidized or broken, and if the casing shows signs of burning or cracking.
- For power FETs, inspect whether the heat sink is firmly soldered to the chip.
2. **Identify Type and Pins**
- FETs are divided into **Junction Field-Effect Transistors (JFETs)** and **Insulated-Gate Field-Effect Transistors (MOSFETs)**, with the latter further categorized into N-channel and P-channel types.
- The pins are typically **Gate (G), Source (S), and Drain (D)**, while some MOSFETs have a substrate pin (B).
- Use the datasheet or the diode test function of a multimeter to preliminarily identify the pins (the gate of a MOSFET is insulated from the source/drain, with infinite resistance).
*II. Testing with a Multimeter (Taking MOSFET as an Example)**
*1. Check the Gate (G) for Normality**
- **Purpose**: Determine if the gate is damaged by breakdown or short-circuit.
- **Method**:
- Set the multimeter to the resistance range (×10kΩ to avoid electrostatic damage to the MOSFET).
- Measure the resistance between the gate and source, and between the gate and drain. Normal values should be **infinite**.
- If the resistance is 0 or low, the gate is broken down, and the device is damaged.
*2. Test the Unidirectional Conductivity of Source (S) and Drain (D) (N-Channel MOSFET)**
- **Principle**: When the MOSFET is conducting, a parasitic diode exists between the source and drain (absent in some models).
- **Method**:
- Set the multimeter to the diode test function. Connect the red probe to the source and the black probe to the drain. A normal reading should show a **voltage drop of 0.5–0.8V** (indicating the parasitic diode is conducting).
- Reverse the probes (red to drain, black to source). For enhancement-mode MOSFETs, the resistance should be infinite when not conducting; for depletion-mode MOSFETs, low resistance may be displayed (verify with the device model).
- **Abnormal Cases**: If both forward and reverse measurements show conduction or no conduction, the device may have internal breakdown or an open circuit.
*3. Trigger Conduction Test (Check Normal Turn-On)**
- **Purpose**: Verify whether the gate voltage can control the conduction between source and drain.
- **Method**:
- Set the multimeter to the resistance range (×1kΩ). Connect the red probe to the source and the black probe to the drain; the resistance should be infinite (not conducting).
- Short the gate and drain with a finger or tweezers (apply a positive voltage to the gate for an N-channel MOSFET) and observe if the resistance drops rapidly (indicating conduction).
- Short the gate and source (discharge), and the resistance should return to infinity.
- **Note**: For P-channel MOSFETs, trigger conduction by applying a negative voltage (connect the gate to the source).
*III. Differentiating Between JFETs and MOSFETs**
| Type | Gate Characteristics | Key Testing Points |
| JFET | PN junction between gate and channel; measurable forward/reverse resistance | Forward/reverse resistance between G-S should differ (PN junction characteristics); D-S can be triggered to conduct |
| MOSFET | Insulated gate with infinite resistance | Avoid electrostatic damage; the gate must not be left floating |
**IV. Precautions**
1. **Anti-Static Measures**
- The gate insulation layer of MOSFETs is vulnerable to electrostatic breakdown. Wear an anti-static wrist strap before operation or short the pins with a multimeter probe to discharge static electricity.
2. **Avoid Misjudgment**
- Some power MOSFETs have an integrated freewheeling diode internally. During testing, distinguish between diode characteristics and MOSFET conduction characteristics.
- If the device is soldered to a circuit board, disconnect one pin before measuring to avoid circuit interference.
3. **Professional Tool Assistance**
- For precise detection of parameters such as transconductance (Gm) and voltage withstand (VDS), use a **transistor characteristic curve tracer** or **specialized testing equipment**.
*V. Summary of Judgment Logic**
1. **Abnormal Gate Resistance**: Directly confirm device damage.
2. **Lack of Unidirectional Conductivity or Failed Conduction Triggering in S-D**: Indicates possible internal open circuit, breakdown, or performance degradation.
3. **Refer to Model and Datasheet Standards**: Compare measured resistance and voltage drop with theoretical values.
Through the above steps, the functionality of a FET can be quickly assessed. For precision circuits or high-power devices, it is recommended to combine circuit principles with dynamic testing (e.g., observing operating status when integrated into a circuit) for further verification.
