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:

1. Preliminary Observation and Preparation
2. 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).

1. Testing with a Multimeter (Taking MOSFET as an Example)
2. 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

1. Precautions
2. 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.

1. Summary of Judgment Logic
2. Abnormal Gate Resistance: Directly confirm device damage.
3. Lack of Unidirectional Conductivity or Failed Conduction Triggering in S-D: Indicates possible internal open circuit, breakdown, or performance degradation.
4. 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.