Electrode Measurement by Resistance Method: Based on the difference in forward and reverse resistance values of the PN junction in a junction field-effect transistor (JFET), the three electrodes of the JFET can be identified. Specific method: Set the multimeter to the R×1k range. Randomly select two electrodes and measure their forward and reverse resistance values. When the forward and reverse resistance values of two electrodes are equal and both are several thousand ohms, these two electrodes are the drain (D) and source (S), respectively. Because the drain and source are interchangeable for JFETs, the remaining electrode must be the gate (G). Alternatively, you can arbitrarily touch one electrode with the black probe of the multimeter (the red probe can also be used), and then touch the other two electrodes in turn, measuring their resistance values. When two approximately equal resistance values are obtained, the electrode touched by the black probe is the gate, and the other two electrodes are the drain and source, respectively. If both measured resistance values are very high, it indicates a reverse PN junction (PN junction), meaning both are reverse resistances, indicating a P-channel MOSFET, and the black probe is connected to the gate. If both measured resistance values are very low, it indicates a forward PN junction (PN junction), meaning both are forward resistances, indicating an N-channel MOSFET, and the black probe is also connected to the gate. If neither of these conditions is met, try swapping the black and red probes and testing as described above until the gate is identified.
Resistance Method for Determining Quality: The resistance method involves using a multimeter to measure the resistance between the source and drain, gate and source, gate and drain, and gate G1 and gate G2 of the MOSFET. Compare the measured resistance values with the resistance values specified in the MOSFET's datasheet to determine if the MOSFET is good or bad. Specific method: First, set the multimeter to the R×10 or R×100 range and measure the resistance between the source (S) and drain (D). This resistance is typically in the range of tens to thousands of ohms (the resistance value varies depending on the transistor model, as can be found in the datasheet). If the measured resistance is higher than normal, it may be due to poor internal contact; if the measured resistance is infinite, it may indicate an internal open circuit. Next, set the multimeter to the R×10k range and measure the resistance between the gates G1 and G2, between the gate and source, and between the gate and drain. If all these resistances are infinite, the transistor is normal; if the measured resistances are too low or show a short circuit, the transistor is faulty. Note that if both gates are open inside the transistor, a component substitution method can be used for testing.
Measuring Amplification Capability
Using the Inductive Signal Method: Using a multimeter set to the R×100 range, connect the red probe to the source (S) and the black probe to the drain (D). Apply a 1.5V power supply to the MOSFET. The meter needle will then indicate the resistance between the source and drain. Next, gently squeeze the gate (G) of the MOSFET, applying the induced voltage signal from your body to the gate. Due to the MOSFET's amplification effect, both the drain-source voltage (VDS) and drain current (Ib) will change, resulting in a change in the source-drain resistance. This will be observed as a significant swing in the meter needle. A small needle swing indicates poor amplification; a large swing indicates high amplification; no movement indicates a faulty MOSFET.
Using this method, test the 3DJ2F MOSFET using the R×100 range of a multimeter. First, with the gate (G) of the transistor open-circuited, the drain-source resistance (RDS) was measured to be 600Ω. Then, by pinching the G terminal, the multimeter needle swung to the left, indicating an RDS of 12kΩ. The significant swing of the needle indicates that the transistor is good and has a large amplification capability.
Several points should be noted when using this method: First, when testing a field-effect transistor (FET), the multimeter needle may swing to the right (resistance decrease) or to the left (resistance increase) when the gate is pinched. This is because the AC voltage induced by the human body is relatively high, and different FETs may have different operating points when measured with resistance (either in the saturation region or the unsaturation region). Experiments show that for most transistors, the RDS increases, causing the needle to swing to the left; for a few transistors, the RDS decreases, causing the needle to swing to the right. However, regardless of the direction of the needle swing, a large swing indicates that the transistor has a large amplification capability. Second, this method is also applicable to MOSFETs. However, it's important to note that MOSFETs have high input resistance, and the allowable induced voltage at the gate (G) should not be too high. Therefore, never touch the gate directly with your hands; instead, use an insulated handle like a screwdriver to touch the gate with the metal rod to prevent induced charge from being directly applied to the gate and causing breakdown. Third, after each measurement, short-circuit the G-S junction. This is because a small amount of charge will accumulate on the G-S junction capacitance, building up a VGS voltage, which may cause the meter needle to not move during subsequent measurements. Only by short-circuiting the G-S junction can this charge be discharged.
Identifying Unmarked Transistors: First, use resistance measurement to identify the two pins with resistance values: the source (S) and drain (D). The remaining two pins are the first gate (G1) and the second gate (G2). Record the resistance value measured between the source (S) and drain (D) using the first two probes. Reverse the probes and measure again, recording the resulting resistance value. The electrode connected to the black probe is the drain (D), and the electrode connected to the red probe is the source (S). The source (S) and drain (D) terminals identified using this method can be verified by estimating the transistor's amplification capability. The black probe, indicating higher amplification, is connected to the drain (D), and the red probe, grounded, is connected to the source (S). Both methods should yield the same result. Once the drain (D) and source (S) positions are determined, the circuit is installed according to their corresponding positions. Generally, G1 and G2 will also align sequentially, thus determining the positions of the two gates, G1 and G2, and consequently, the order of the D, S, G1, and G2 pins.
Determining Transconductance: The transconductance is determined by measuring the change in reverse resistance. When measuring the transconductance of a VMOS V-channel enhancement-mode MOSFET, the red probe can be connected to the source (S) and the black probe to the drain (D). This is equivalent to applying a reverse voltage between the source and drain. At this time, the gate is open-circuited, and the reverse resistance value of the transistor is very unstable. The multimeter should be set to the high resistance range of R×10kΩ, where the internal voltage is relatively high. When the gate G is touched by hand, the reverse resistance of the transistor will change significantly. The greater the change, the higher the transconductance of the transistor. If the transconductance of the transistor under test is very small, the reverse resistance will not change much when measured by this method.
