Junction Field-Effect Transistor (JFET) Pin Identification
The gate of a JFET is equivalent to the base of a transistor, while the source and drain correspond to the emitter and collector, respectively. Set a multimeter to the R×1k range and measure the forward and reverse resistance between each pair of pins. When the forward and reverse resistances between two pins are equal, both several kΩ, these two pins are the drain (D) and source (S) (interchangeable). The remaining pin is the gate (G). For JFETs with four pins, the remaining pin is the shield (grounded during use).
Gate Determination
Touch one electrode of the transistor with the black probe of the multimeter, and touch the other two electrodes with the red probe. If both measured resistances are very high, it indicates reverse resistance, meaning the transistor is an N-channel JFET, and the black probe is connected to the gate. The manufacturing process dictates that the source and drain of a JFET are symmetrical and interchangeable without affecting circuit operation; therefore, differentiation is unnecessary. The resistance between the source and drain is approximately several thousand ohms.
Note that this method cannot be used to determine the gate of an insulated-gate field-effect transistor (IGFET). This is because the input resistance of such transistors is extremely high, and the gate-source capacitance is very small. During measurement, even a small amount of charge can create a very high voltage across the gate-source capacitance, easily damaging the transistor.
Estimating Amplification Capability
Set the multimeter to the R×100 range. Connect the red probe to the source (S) and the black probe to the drain (D), effectively applying a 1.5V power supply voltage to the IGFET. The meter needle will then indicate the D-S resistance value. Next, pinch the gate (G) with your finger, applying the induced voltage from your body as an input signal to the gate. Due to the transistor's amplification effect, both UDS and ID will change, which is equivalent to a change in the D-S resistance. A significant swing in the meter needle can be observed. If the needle swings very little when the gate is pinched, the transistor's amplification capability is weak; if the needle does not move, the transistor is damaged. Because the 50Hz AC voltage induced by the human body is relatively high, and the operating point of different MOSFETs may differ when measured with a resistance range, the meter needle may swing to the right or left when the gate is squeezed by hand. A few MOSFETs will have a decreased RDS, causing the needle to swing to the right; most MOSFETs will have an increased RDS, causing the needle to swing to the left. Regardless of the direction of the needle's swing, as long as there is a noticeable swing, it indicates that the MOSFET has amplification capability.
This method also applies to measuring MOSFETs. To protect the MOSFET, the insulated handle of the screwdriver must be held by hand, and the gate must be touched with a metal rod to prevent the induced charge from being directly applied to the gate and damaging the MOSFET.
After each measurement of a MOSFET, a small amount of charge will accumulate on the G-S junction capacitance, establishing a voltage UGS. When measuring again, the meter needle may not move. In this case, short-circuiting the G-S terminals will resolve the issue.
