How it works
The JFET channel is a bar of N-type (or P-type) silicon with ohmic contacts at drain and source. The gate is a P-type region diffused on both sides, forming two PN junctions that are always reverse biased during normal operation. V_GS controls the depletion region width — more negative V_GS widens the depletion regions, narrows the channel, and reduces I_D. The Shockley equation governs this: I_D = I_DSS · (1 − V_GS/V_P)². Transconductance g_m = 2I_DSS/|V_P| · (1 − V_GS/V_P) peaks at V_GS = 0 and reaches a typical value of 2–4 mA/V for small-signal JFETs.
Key points to remember
JFETs are depletion-mode only — they are ON at V_GS = 0 and turned off by increasing the gate voltage magnitude. N-channel JFETs require V_GS between 0 and V_P (negative); P-channel JFETs require positive V_GS. The drain resistance r_d in the saturation region is typically 10 kΩ to 1 MΩ. Because gate current is essentially zero (typically < 1 nA), JFET amplifiers present extremely high input impedance — up to 10⁸ Ω — making them ideal for pH meters and electrometer circuits. The 2N5484 and BFW10 are commonly cited in Indian textbook examples and exam problems.
Exam tip
The examiner always asks you to derive the transconductance expression g_m from the Shockley equation by differentiation — show the step from I_D = I_DSS(1 − V_GS/V_P)² to g_m = dI_D/dV_GS clearly.