Electronics Basics #21 | JFET & Shockley’s Equation — Step by Step

Instructor: Dr. Süleyman Burak ÇELİK

How can a voltage control a current while drawing almost no steady gate current? Start with the physical goal, explore an n-channel JFET in a conceptual 3D cutaway, then work through Shockley’s equation one arithmetic step at a time. Teaching example: IDSS = 8 mA, VP = −4 V, VGS = −2 V, VDS = 5 V. The ideal saturation result is ID = 2 mA; the required drain-voltage threshold is 2 V. We distinguish pinch-off from cutoff and show why a formula must not be extrapolated beyond its valid range. The 3D model is explanatory, not a semiconductor simulation. Chapters: 0:00 The goal: control current with voltage 0:49 Three terminals, a different control mechanism 2:08 Why almost no gate current? 3:27 Two landmarks and Shockley’s equation 4:53 Work it out: every step 6:59 Pinch-off is not the same as cutoff 8:38 The calculator does not know the device 10:21 Your JFET analysis checklist