Describe, with circuit diagrams, how you would plot the forward and the reverse V-I curves of a p-n junction diode. Sketch the curves you expect.

1. 1 · State the idea. A diode's V-I characteristic shows how its current changes with the voltage across it.
2. 2 · Draw both circuits. [Diagram: forward circuit with battery, rheostat, protective resistor, diode with p-side to +, milliammeter in series and voltmeter across the diode; reverse circuit with the diode turned round and a microammeter]
3. 3 · Describe the forward readings. Raise V in 0.1 V steps. The current stays near zero up to the knee voltage (Si 0.7 V, Ge 0.3 V), then rises steeply in mA.
4. 4 · Describe the reverse readings. Raise V in 1 V steps. A tiny, nearly constant current in μA flows until the breakdown voltage.
5. 5 · Draw the characteristic. [Graph: forward curve in the first quadrant with the knee voltage marked; reverse curve in the third quadrant, nearly flat, dropping sharply at breakdown; separate mA and μA scales]
6. 6 · Real life. This one-way action lets diodes in phone chargers turn AC into DC.
7. 7 · Close in one line. So, a diode has low resistance in forward bias and very high resistance in reverse bias.

Scoring tip: Show the protective resistor, and write mA on the forward axis and μA on the reverse axis. Examiners look for both.