Draw the circuit diagrams used to study the forward and reverse bias characteristics of a p-n junction diode. Draw the characteristics and explain their shape.

1. 1 · State the aim. To study how the current through a p-n junction diode changes with voltage in forward and reverse bias.
2. 2 · Draw the forward-bias circuit. [Circuit: battery, rheostat as potential divider, diode with p to +, milliammeter in series, voltmeter across the diode]
3. 3 · Draw the reverse-bias circuit. [Circuit: higher-voltage battery, rheostat, diode with p to −, microammeter in series, voltmeter across the diode]
4. 4 · Explain the forward curve. Almost no current until the threshold (about 0.7 V for Si), because the barrier must be overcome; then the current rises steeply.
5. 5 · Explain the reverse curve. Barrier rises, so only minority carriers flow: a tiny, nearly constant current (μA), until it rises sharply at breakdown voltage.
6. 6 · Draw the graph. [Graph: forward curve in the first quadrant (I in mA), reverse curve in the third quadrant (I in μA), threshold voltage and Vbr marked]
7. 7 · Real life. A diode guards torches and toys against cells put in the wrong way.
8. 8 · Close in one line. So, a diode conducts easily in forward bias and hardly at all in reverse bias.

Scoring tip: Draw both circuits separately and state the different current scales on the graph. Many students lose marks by using the same scale.