Junction diode in reverse bias, and the I-V experiment

What reverse bias does

In reverse bias, the applied voltage V adds to the barrier, so it becomes V₀ + V, and the depletion region widens. Majority carriers can no longer diffuse across. Only the few minority carriers are swept across by the field, giving a tiny current of a few microamperes (μA). This current is limited by the number of minority carriers, not by the voltage, so it stays nearly constant: the reverse saturation current. At the breakdown voltage Vbr, the reverse current suddenly becomes large. An ordinary diode can be damaged if it is used beyond this point.

Forward bias and reverse bias

Activity: plotting the I-V characteristic of a p-n junction diode

Aim: to draw the I-V characteristic curve of a p-n junction diode in forward and reverse bias.

1. 1
   Forward-bias circuit: join a low-voltage battery, a rheostat used as a potential divider, a protective resistor, the diode (p-side towards +) and a milliammeter in series. Connect a low-range voltmeter (for example 0 to 3 V) across the diode.
2. 2
   Raise the voltage from zero in steps of about 0.1 V. Record the forward current I (mA) each time. Observe: the current stays near zero until about 0.7 V (Si), then rises steeply.
3. 3
   Reverse-bias circuit: turn the diode round (p-side towards −). Use a higher-voltage battery, a microammeter in place of the milliammeter, and a voltmeter of larger range (for example 0 to 15 V).
4. 4
   Raise the reverse voltage in steps of about 1 V and record the reverse current (μA). Observe: it stays tiny and almost constant. Do not go near the breakdown voltage.
5. 5
   Plot both sets of readings on one graph: forward readings in the first quadrant (I in mA) and reverse readings in the third quadrant (I in μA). Write the two scales clearly.
6. 6
   Record the result: the threshold voltage, the dynamic resistance from the slope of the forward curve, and the reverse current. Conclusion: the diode conducts well in one direction only.

How to draw the full I-V characteristic

• Draw axes crossing at the origin. Label the right and up directions forward V (V) and forward I (mA).
• Label the left and down directions reverse V (V) and reverse I (μA), and note that the two current scales differ.
• First quadrant: draw the forward curve, flat until the threshold voltage, then rising steeply.
• Third quadrant: draw a small, nearly flat reverse current just below the axis.
• At a large reverse voltage, bend the curve sharply downward and mark that voltage Vbr.

Worked example: forward and reverse resistance