Question

Draw a graph between voltage and current of a p-n junction diode in forward bias.

Hint:

Recall that a forward-biased diode conducts only after the knee/threshold voltage (~0.7 V for Si, ~0.3 V for Ge); the I-V curve stays flat then rises steeply.

Answer 1

Step 1: Set up the experiment.
Connect the diode in forward bias (p to battery +, n to battery −) with a variable voltage source, a milliammeter to read current \(I\), and a voltmeter to read the applied voltage \(V\).

Step 2: Take readings.
Slowly increase \(V\) from zero. Note that up to a certain small voltage the current recorded by the milliammeter is negligibly small, because the junction barrier field still opposes the flow of majority carriers.

Step 3: Identify the threshold.
At the threshold or cut-in voltage the barrier is effectively cancelled. For silicon this is near \(0.7\) V and for germanium near \(0.3\) V. After this point even a tiny increase in \(V\) causes a large increase in \(I\).

Step 4: Plot the trace.
Taking \(V\) on the horizontal axis and \(I\) on the vertical axis, the plotted points give a curve that clings to the voltage axis at first and then swings upward almost vertically after the cut-in voltage. This upward-bending, exponential-looking curve is the forward characteristic of the diode.
\[\boxed{I\text{ is very small below }V_k\text{ and increases rapidly above }V_k}\]