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: Understand forward bias.
In forward bias, the p-side of the junction is connected to the positive terminal of the battery and the n-side to the negative terminal. This external voltage opposes the built-in potential barrier of the depletion region and reduces its width, so the diode starts to conduct.

Step 2: Describe the axes of the graph.
Plot the forward voltage \( V \) (in volts) along the positive x-axis and the forward current \( I \) (in milliamperes) along the positive y-axis.

Step 3: Shape of the curve.
For small applied voltage (below the knee voltage) the current is almost zero because the potential barrier is not yet overcome. Beyond the knee (threshold) voltage the current rises sharply and almost exponentially. The knee voltage is about \( 0.7 \) V for a silicon diode and about \( 0.3 \) V for a germanium diode.

Step 4: Draw and label.
The forward characteristic is a curve starting near the origin, staying nearly flat along the voltage axis up to the knee voltage \( V_k \), then bending upward steeply. Mark the knee voltage \( V_k \) on the voltage axis where the sharp rise begins.

Described graph: a nearly horizontal line hugging the V-axis from \( 0 \) to \( V_k \), followed by a steeply rising curve for \( V > V_k \).
\[\boxed{\text{Current stays ~0 until knee voltage } V_k,\ then rises sharply.}\]