CBSE Class 12 Physics: Forward Biased p-n Junction Characteristics
Step 1 (Circuit Configuration & Principle): [1 Mark]
To obtain the forward bias characteristics, the p-n junction is connected in a circuit where the p-terminal is connected to the positive terminal of an external DC voltage source and the n-terminal is connected to the negative terminal. A variable DC source (potentiometer) is used to vary the applied voltage ($V$), and a milliammeter ($\text{mA}$) is connected in series to measure the forward current ($I_f$), while a voltmeter ($V$) is connected in parallel to measure the potential drop across the diode.
Circuit Diagram Description:
- Diode: p-n junction diode.
- Series Resistor: Used to limit the current and prevent damage to the diode.
- Voltmeter: Connected across the diode to measure $V_f$.
- Milliammeter: Connected in series to measure $I_f$.
Step 2 (Experimental Procedure & Working): [0.5 Mark]
The applied voltage ($V_f$) is increased in small, equal steps. For each step, the corresponding forward current ($I_f$) is recorded. Initially, the current increases very slowly (until the barrier potential is overcome). Once the applied voltage exceeds the barrier potential (approx. $0.7\text{ V}$ for Silicon, $0.3\text{ V}$ for Germanium), the current increases exponentially with a small increase in voltage.
Step 3 (Characteristic Curve & Conclusion): [0.5 Mark]
The graph is plotted with $V_f$ on the x-axis and $I_f$ on the y-axis. The curve shows a non-linear rise, characteristic of a semiconductor junction.
The curve starts from the origin, remains nearly flat until the knee voltage, and then rises sharply.
💡 EL10 Concept Insight:
Think of the p-n junction like a gate with a spring-loaded door; you have to push with a certain amount of "force" (voltage) to overcome the barrier, but once the door is open, even a tiny extra push makes the "crowd" (current) rush through very quickly!