PN Junction Diode: I–V Characteristics and Applications Quiz Quiz

Explore the essential concepts of PN junction diode I–V characteristics, including forward and reverse bias behavior, threshold voltage, breakdown mechanisms, and real-world applications. Evaluate your understanding of how diodes function in electronic circuits with these carefully designed questions.

1. Forward Bias Current Flow

   When a silicon PN junction diode is connected in forward bias with a voltage greater than its threshold (e.g., 0.8 V), what is most likely to happen to the current across the diode?

   1. The current will increase rapidly.
   2. The current will become negative.
   3. The current will remain zero.
   4. The current will decrease gradually.

   Explanation: In forward bias, once the applied voltage exceeds the threshold (also known as cut-in or knee voltage, about 0.7 V for silicon), the potential barrier is overcome, and current increases sharply. The current does not remain zero because the barrier is breached and carriers flow freely. A decrease in current is incorrect; forward bias encourages carrier movement. The current will not become negative, since negative current indicates reverse bias conditions.

2. Reverse Breakdown Phenomenon

   What primarily causes the sharp increase in reverse current when the reverse voltage across a PN junction diode exceeds a certain value, such as the breakdown voltage?

   1. Formation of new junctions
   2. Avalanche multiplication of carriers
   3. Saturation of the depletion layer
   4. Ohmic contact between P and N regions

   Explanation: When the reverse bias voltage exceeds the breakdown voltage, the electric field becomes strong enough to accelerate carriers, leading to avalanche multiplication and a sudden rise in current. Saturation of the depletion layer is not responsible for the spike in current. Ohmic contact refers to the electrode-junction quality and does not induce breakdown. New junctions are not formed during breakdown; only existing carrier processes are amplified.

3. Threshold Voltage Effect

   Consider a germanium diode used in a circuit; what is its typical threshold or cut-in voltage required before significant current flows in forward bias?

   1. About 2.2 V
   2. About 1.2 V
   3. About 0.3 V
   4. About 0.7 V

   Explanation: Germanium diodes generally have a lower threshold voltage, around 0.3 V, compared to silicon diodes (roughly 0.7 V). Option 0.7 V is correct for silicon, not germanium. Higher values like 1.2 V or 2.2 V are incorrect, as they're much above typical diode threshold values and could result in damage before conduction starts. The correct answer distinguishes germanium from other commonly used materials.

4. Diode Application Scenario

   In which of the following electronic circuits is a standard PN junction diode commonly used to allow current flow only during the positive half-cycles of an AC signal?

   1. LED indicator circuit
   2. Half-wave rectifier
   3. LC oscillator circuit
   4. Voltage regulator

   Explanation: A half-wave rectifier uses a PN junction diode to convert AC into pulsating DC by blocking current during negative half-cycles and allowing it during positive ones. An LC oscillator circuit uses inductors and capacitors to generate oscillations, not for rectification. A voltage regulator is designed for maintaining constant voltage and generally involves other components. An LED indicator emits light rather than controlling AC direction.

5. Reverse Saturation Current Nature

   If a small reverse bias is applied to a PN junction diode and temperature increases, what happens to the reverse saturation current primarily due to thermally generated carriers?

   1. It increases exponentially.
   2. It decreases gradually.
   3. It instantly drops to zero.
   4. It remains the same.

   Explanation: Reverse saturation current is dominated by thermally generated minority carriers; as temperature rises, the number of such carriers increases exponentially, causing a significant rise in current. A decrease does not occur; higher heat means more carriers. The current doesn't remain constant because thermal effects become pronounced. Instant drop to zero is impossible unless the diode is disconnected.