a) During forward biasing of p n junctions diode, the width of depletion layer decreases. Justify. [2] b) Write the truth table for the combination of gates given in the figure below. [2] [figure in…

a) Justification for depletion layer width decrease under forward bias

Under forward bias, the positive terminal of the battery is connected to the p-side and the negative terminal to the n-side. This reduces the potential barrier at the junction by applying an external electric field opposite to the built-in field. As a result:

• Majority carriers (holes in p-side, electrons in n-side) are pushed toward the junction by the external field.
• The recombination rate of electrons and holes increases near the junction, reducing the space-charge region (depletion layer).
• The width of the depletion layer decreases because the depletion region is formed by the diffusion of majority carriers, which is now enhanced by the forward bias.

Mathematically, the depletion width $W$ under bias is given by: $$W=W_0\sqrt{1-\frac{V}{V_0}}$$ where $W_0$ is the width at zero bias, $V$ is the applied forward voltage, and $V_0$ is the built-in potential (~0.7 V for Si). As $V$ increases, $W$ decreases.

b) Truth table for the given gate combination

Since the original figure is not provided, we assume the combination is a NAND gate followed by a NOT gate (a common configuration yielding a NOR gate). The figure would look like this:

graph LR
  A["Input A"] --> B["NAND gate"]
  C["Input B"] --> B
  B --> D["NOT gate"]
  D --> E["Output Y"]

Truth table:

c) Identified gate

The combination of a NAND gate followed by a NOT gate is equivalent to a NOR gate. This is because: $$\text{NOT}(\text{NAND}(A,B))=\overline{\overline{A\cdot B}}=A+B$$ which is the definition of a NOR gate.

Note: If the original figure was different (e.g., an AND-OR combination), the truth table and gate would change accordingly. Always verify the figure in the exam paper. For this model answer, we assumed a NAND-NOT configuration.