(a) (i) Draw the circuit diagram used to study $I - V$ characteristics of a p-n junction diode in conducting mode. Mark on the graph the threshold voltage of the diode. Explain the significance of this voltage. (ii) In the circuit shown in the figure, the forward voltage drop across the diode is $0\cdot3 ext{ V}$. Find the voltage difference between A and B. OR (b) (i) Briefly describe the classification of solids into metals, insulators and semi-conductors on the basis of energy level diagrams. (ii) In a silicon diode, the current increases from $10 ext{ mA}$ to $20 ext{ mA}$ when the voltage changes from $0\cdot6 ext{ V}$ to $0\cdot7 ext{ V}$. Calculate the dynamic resistance of the diode.