Three photo diodes $D_1, D_2$ and $D_3$ are made of semiconductor having band gaps of $2.5\mathrm{eV}, 2\mathrm{eV}$ and $3\mathrm{eV}$, respectively. Which ones will be able to detect light of wavelength $60000 \stackrel{\mathrm{o}}{\mathrm{A}}$?

If the resistance ${\mathrm{R}}_i$ is increased in the given figure, how will the readings of the ammeter and voltmeter change?

Explain why elemental semiconductor cannot be used to make visible $\mathrm{LEDs}$.

Write the truth table for the circuit shown in figure. Name the gate that the circuit resembles.

A Zener of power rating $1 \mathrm{W}$ is to be used as a voltage regulator. If zener has a breakdown of $5\mathrm{V}$ and it has to regulate voltage which fluctuated between $3\mathrm{V}$ and $7\mathrm{V}$, what should be the value of $R_s$ for safe operation ?

If each diode in given figure has a forward bias resistance of $25\Omega$ and infinite resistance in reverse bias, what will be the values of the current $I_1, I_2, I_3$ and $I_4$?

In the circuit shown in figure, when the input voltage of the base resistance is $10 \mathrm{V}$, $V_{BE}$ is zero and $V_{CE}$ is also zero. Find the values of $I_B, I_C$ and $\beta$.