How recombination process produces light in the case of Light emitting diodes (LEDs)?

The value of the resistor, $R_S$, needed in the DC voltage regulator circuit shown here, equals:

$Ge$ and $Si$ diodes start conducting at $0.3 V$ and $0.7 V$ respectively. In the following figure if $Ge$ diode connection are reversed, the value of $V_0$ changes by: (assume that the Ge diode has large breakdown voltage)

LED emits light when connected in _____.

(Fill in the blank using 'forward bias' or 'reverse bias')

Identify the semiconductor devices whose characteristics are given below, in the order $\left(\mathrm{a}\right),\mathrm{ }\left(\mathrm{b}\right),\mathrm{ }\left(\mathrm{c}\right),\mathrm{ }\left(\mathrm{d}\right)$:

Take the breakdown voltage of the zener diode used in the given circuit as $6 \mathrm{V}$. For the input voltage shown in the figure below, the time variation of the output voltage is: (Graphs drawn are schematic and not to the scale)

Two Zener diodes ($A$ and $B$) having breakdown voltages of $6 \mathrm{V}$ and  $4 \mathrm{V}$ respectively, are connected as shown in the circuit below. The output voltage $V_0$ variation with input voltage linearly increasing with time, is given by ($V_{input} = 0 \mathrm{V} \text{at} t = 0$)