How does the depletion region of a $p-n$ junction diode get affected under reverse bias?

Why does the base region of a transistor is thin and lightly doped ?

The output of an unregulated dc power supply needs to be regulated. Name the device that can be used for this purpose and draw the relevant circuit diagram.

Two semiconductor materials $X$ and $\underset{¯}{Y}$ shown in Fig. $21.80$ are made by doping germanium crystal with indium and arsenic respectively. The two are joined end to end and connected to a battery as shown.

Sketch a $\mathrm{V}$-$\mathrm{I}$ graph for this arrangement. 

What do the terms ‘depletion region’ and ‘barrier potential' mean for a $p-n$ junction?

In a transistor, the doping level in the base is increased slightly. How will it affect

$\left(\mathrm{i}\right)$ collector current and 

In a transistor, doping level in base is increased slightly. How will it affect

$\left(\mathrm{ii}\right)$ base current.

What happens to the width of depletion layer of a $p-n$ junction when it is

(i) forward biased 

What happens to the width of depletion layer of a $p-n$ junction when it is

 (ii) reverse biased?