$A$ Zener diode acts as a/an

Which of the following semi-conducting devices is used as a voltage regulator?

An $8\,V$ Zener diode along with a series resistance $R$ is connected across a $20\,V$ supply as shown in the figure. If the maximum Zener current is $25\,mA$ and the load current is negligible, then the minimum value of $R$ will be $\Omega$.

In the following circuit,the reading of the ammeter will be (Take Zener breakdown voltage $= 4 \ V$) (in $mA$)

An ideal Zener diode with a breakdown voltage of $3 \text{ V}$ is reverse-biased with an input voltage $V_i = 5 \text{ V}$. The magnitude of the voltage difference between points $B$ and $A$ is: (in $\text{ V}$)

If the current in the Zener diode is five times the current in $R_1$ and the breakdown voltage of the diode is $6 \text{ V}$,find the value of $R$.