Can the potential barrier across a $p-n$ junction be measured by simply connecting a voltmeter across the junction?

In a $P-N$ junction,avalanche current flows in the circuit when the biasing is:

In the given circuit,the voltage across the $P-N$ junction diode remains constant at $0.5 \ V$ regardless of the current. The maximum power rating of the diode is $100 \ mW$. What should be the value of $R$ in $\Omega$ to obtain the maximum current in the circuit?

In a $P-N$ junction,which of the following statements is correct regarding the potential?

$A$ junction diode has a resistance of $50 \, \Omega$ when forward biased and $5000 \, \Omega$ when reverse biased. The current in the arrangement shown in the figure will be

Assuming that the junction diode is ideal, the current in the arrangement shown in the figure is: (in $\text{mA}$)