What is the order of resistance of a forward-biased junction diode and a reverse-biased junction diode?

In which of the solids (semiconductors, conductors or insulators) do conduction band and valence band overlap?

How does the thickness of the depletion layer in a $p$-$n$ diode vary with increase in reverse bias? With increase in forward bias?

Can we measure the potential difference across an unbiased $p$-$n$ junction by connecting a sensitive voltmeter across its terminals?