why pn junction turn ON for lower voltages at higher temperatures?
In a cold semiconductor below the knee voltage electrons from the n side haven’t enough energy to enter the p side and, likewise, holes from the p side haven’t enough energy to enter the n side. When you heat it, a large fraction of charge carriers (on one of these sides, or both) acquire necessary energy. The Fermi–Dirac distribution can provide an approximation, although forward-biased junction is a strongly non-equilibrium system.

In terms of the Shockley diode equation, it is described as “the saturation current \$I_{\mathrm S}\$ increases with \$T\$ faster than \$\exp(V_{\mathrm D} / n V_T)\$ decreases”.