Why A PN junction has to be forward biased past its built-in potential before it conducts appreciably. For silicon that knee is right around 0.7 volts, so a conducting silicon diode drops roughly 0.7 V no matter how much current flows. That is why a silicon diode bridge loses about 1.4 V total (two diodes in series conduct on each half cycle).
Watch out The 0.3 volt figure is the threshold for a germanium junction, which is why germanium diodes are preferred in low-level detector circuits where a small signal must still turn the diode on.
Silicon = 0.7 V, germanium = 0.3 V. Si has more letters and a bigger drop.