In a silicon diode, that 0.7V is needed to overcome the built-in potential of the p-n junction, allowing charge carriers to flood across. In reverse bias, you're just widening that depletion region, so ideally, no current flows. Of course, until you blow it!
But you see, I learned this not just in a textbook, but while fixing my old transistor radio under the dim light of my chai stall, trying to save every rupee. That "ideal" 0.7V? In the real heat and dust, with components bought from the weekly haat, it's never that neat. The theory is perfect, but the zameen, my friend, is always a little different.
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