Why must the forward bias voltage across a pn junction diode always be less than the built-in potential barrier? What happens when you apply a forward bias voltage across a pn junction diode that is larger than the built-in potential barrier? Where does the rest of the voltage go?
If you attempt to apply a forward bias voltage larger than the diode's "natural" forward voltage, the diode will draw more current in an attempt to bring the applied voltage down to its "natural" voltage. If the power supply is sufficiently "stiff" (maintains its output voltage regardless of current), the diode will release its magic smoke, and become an open circuit.