Why does a PN-junction diode not conduct current when reverse biased?

A PN-junction diode consists of P-type semiconductor material (which has an excess of positive charge carriers called holes) and N-type material (which has an excess of negative electrons). The boundary between them is called the depletion region, where mobile charge carriers have neutralized each other. When a diode is reverse biased, the positive terminal of the voltage source is connected to the N-type material, and the negative terminal is connected to the P-type material. This polarity attracts the electrons in the N-type material and the holes in the P-type material away from the junction. As these charge carriers are pulled apart by the applied voltage, the depletion region widens. A wider depletion region lacks free charge carriers, meaning it acts as an insulator and effectively stops current from flowing across the junction.