Why does the current in a diode flow in only one direction?

Why does the current in a diode flow in only one direction?

Why does the current in a diode flow in only one direction? In response to this problem, the editor summarized some problems, and made the following circuit diagram for analysis:

As shown in Figure 1, the P-type carriers and N-type carriers are randomly in the crystal when no electric field (voltage) is applied. If a positive voltage is applied to the N terminal in Figure 2, and a negative voltage is applied to the P terminal, the internal carriers, electrons are pulled to the positive voltage side, and the empty core is pulled to the negative voltage side, thereby combining the carriers on the surface The number is greatly reduced, and the resistance is increased. Apply the opposite voltage as shown in Figure 3, at this time, the internal carriers become easy to flow through the junction surface. In other words, the resistance becomes smaller and the current flows in the forward direction.

Please remember: the forward conduction of a  diode flow  is from P-type to N-type. The international standard is: a triangle indicates P-type, and a horizontal line indicates N-type. The current of the  diode can move sharply at a voltage above 0.6V, but nothing in the reverse direction!