Working Of a Tunnel Diode

Question

According to Wikipedia,

The reason that a tunnel diode conducts current in reverse direction is :

1)In the tunnel diode, the dopant concentrations in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction.

2)Quantum Tunneling : Under reverse bias, filled states on the p-side become increasingly aligned with empty states on the n-side and electrons now tunnel through the pn junction barrier in reverse direction.

Which is the more appropriate and more dominant reason, i.e. which effect contributes more current ?

Answer 1

According to this lecture in a reverse-biased tunnel diode the current is due to the alignment of conduction band in the p-region and the valence band in the n-region. Because of that, the electrons in the valence band of the p-region are tunneled to the holes in the valence band of the n-region.

Since this current has the same characteristic of the zener diode, it's considered as a zener with zero breakdown voltage. So the physical reason is the 2) but the alignment of the bands is due to the dopant concentration, as you say in 1).