When the Light Angle of a directional light is set to a high number, shadow banding occurs.
Avoid a Light Angle of higher than 10 degrees. The Light Angle fakes a light source with a certain shape by bending shadow ray directions, thus promoting the common shadow raytracing artifacts with coarse models.
Alternatively, you can avoid this problem by using a "shape" area light (e.g. rectangle, sphere) instead of a light angle on a directional light.
