The electronic band structure is dependent on the static electric bias voltage, which determines the electron and hole current densities?

I am currently studying the textbook Physics of Photonic Devices, second edition by Shun Lien Chuang. Chapter 1.4 Overview of the Book says the following:

In the presence of injection of electrons and holes using a current bias or an optical source, the semiconductor materials may change from being absorptive to gain media due to population inversion effect. This implies that the optical dielectric function is also changed. This change can be modeled with the knowledge of the electronic band structures, which require the solutions of the Schrödinger equation or the so-called effective-mass equation for the given bulk or quantum-well semiconductors. By solving Maxwell’s equations, we obtain the optical field from the dielectric function of the semiconductors. The electronic band structure is also dependent on the static electric bias voltage, which determines the electron and hole current densities.

I didn’t understand this part:

The electronic band structure is also dependent on the static electric bias voltage, which determines the electron and hole current densities.

What is meant by this? What is the "static electric bias voltage", and how does it determine the electron and hole current densities?