Drift current in germanium is caused by the applied electric field.

Explanation:
The drift current in a semiconductor like germanium is a result of the movement of charge carriers in response to an applied electric field. This electric field can be created by an external power source or by the presence of a voltage across the semiconductor material.

1. What is drift current?
Drift current refers to the movement of charge carriers, such as electrons or holes, in a semiconductor material due to an applied electric field. It is called drift current because the charge carriers "drift" in response to the electric field.

2. Role of electric field:
The electric field exerts a force on the charge carriers, causing them to move in a particular direction. In the case of germanium, both electrons and holes can contribute to the drift current.

3. Movement of electrons:
In germanium, which is an intrinsic semiconductor, thermal agitation of the crystal lattice can generate free electrons in the conduction band. When an electric field is applied, these free electrons experience a force in the direction opposite to the electric field and move towards the positive terminal of the applied voltage source.

4. Movement of holes:
Similarly, in germanium, the thermal agitation of the crystal lattice can create "holes" in the valence band. These holes can be thought of as the absence of an electron in a particular position. When an electric field is applied, the holes experience a force in the direction of the electric field and move towards the negative terminal of the applied voltage source.

5. Combined effect:
The combined movement of both electrons and holes contributes to the drift current. The net result is a current flow in the direction of the applied electric field.

6. Other factors:
While thermal agitation of the crystal lattice does play a role in the generation of charge carriers, it is not the primary cause of the drift current in germanium. The concentration gradient of charge carriers also does not directly cause the drift current. Instead, it is the applied electric field that provides the driving force for the movement of charge carriers.

In conclusion, the drift current in germanium is primarily caused by the applied electric field. The electric field exerts a force on the charge carriers, causing them to move and contribute to the current flow.