In semiconductor physics, how many distinct types of charge flow contribute to conduction inside a semiconductor crystal?

Introduction / Context:
Conduction in semiconductors differs from metals because two kinds of mobile charge carriers participate. Recognizing both carrier types is critical for understanding p–n junctions, bipolar transistors, and diode I–V behavior.

Given Data / Assumptions:

• Material may be intrinsic or doped.
• We consider standard band theory without exotic effects.
• Thermal equilibrium or steady-state bias with carriers present.

Concept / Approach:
There are two carrier types: electrons in the conduction band and holes in the valence band. Electrons carry negative charge, while holes act as positive charge carriers representing the absence of an electron. Both contribute to current flow under electric fields and concentration gradients, leading to drift and diffusion components in device equations.

Step-by-Step Solution:
1) Identify available mobile carriers: electrons and holes.2) Note that both can move under an applied electric field, producing current.3) Conclude there are two types of flow contributing to conduction.

Verification / Alternative check:
In p–n junction forward bias, both electron injection into p-side and hole injection into n-side produce the total current, directly demonstrating two-carrier conduction.

Why Other Options Are Wrong:

• 0 or 1: Metals have effectively one type of carrier; semiconductors typically have two.
• 3: There are not three fundamental carrier types in standard semiconductor physics.
• None of the above: Incorrect because the correct count is two.

Common Pitfalls:
Assuming holes are merely mathematical is misleading; hole motion and recombination are physically observable through device behavior and measured currents.

Final Answer:
2.