Step 1: Concept Overview:
The question identifies which type of current is not present in a uniformly doped semiconductor under thermal equilibrium (without external fields or gradients).
Step 2: Detailed Explanation:
Semiconductor current flow occurs primarily through two mechanisms:
Drift Current: This current is due to the movement of charge carriers (electrons and holes) caused by an applied electric field. Without an electric field, there is no net drift, and the drift current is zero.
Diffusion Current: This current results from the movement of charge carriers from areas of high concentration to areas of low concentration, driven by a concentration gradient (\(dn/dx\) or \(dp/dx\)).
The problem specifies a uniformly doped semiconductor. This implies a consistent dopant concentration (and thus majority carrier concentration) throughout the material. At thermal equilibrium, this leads to a uniform carrier concentration.
Because there is no concentration gradient (\(dn/dx = 0\) and \(dp/dx = 0\)), there's no driving force for diffusion. Therefore, the diffusion current is zero in a uniformly doped semiconductor at equilibrium.
Options A and D represent types of electrical current in a general sense but are not fundamental transport mechanisms within the semiconductor.
Step 3: Conclusion:
Diffusion current relies on a carrier concentration gradient, which is absent in a uniformly doped semiconductor at equilibrium.