Question

After a short time, an equilibrium is reached between the diffusion currents (also called recombination currents) and a second, opposing type of current. What causes these secondary currents?

	A)The diffusion currents cause buildup of positive charge in the n-type material and negative charge in the p-type material. This makes an electric field.
	B)The diffusion currents cause buildup of negative charge in the n-type material and positive charge in the p-type material. This makes an electric field.
C)	Minority carriers diffuse from each material to the opposite material.
D)	An excess of holes builds up on the n-type side and causes a reverse diffusion; similarly electrons build up in the p-type material.

Answer 1

The secondary currents are called generation currents or drift currents. The charge that moves in the generation currents are electrons and holes simultaneously generated in the junction zone by themal excitation. In this phenomenon, the temperature measures average kinetic energy. Even at room temperature, it is possible for an electron to gain enough energy to cross the band gap of the semiconductor. When this phenomenon takes place, a conduction electron abruptly becomes available as does a hole. The electric field set up by the diffusion currents then sweep the electron and hole out of the junction zone to then n - side and p - side, respectively. The diffusion currents cause buildup of positive charge in the n-type material and negative charge in the p-type material. This makes an electric field. Therefore, the correct response is option (A).