Transferred-Electron (Gunn) Effect: Direction of Electron Transfer In a Gunn diode operating above threshold field, electrons transfer between energy valleys in the conduction band. From which type of valley to which type do they predominantly transfer?

Introduction / Context:
Gunn diodes are transferred-electron devices. Their negative differential mobility arises from field-induced transfer of carriers among multiple conduction-band valleys in certain semiconductors (e.g., GaAs).

Given Data / Assumptions:

• Material with at least one low-energy central valley and higher-energy satellite valleys.
• Electric field exceeds a threshold so intervalley transfer occurs.
• Mobility in satellite valleys is lower due to higher effective mass.

Concept / Approach:

At low fields, electrons occupy the central valley and exhibit high mobility. At high fields, a significant fraction transfers to satellite valleys with lower mobility. Net drift velocity can then decrease with increasing field, producing negative differential resistance and enabling oscillation.

Step-by-Step Solution:

Verification / Alternative check:

Velocity–field characteristics in GaAs show transfer at fields of the order of a few kV/cm, aligning with Gunn effect theory.

Why Other Options Are Wrong:

Option B reverses the physics; options C and D confuse band transport with device-level domain terminology; option E describes tunneling (tunnel diode), not the Gunn effect.

Common Pitfalls:

Assuming a pn junction is required; Gunn diodes are bulk devices without a junction.

Final Answer:

from high-mobility central valley to low-mobility satellite valley