Question

At very high electric fields in silicon, the drift velocity of electrons becomes independent of the electric field. This phenomenon is known as:

• A. Velocity Saturation
• B. Avalanche Breakdown
• C. Negative Differential Resistance
• D. High-field Ionization

Hint:

At high electric fields, carrier velocity stops increasing linearly and levels off. This limit is called Velocity Saturation (\(v_{\text{sat}} \approx 10^7\text{ cm/s}\) in Si).

Answer 1

The Correct Option is A

Understanding the Concept: The drift velocity (\(v_d\)) of charge carriers within a semiconductor material normally follows a linear relationship with the applied internal electric field (\(E\)) under low to moderate field intensities. This relationship is defined by: \[ v_d = \mu E \] Where \(\mu\) represents the low-field carrier mobility. However, as the electric field intensity escalates to high levels (approaching and exceeding roughly \(10^4\text{ V/cm}\) in silicon), the carriers acquire significant kinetic energy between scattering events.

Step 1: Explaining high-field carrier scattering dynamics.
At these high energy levels, the carrier scattering rate with the crystal lattice increases dramatically. Specifically, electrons begin interacting heavily through optical phonon emission, which efficiently dissipates their excess kinetic energy. This extra energy loss counteracts any further acceleration that would be provided by an increasing electric field.

Step 2: Defining the plateau limit.
Because of this balanced energy exchange mechanism, further increases in the electric field fail to increase the average drift velocity. The drift velocity levels off and saturates at a maximum value known as the saturation velocity (\(v_{\text{sat}} \approx 10^7\text{ cm/s}\) for silicon). This behavior is called Velocity Saturation, which corresponds exactly to Option (A).