Current Conduction in Semiconductors: Drift and Diffusion Explained

Current conduction in semiconductors, unlike in metals, is influenced not only by electric fields but also by how charge carriers—electrons and holes—are distributed spatially.

Two primary mechanisms govern this conduction:

  • ✅ Drift Current – Carrier motion under the influence of an electric field

  • ✅ Diffusion Current – Carrier motion due to concentration gradients

These two mechanisms are fundamental in the operation of key electronic components such as diodes, transistors, and solar cells.

1. Drift Current – Motion Under an Electric Field

When an electric field (E) is applied across a semiconductor, it exerts a force on mobile charge carriers—electrons and holes. This causes them to accelerate and generate a net flow of charge, which we call drift current.

Drift Current Formula:

Where:

  • q = charge of an electron (1.6 × 10⁻¹⁹ C)

  • n = carrier concentration (per m³)

  • ΞΌ = mobility of carriers (m²/V·s)

  • E = applied electric field (V/m)

What Happens Without an External Field?

Even in a pure (intrinsic) semiconductor:

  • Thermal energy generates electron-hole pairs.

  • Each pair behaves like a tiny electric dipole, with electron and hole slightly separated.

  • These dipoles are randomly oriented in all directions.

Key Insight:

Because the dipoles are randomly aligned, their effects cancel each other, and no net drift current exists. The system remains electrically neutral and isotropic.

What If an Electric Field Is Applied?

When a uniform electric field is applied:

  • Dipoles begin to align with the field.

  • Electrons drift opposite to the field direction.

  • Holes drift along the field direction.

This alignment creates asymmetric carrier motion, breaking the previous randomness and resulting in a net current, called drift current.

What If the Field Is Non-Uniform?

If the electric field varies with position (as in a pn-junction or graded semiconductor):

  • Carrier drift velocity also changes across the material.

  • Regions with stronger fields produce more acceleration.

  • This causes the carrier distribution to become position-dependent, impacting both drift and diffusion currents.

Such scenarios require solving:

  • Poisson’s Equation

  • Continuity Equations

These are essential tools in semiconductor device simulation.

Drift Current and Velocity Saturation

At low electric fields:

  • Carrier velocity increases linearly with the field (Ohm’s Law region).

At high electric fields:

  • Carriers undergo increased scattering with lattice vibrations (phonons).

  • Eventually, velocity reaches a maximum limit known as saturation velocity (V_sat).

This sets a fundamental speed limit in high-speed devices like MOSFETs and CMOS inverters.

2. Diffusion Current – Carrier Flow Due to Concentration Gradient

Even without an external electric field, current can still arise due to non-uniform carrier concentration. This is known as diffusion.

Origin of Diffusion Current

Imagine two regions:

  • Region A: High electron concentration (n₁)

  • Region B: Low electron concentration (n₂)

Electrons naturally move from A to B to equalize the imbalance. Since electrons are negatively charged, their movement from A to B causes current in the opposite direction (B to A).

Similarly, holes diffuse from regions of high hole concentration to regions of low hole concentration—and hole current follows the same direction as hole movement.

Electron Motion vs Electron Current


This opposite behavior between electrons and holes is a key aspect of semiconductor operation.

Real Devices Combine Both Mechanisms

In practical semiconductor devices like diodes, transistors, and solar cells:

Both drift and diffusion currents occur simultaneously.

Example: PN-Junction Diode

Why This Matters

A clear understanding of drift and diffusion currents is critical to mastering electronic device physics. These two currents are responsible for how semiconductors behave under varying conditions and form the backbone of modern electronics.

Summary

  • Drift Current: Carrier movement due to electric fields.

  • Diffusion Current: Carrier movement due to concentration gradients.

  • Both are present in all real devices.

  • Essential for understanding diodes, MOSFETs, BJTs, and photovoltaic cells.

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