Types of Conductivity in Semiconductors: Drift Current and Diffusion Current

Semiconductors are the backbone of modern electronics, enabling the operation of devices like transistors, diodes, and integrated circuits. One of the most critical aspects of semiconductors is how they conduct electricity, which primarily depends on the movement of charge carriers electrons and holes.

In semiconductor materials, electrical conduction occurs through two key mechanisms:

1. Drift current
2. Diffusion current

Understanding these types of conductivity is essential for grasping the working principles of electronic components.

Drift current refers to the flow of charge carriers (electrons and holes) in response to an externally applied electric field across a semiconductor.

When a voltage is applied across a semiconductor, an electric field is established. This field exerts a force on the free electrons in the conduction band, causing them to accelerate toward the positive terminal. Simultaneously, holes in the valence band move toward the negative terminal.

At ambient conditions (around 25 °C), this drift of charge carriers results in an electric current. Although the motion of electrons is scattered due to collisions with atoms and other carriers, there is a net movement in the direction of the field, producing a drift current. This phenomenon is observed in both conductors and semiconductors.

The velocity of the carriers under the influence of the electric field is expressed as:

v_n=−μ_nE

v_p=μ_pE

Where:

• v_n​ = drift velocity of electrons in metre/second
• v_p= drift velocity of holes in metre/second
• μ_n = mobilities of electrons and holes, respectively metre²/volts·second
• E = electric field intensity in volts per metre

In essence, higher mobility and stronger electric fields result in faster carrier movement, increasing the drift current.

Diffusion current arises from the non-uniform distribution of charge carriers (electrons or holes) in a semiconductor.

If one part of a semiconductor has a higher carrier concentration than another, carriers will naturally move from the high-concentration region to the low-concentration region. This movement is not driven by an electric field but by the carrier concentration gradient a purely thermodynamic process.

• Electron and hole pairing occurs when carriers move between regions of differing carrier densities.
• Repulsive forces between like charges (e.g., electrons repelling other electrons) also drive diffusion.

This spontaneous movement of carriers generates a diffusion current, which is especially important in PN junctions and semiconductor diodes.

Both drift and diffusion currents play vital roles in determining how a semiconductor conducts electricity:

• Drift current is influenced by an applied electric field.
• Diffusion current arises from carrier concentration gradients.

In practical semiconductor devices, both drift current and diffusion current mechanisms often act simultaneously, and understanding their interaction is crucial for designing efficient electronic circuits.