10.5
Carrier Transport
The generation of electrical current in semiconductors is fundamentally driven by two mechanisms: drift and diffusion. These processes are essential f…
Carrier transport in semiconductors generates current, through drift and diffusion mechanisms.
Drift current arises when an external electric field causes charged particles to accelerate between collisions.
Considering the electrons, the additional velocity added to the electrons' thermal motion is called the drift velocity. It can be obtained by equating the momentum lost during collisions and the momentum gained from the applied field.
The proportionality factor-mobility- characterizes electron and hole transport due to drift.
The drift current densities for both carriers can be expressed as the product of charge density, mobility, and electric field intensity. The sum of these two gives the total drift current density. Here, the term in parentheses gives conductivity.
Diffusion current results from the random thermal motion of carriers from high to low carrier density regions.
The diffusion current density equals the product of carrier flux and proportionality constant, which includes elementary charge and diffusion coefficient. Its ratio with mobility gives the Einstein relation.
If both an electric field and concentration gradient are present, both the drift and diffusion current contribute to the total current.
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Q1: What is drift current in semiconductors?
Drift current arises when an external electric field causes charged particles to accelerate between collisions with lattice atoms. Electrons and holes gain drift velocity proportional to the applied field intensity and their mobility. The drift current density equals the product of charge density, mobility, and electric field intensity for each carrier type.
Q2: How does mobility relate to drift velocity?
Mobility is the proportionality factor that characterizes how easily electrons and holes respond to an applied electric field. Drift velocity is directly proportional to mobility and electric field intensity. Higher mobility means carriers accelerate more readily, resulting in greater drift velocity and increased current density in the semiconductor.
Q3: What causes diffusion current in semiconductors?
Diffusion current results from the random thermal motion of carriers moving from regions of high concentration to regions of low concentration. This concentration gradient drives carriers without requiring an external electric field. The diffusion current density depends on the diffusion coefficient and the concentration gradient of electrons and holes.
Q4: What is the Einstein relation in semiconductor physics?
The Einstein relation links the diffusion coefficient and mobility for both electrons and holes through fundamental constants. It shows that the ratio of diffusion coefficient to mobility equals the thermal voltage, which depends on Boltzmann's constant and absolute temperature. This relationship connects drift and diffusion transport mechanisms in semiconductors.
Q5: How is total conductivity determined in semiconductors?
Total conductivity is the sum of contributions from both electrons and holes, calculated as the product of charge density and mobility for each carrier type. The conductivity term appears in the total drift current density expression. This combined conductivity determines how readily a semiconductor conducts current under an applied electric field.
Q6: What happens when both electric field and concentration gradient are present?
When both an external electric field and a concentration gradient exist simultaneously, the total current density is the sum of drift and diffusion components. Drift current dominates in regions with strong electric fields, while diffusion current dominates where concentration gradients are steep. Understanding both mechanisms is essential for analyzing carrier generation and recombination processes.
Q7: Why is drift velocity important for semiconductor current flow?
Drift velocity represents the net motion of charge carriers superimposed on their random thermal motion due to an applied electric field. It directly determines the magnitude of drift current density through its relationship with mobility and field intensity. Calculating drift velocity is fundamental to predicting current behavior in semiconductor devices and understanding device performance.