What is Carrier Lifetime (Part 2 of 10)

Carrier Lifetime is a key parameter in semiconductor physics, used to describe the average time that non-equilibrium carriers (electrons or holes) survive in a material before recombination. Its value directly reflects the quality and purity of the semiconductor material, as well as the potential performance of devices. Below is a detailed explanation:

1. Basic Definition

Carriers:
Conductive particles in semiconductors, including electrons (negative charge) and holes (positive charge). When excited by light, electricity, or heat, electrons transition from the valence band to the conduction band, generating electron-hole pairs (i.e., non-equilibrium carriers).

Carrier Lifetime:
The average time from when these non-equilibrium carriers are generated until they recombine (electrons filling holes), measured in microseconds (μs) or milliseconds (ms). The longer the lifetime, the higher the typical material quality.

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2. Why is it Important?

Semiconductor Device Performance:

• Solar Cells: The longer the carrier lifetime, the more opportunities photogenerated electron-hole pairs have to be collected by electrodes, improving conversion efficiency.
• Power Devices (e.g., IGBT, SiC MOSFET): A higher lifetime reduces switching losses and improves voltage withstand capability.
• Sensors/Detectors: Influences response speed and signal-to-noise ratio.

Process Monitoring:
A decrease in lifetime may indicate material contamination (such as metal impurities), crystal defects, or process damage (such as excessive ion implantation).

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3. Factors Affecting Carrier Lifetime

(1) Intrinsic Material Properties

• Bandgap Width (Eg): Wide-bandgap materials (e.g., SiC, GaN) generally have shorter carrier lifetimes (nanoseconds), whereas silicon (Si) can reach milliseconds.
• Crystal Quality: Single-crystal silicon has a much longer lifetime than polycrystalline silicon (due to grain boundary recombination).

(2) Impurities and Defects

• Metal Impurities (Fe, Cu, etc.): Create recombination centers and accelerate carrier recombination.
  Example: In silicon, just 1 ppb (one part per billion) of iron impurity can reduce the lifetime from 1000 μs to 10 μs.
• Dislocations/Vacancies: Crystal defects capture carriers, shortening their lifetime.

(3) Surface and Interface

• Surface Recombination: Unpassivated silicon wafer surfaces contain dangling bonds that serve as recombination centers (can be suppressed using SiNx/Al₂O₃ passivation layers).
• Oxide Layer Charge: SiO₂/Si interface charges increase interface recombination rates.

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4. Measurement Methods

Method	Principle	Application Scenario
μ-PCD	Microwave-detected photoconductivity decay	Rapid online testing (solar silicon wafers)
QSSPC	Quasi-steady-state photoconductance measuring minority carrier diffusion length	High-precision laboratory measurement
PL (Photoluminescence)	Infers lifetime from photon intensity emitted during carrier recombination	Non-contact, suitable for thin-film materials
TRPL (Time-Resolved PL)	Measures fluorescence decay time to directly obtain lifetime	For direct bandgap semiconductors (e.g., GaAs)

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5. Practical Case: How Quartz Tubes Affect Carrier Lifetime

• Contamination Transfer: At high temperatures, Na⁺ from the quartz tube can diffuse into silicon wafers, forming recombination centers → reduced lifetime.
• Crystallization Particles: Devitrification (formation of cristobalite) in quartz tubes can cause particles to detach and adhere to wafer surfaces → increased surface recombination rate.

Solution: Use ultra-high-purity synthetic quartz tubes (metal impurities <0.1 ppm) and control process temperatures.

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6. Typical Industry Reference Values

• Photovoltaic-grade silicon wafers: >100 μs (high-efficiency PERC cells require >500 μs).
• Semiconductor-grade silicon: >1 ms (high-resistivity silicon for integrated circuits).
• SiC epitaxial layers: ~0.1–1 μs (faster recombination due to wide-bandgap nature).

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Summary

Carrier lifetime is the “health indicator” of semiconductor materials. Its value is jointly influenced by the base material, impurities, interfaces, and process environment. By optimizing the purity of quartz tubes, flange sealing quality, and other peripheral components, this parameter can be indirectly preserved, thereby enhancing device performance.