Charge Carrier Concentration Measurement (THz)

Charge carrier concentration measurement with terahertz technologies

Non-contact semiconductor characterization for the next generation of power electronics

As semiconductor devices become smaller, faster, and more powerful, accurate knowledge of material properties such as charge carrier concentration and mobility becomes critical. These parameters define the performance, efficiency, and reliability of power devices in electric vehicles, renewable energy converters, and high-voltage electronics.

Traditional methods such as mercury capacitance-voltage (mCV) or four-point probe techniques require physical contact with the sample. They are slow, often destructive, and susceptible to contamination, an issue that is especially problematic for high-purity materials like silicon carbide (SiC) or gallium nitride (GaN).

Terahertz time-domain spectroscopy (THz-TDS) changes the game. It enables contact-free, fast, and spatially resolved measurement of charge carrier density and mobility directly on wafers, under ambient conditions, without any electrodes or surface preparation.

Principle: THz time-domain spectroscopy in reflection geometry

THz-TDS measures how broadband terahertz pulses interact with a semiconductor. When a THz pulse strikes the surface, a part of the pulse reflects from each internal interface and is modified by the free carriers within the material.

From these reflected signals, both the amplitude and phase can be analyzed to extract electrical conductivity, plasma frequency, and ultimately charge carrier concentration (N). The underlying model follows Drude’s theory, linking the material’s dielectric response to the density and mobility of charge carriers.

By comparing the measured and simulated reflection waveforms, the charge carrier concentrations of both the epilayer and the buffer can be determined simultaneously. This unique capability enables full-stack characterization of semiconductor wafers such as 4H-SiC, even when the epilayers differ by three orders of magnitude in their doping concentration (J. Hennig et al., Opt. Express 33:22 (2025) 45828).

Why THz? Contact-free, fast, and quantitative

Unlike traditional electrical methods, THz spectroscopy works without electrodes or chemical/physical contact. The sample remains pristine, avoiding contaminations or surface damage.
Even more importantly, THz-TDS provides depth-sensitive information, being able to probe through multilayer stacks that are opaque to visible and infrared light.

Key benefits:

  • Contactless measurement: No physical contact, no chemical residue, no sample preparation.
  • Simultaneous layer evaluation: Access both epilayer and substrate properties in one step.
  • Wide dynamic range: Detects carrier concentrations across several orders of magnitude.
  • High spatial resolution: Enables full wafer mapping with sub-millimeter precision.
  • Non-destructive: Suitable for production-line quality control and research metrology alike.

High-speed ECOPS: Enabling industrial throughput

In industrial semiconductor environments, measurement speed and robustness are as crucial as accuracy. This is where Electronically Controlled Optical Sampling (ECOPS) has revolutionized THz metrology.

ECOPS-based THz systems such as TOPTICA’s TeraFlash smart acquire complete THz time-domain waveforms at kHz sampling speeds. This enables the rapid scanning of entire wafers.

Advantages of high-speed ECOPS for charge carrier mapping:

  • Faster than contact-based methods,  up to 200× shorter measurement times than mCV.
  • Insensitive to acoustic noise or mechanical vibrations. The fast sampling “freezes” disturbances, ensuring stable results in industrial environments.
  • Real-time wafer mapping, with thousands of points measured in minutes.

In a collaboration between Fraunhofer Institute for Industrial Mathematics (ITWM), Fraunhofer Institute for Integrated Systems and Device Technology (IISB), Technical University Freiberg, Freiberg Instruments and TOPTICA Photonics, an entire SiC wafer was mapped in under 70 minutes with more than 17,000 data points, compared to 20 minutes for only 25 points using mCV. The results demonstrated a good agreement between THz-derived carrier densities and mCV reference measurements.

Comparison with conventional methods

Feature THz-TDS (with ECOPS) Mercury C-V / Four-point probe
Contact required No Yes
Sample preparation None Surface polishing / cleaning
Measurement time Milliseconds .. seconds per point Minutes per point
Contamination risk None Mercury residue or damage
Spatial mapping Full wafer (X-Y scan) Point-by-point
Multilayer capability Yes (epi + substrate) No
Operation environment Ambient Controlled lab setup
Automation potential High Low

Conclusion

The transition to SiC and other wide-bandgap semiconductors demands new metrology tools that match the materials’ complexity and production speed. Terahertz time-domain spectroscopy fulfills this need, delivering non-contact, broadband, and high-speed measurements of charge carrier concentration and wafer uniformity.

With ECOPS-based THz systems like TOPTICA's TeraFlash smart, wafer mapping that once took hours now takes minutes. The result: precise, contamination-free, and industrially viable semiconductor inspection, paving the way towards future high-efficiency power devices.

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Application Notes