Nonlinear Piezoresistive Effect of 4H-SiC and Its Application +

PAM-XIAMEN can grow 4H-SiC epitaxial wafers specifically for pressure sensors. For specific parameters, please consult [email protected]

The performance of traditional silicon-based MEMS pressure sensors significantly decreases above 150℃, mainly due to issues such as PN junction leakage and material oxidation caused by the narrow bandgap of silicon. To address the challenges of high-temperature measurement, research has shifted towards wide bandgap semiconductors such as silicon carbide (SiC), among which 4H-SiC has become the preferred material due to its high carrier mobility and mature crystal growth process. It is worth noting that 4H-SiC exhibits significant nonlinear piezoresistive effects at high temperatures: its piezoresistive coefficient varies non monotonically from -50℃ to 500℃, and the resistance first decreases and then increases, with a turning point of about 207.5℃. This nonlinearity arises from the competitive mechanism between carrier concentration and mobility, and is regulated by ionization energy – high ionization energy helps maintain high temperature stability. This controllable nonlinear characteristic makes 4H-SiC an ideal choice for extreme environmental pressure sensing.

1. Piezoresistive Effect and Nonlinear Origin of 4H-SiC

1.1 Piezoresistance Coefficient and Gauge Factor

The piezoresistive effect refers to the physical phenomenon in which the electrical resistivity of a material changes under stress. The Gauge Factor (GF) is a key parameter for measuring the strength of the piezoresistive effect, defined as:

In the formula, Δ R/R is the relative change in resistance, and ε is the strain.

Wu et al. studied the GF values of n-type 4H-SiC under different sizes of piezoresistive strips through cantilever beam bending experiments, and found that the absolute value of transverse GF is always greater than longitudinal GF, and the GF value varies with the size of the piezoresistive strip (see Fig. 1 and 2).

Fig. 1 Fitting curves of (a) transverse and (b) longitudinal piezoresistive coefficients

Fig. 2 GF values of different sizes of piezoresistive strips

1.2 Nonlinear Effect of Temperature on Piezoresistive Effect

The piezoresistive effect of 4H-SiC exhibits significant nonlinear characteristics at high temperatures. Wu et al. found through experiments that the resistance value of n-type 4H-SiC exhibits a non monotonic trend with temperature variation within the range of -50 ℃ to 500 ℃:

When below room temperature, the resistance increases as the temperature decreases;

Between 100℃ and 200℃, the resistance decreases;

After exceeding 200℃, the resistance rises again, with an average turning point of about 207.5℃.

Sugura et al. further pointed out that the temperature dependence of the piezoresistive coefficient is closely related to the ionization energy of the material. They proposed a linear model based on ionization energy:

This model indicates that the higher the ionization energy, the more stable the piezoresistive coefficient becomes at high temperatures.

2. Microscopic Mechanism of Nonlinear Piezoresistance: Scattering Theory and Carrier Behavior

Wu et al. established a model for the conductivity variation of 4H-SiC based on scattering theory, revealing the physical essence of its nonlinear behavior. The semiconductor conductivity σ can be expressed as:

In the formula, n and p are carrier concentrations, and μ is mobility. Conductivity is influenced by both carrier concentration and mobility, which in turn is constrained by mechanisms such as lattice vibration scattering, ionizing impurity scattering, and inter valley scattering.

Fig. 3 Microscopic motion mechanism of charge carriers in 4H-SiC at different temperatures

3. Design and Performance of MEMS Pressure Sensor Based on 4H-SiC

3.1 Sensor Structure Optimization

Wu et al. designed an NPN type electrically isolated 4H-SiC piezoresistive pressure sensor chip, which adopts a circular sensitive diaphragm structure. Four groups of piezoresistive strips are symmetrically arranged at the edge of the diaphragm and connected by a Wheatstone bridge. By using COMSOL multi physics simulation to optimize the membrane size (radius 900μm, thickness 51μm), the highest sensitivity can be achieved while ensuring linear deformation.

Fig. 4 (a) Stress and (b) displacement distribution of diaphragm under pressure

3.2 High Temperature and Extreme Environmental Performance

The sensor exhibits:

High output sensitivity: 3.38 mV/V/MPa (-50 ℃)

Low temperature sensitivity coefficient (TCS): -0.067% FS/℃

Accuracy: 0.56% FS (25 ℃)

In addition, after soaking the sensor in H₂SO₄ at pH=1.05 and NaOH solution at pH=12.52 for 60 hours, the resistance changes were only 1.398% and 0.343%, respectively; Under 5W X-ray radiation, the maximum change in zero point output is only 1.93%, demonstrating excellent corrosion and radiation resistance.

The nonlinear piezoresistive effect of 4H-SiC is the key to maintaining stable sensing performance in high temperature and extreme environments. The nonlinear behavior of resistance with temperature can be well explained through scattering theory models and ionization energy related temperature models. MEMS pressure sensors developed based on this effect have high sensitivity, low temperature drift, and excellent environmental tolerance over a wide temperature range, making them suitable for scenarios such as geothermal energy extraction, deep well drilling, aircraft engines, and gas turbines.

Whether you need 4H-SiC epitaxy for research or for industrial applications, please contact us email at [email protected] and [email protected].

 

References:

1. Li, Y., Liang, T., Lei, C., Hong, Y., Li, W., Li, Z., … & Xiong, J. (2019). Quantitative analysis of piezoresistive characteristic based on a P-type 4H-SiC epitaxial layer. Micromachines, 10(10), 629.
2. Wu, C., Fang, X., Kang, Q., Fang, Z., Wu, J., He, H., … & Jiang, Z. (2023). Exploring the nonlinear piezoresistive effect of 4H-SiC and developing MEMS pressure sensors for extreme environments. Microsystems & Nanoengineering, 9(1), 41.
3. Sugiura, T., Takahashi, N., Sakota, R., Matsuda, K., & Nakano, N. (2022). High-temperature piezoresistance of silicon carbide and gallium nitride materials. IEEE Journal of the Electron Devices Society, 10, 203-211.