Capacitive vs Piezoresistive MEMS Sensors: Key Differences
MEMS accelerometers and pressure sensors convert how much a proof mass or diaphragm moves or bends under force into an electrical signal. Capacitive sensors measure displacement from the change in capacitance between movable and fixed electrodes. Power consumption is low and temperature has little effect, but the change is tiny, so a dedicated high-sensitivity circuit is needed. Piezoresistive sensors use the effect that silicon's resistivity changes under stress and read it with a bridge circuit. The circuit is simple and the output is easy to handle, but current flows continuously, so power consumption is higher, and compensation for temperature-induced changes is essential.
| Aspect | Capacitive | Piezoresistive |
|---|---|---|
| Sensing principle | Change in capacitance between movable and fixed electrodes | Change in resistivity caused by stress (piezoresistive effect) |
| Accelerometer structure | Comb-shaped movable electrodes are attached to a spring-supported proof mass, and the difference in capacitance to fixed electrodes on both sides is measured (differential) | Resistors are built into the beams that support the proof mass, and the stress when the beams bend is measured |
| Pressure sensor structure | Measures the capacitance between a diaphragm that bends under pressure and an opposing electrode | Four resistors are built into the edges of a thin diaphragm to form a Wheatstone bridge |
| Signal read | Very small capacitance changes (in accelerometers, sometimes below a fF). A dedicated ASIC that converts capacitance to voltage is required | The resistance change is taken out as a bridge voltage. The circuit is simple |
| Power consumption | Low. No DC current flows | Higher. DC current flows through the bridge continuously |
| Temperature effects | Little affected | Resistance and sensitivity change significantly with temperature, so temperature compensation is essential |
| Sensitivity and resolution | Differential electrodes and a high-sensitivity circuit make it easy to measure small accelerations and tiny pressures with high resolution | Good output linearity. Sensitivity is relatively low, but it also suits measuring high pressures and shocks |
| Static (constant force) measurement | Possible (can also measure gravity) | Possible (can also measure gravity) |
| Robustness to disturbances | Susceptible to parasitic capacitance of the wiring and electrical noise, so the circuit is placed close to the sensor | Low impedance, so it is hardly affected by parasitic capacitance |
| Manufacturing | Made by surface micromachining and similar methods, and easy to put in a small package together with the circuit chip | Diaphragms and beams are made mainly by bulk micromachining, and resistors are formed by ion implantation or diffusion |
| Typical uses | Accelerometers and gyroscopes in smartphones and similar devices, MEMS microphones, low-power barometric pressure sensors | Automotive engine and tire pressure, blood pressure monitors, industrial pressure gauges, shock accelerometers |
Figures are typical orders of magnitude and vary by process generation and product.