Barometric Pressure Sensor

Squeeze a doped silicon bar and its resistance jumps far more than the squeeze alone explains. C. S. Smith measured that in 1962.

A bathroom scale turns your weight into a number by stretching a metal strip until it resists current a little more. Silicon does the same job, but wildly better. The piezoresistive effect, the way a material's electrical resistance changes when you bend it, runs about 50 times stronger in doped silicon than in metal. Without semiconductor piezoresistance, a barometric sensor would be about 50 times less sensitive, dropping its output from roughly 10 mV/V to around 0.2 mV/V for the same strain. That gap decides everything downstream. A weak signal needs a huge membrane and hungry electronics to be readable; a strong one fits inside a phone running on a coin cell. Four resistors wired into a Wheatstone bridge, a simple balance that flags tiny imbalances, turn membrane stress into a clean voltage.

Wet acid eats silicon sideways as fast as it eats down, rounding every wall. In 1995 the Bosch process etched straight trenches instead.

Older chemical etching dissolves silicon in all directions at once, so it cannot cut a narrow deep slot without widening the top. Deep reactive ion etching, a method that alternates a cutting gas with a protective coating, carves nearly vertical walls one thin layer at a time. Without DRIE grooves reaching a 300 micrometer depth-to-width ratio, sensor cavities cannot shrink, forcing larger membrane openings and sharply weaker response to small pressure changes. That control is what lets a diaphragm be both small and thin at once.

A drum skin pulled too tight tears; pushed too gently it barely moves. A sensor membrane lives in the same narrow band.

Push on a thin plate and it bows in the middle. Push too hard and pressure and stress stop tracking in a straight line, which wrecks accuracy. Thin plate theory, the math Kirchhoff and Love worked out for how flat sheets bend, predicts deflection and peak stress from radius and thickness. Size the membrane by naive scaling instead, and central deflection jumps 5 times for the same pressure, pushing it past the linear range toward burst failure. The theory tells engineers exactly how thin is too thin.

Leave a sealed sensor in a hot car and the trapped air expands, faking a pressure drop. Heat and altitude look identical.

Air pressure and air temperature are bound together by the same gas law, so a warming sensor reports altitude change that never happened. Self-temperature compensation models that thermal expansion and subtracts it before the number leaves the chip. Without compensation drawn from the gas laws, a barometer drifts up to 1 hPa across a minus 20 to plus 60 degrees C range, which fakes roughly 8 meters of altitude. Eight meters is more than two floors, enough to ruin the elevator trick the whole package exists to do. Zhang's 2024 quartz-resonant design folds the correction into the sensor itself.