How Fingerprint Scanners in mobile phones Work?

Fingerprint Scanners in smart phones

Latest smartphones like google pixel feature capacitance sensor based fingerprint scanners.

Content Highlights

• Fingerprint scanners in smartphones
• Structure and working of fingerprint sensors in smartphones

Underlying technology of Fingerprint Scanners in smart phones are quite different from ordinary fingerprint sensors.Fingerprint sensors can be broadly classified into two.Optical sensors and capacitive sensors.

An optical fingerprint sensor is an electronic device used to capture a digital image of the fingerprint pattern. This image is digitally processed to create a biometric template (a collection of extracted features) which is stored and used for matching.Fingerprint sensors in modern smartphones are capacitive sensors.

The diagram below shows a simple capacitive finger print sensor. The sensor is an array of tiny sensing cells. Each cell has two conductor plates.Working of this sensors are similar to the working of touch sensors.

capacitance based finger print sensor cells

The sensor is connected to an electronic circuit which sense even minute current variations.Working of this sensing circuit is beyond the scope of this article.Main component of this sensing circuit is an amplifier which amplifies the minute current variations.You might have studied in your high school physics "The factors affecting capacitance":

• Plate Area.
• Plate Spacing ( distance between the plates )
• Dielectric Material.

Finger you are placing over the sensor will act as dielectric.When finger's ridges are placed over conductive plates, it increase the capacitance between plates, while an air gap(valley) will leave the charge at the capacitor relatively unchanged.In short the capacitor in a cell under a ridge will have a greater capacitance than the capacitor in a cell under a valley.Variation in capacitance will result in current flow.This variation is amplified by the sensing circuit.This analog signals are converted into digital signals using an ADC(Analog to Digital Converters) and required signal processing is carried out to extract the feature of a fingerprint.