Novel Micro-LED Display Featuring Fingerprint Recognition Without Additional Sensors

This paper proposes a new architecture for a micro light emitting diode (micro-LED) display featuring fingerprint detection without any additional light emitting and sensing devices in mobile applications. This paper proposes that conventional LEDs can be used as photodetectors in active-matrix pixel circuits. In the area where a finger touched, some LEDs are used as photodetectors and others are used as light emitting sources. The photocurrents of reverse-biased LEDs under various illumination conditions were investigated. The active-matrix circuit was fabricated using a conventional p-type low-temperature polycrystalline silicon (LTPS) process. The fabricated pixel circuit could distinguish the intensity of the incident light to the sensing LED. The voltage across the sensing LED (V_LED) was investigated when the pixel density was 400 pixels per inch (ppi) and the cover glass thickness was $30 ~\mu \text{m}$ . The experimental results showed that the V_LED changed by 0.5 V for 16.7 ms when red, green, or blue LEDs emitted 150 cd/m², 400 cd/m², or 100 cd/m², respectively. $1\times 9$ LED array with 1.5 mm pitch (p) and 2 mm thick cover glass was fabricated to emulate the LED display with 400 ppi and $30 ~\mu \text{m}$ thick cover glass. It is found that an emitting LED affected the sensing LEDs within the distance of 3p by comparing V_LED3. V_LED3 were measured when the material on the cover glass was air and water with white pigments (water/W). $\Delta \mathrm {V} _{\mathrm {LED}}\text{s}$ were 0.29 V and 1.13 V for glass-air and glass-water/W cases, respectively. It is found that the lights incident on the sensing LED comprised not only reflected lights by Fresnel reflection at the glass interface but also scattered lights in the water/W. A $75\times $ magnified fake fingerprint was fabricated. Its material was poly-lactic acid with refractive index of 1.465 which is very close to that of a fingertip. Measured $\Delta \mathrm {V}_{\mathrm {LED}}\text{s}$ at ridges and valleys were 0.48 V and 0.22 V, respectively. The ratio of 0.48/0.22 was 2.18 that is very close to 1.91 that is the ratio of light intensity at the ridge to that at the valley obtained by the calculation. This verifies that the proposed micro-LED display distinguishes the ridges and valleys of fingerprint without any help from any additional devices.