Biometric skin contact sensing apparatus and method

A method of operating a biometric skin-contact sensor comprising a contact sensing area comprising an array of pixels, each pixel comprising a thin film transistor and a capacitive sensing electrode for sensing contact, the method comprising:

• operating the array in a low resolution mode to obtain an indication of a sub-region of the contact sensing area in which contact is sensed;
• operating the array in a high resolution mode to obtain biometric data for skin interacting with the contact sensing area in a first sub-region selected based on the sub-region of the contact sensing area in which contact is sensed, wherein obtaining the indication of the sub-region of the contact sensing area in which contact is sensed comprises using only a subset of the pixels in the array to sense contact
• wherein operating in the low resolution mode comprises performing a scan across the sensor array for determining whether or not one or more sub-regions of the contact sensing area are being contacted, and wherein only a subset of the sensor pixels are activated for each scan of the sensor array;
• and
• wherein, when a pixel is activated, a conductive channel of the transistor of the pixel is connected to a reference signal supply and the transistor outputs a read-out signal to an input of a read-out circuit.

A biometric skin-contact sensor comprising:

• a contact sensing area comprising an array of pixels, each pixel comprising a thin film transistor and a capacitive sensing electrode for sensing contact;
• and
• a controller coupled to the contact sensing area and configured to operate the array of pixels to switch between a low resolution mode and a high resolution mode;
• wherein in the low resolution mode the sensor is operable to obtain contact location data comprising an indication of a sub-region of the contact sensing area in which contact is sensed;
• wherein in the high resolution mode the sensor is operable to obtain biometric data for skin interacting with the contact sensing area by identifying a difference in capacitance between valleys and ridges of the skin;
• wherein when operating in the low resolution mode, the sensor is configured to determine whether or not one or more sub-regions of the contact sensing area are being contacted by activating a subset of the sensor pixels of the sensor array;
• wherein at least one of the activated sensor pixels is separated from another activated sensor pixel by one or more inactivated sensor pixels;
• wherein, for each pixel, a conductive channel of the transistor is connected between a reference signal supply and an input to a read-out circuit of the sensor, and the capacitive sensing electrode is connected to a control terminal of the transistor;
• and
• wherein activating a pixel comprises applying a signal to that pixel so that the transistor outputs a read-out signal to the input of the read-out circuit.