Capacitive fingerprint sensor with improved sensing element

A capacitive fingerprint sensing device for sensing a fingerprint pattern of a finger, said fingerprint sensing device comprising a semiconductor substrate;

• and an array of sensing elements formed on said semiconductor substrate,
• wherein each of said sensing elements comprises:
• a protective dielectric top layer to be touched by said finger;
• an electrically conductive sensing structure arranged underneath said top layer;
• and
• a charge amplifier connected to said sensing structure for providing a sensing signal indicative of a change of a charge carried by said sensing structure resulting from a change in a potential difference between said finger and said sensing structure, said charge amplifier comprising:
• a negative input connected to said sensing structure;
• a positive input;
• an output providing said sensing signal;
• a feedback capacitor connected between said negative input and said output;
• and
• a sense transistor having a gate constituting said negative input,
• wherein said sense transistor is formed in a well in said semiconductor substrate, an interface between said well and said substrate being configured in such a way that current can be prevented from flowing between said well and said substrate;
• and
• wherein said charge amplifier is configured in such a way that a potential at said negative input substantially follows a potential at said positive input,
• wherein said fingerprint sensing device further comprises:
• excitation signal providing circuitry, said excitation signal providing circuitry being:
• connected to said positive input and configured to change a potential at said positive input from a first potential to a second potential, to thereby change a potential of said sensing structure, thereby providing said change in potential difference between said finger and said sensing structure;
• and
• connected to said well for changing a potential of said well from a third potential to a fourth potential, a difference between said third potential and said fourth potential being substantially equal to a difference between said first potential and said second potential, to thereby reduce an influence of a parasitic capacitance between said sensing structure and said well.