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Review Midterm Questions (Ch. 5) February 7th, 2005 ECE493 Topic 2 1) What would you call a sensor that converts photons into electron-hole pairs in the depletion region of a diode (solar cell), and why would you call it this (direct or modulating)? This kind of sensor would be a direct sensor since it is directly converting light energy (photons) into electrical current. 2) Between CCD and CMOS cameras, which one gives a better picture? Which one is less expensive? CCD cameras give a better resolution, but cannot integrate driver and readout circuits on-chip. CMOS cameras have a poorer picture, but are cheaper because driver and readout circuits are incorporated onchip. 3) What is a Fill Factor (FF)? Why is a high FF good? For each pixel on a chip a certain percentage of the area is dedicated to absorbing photons, and a certain area is dedicated to driver circuits. It is equal to the area of the pixel dedicated to absorbing photons divided by the total area of the pixel. A high FF is good because it means that more photons are absorbed per pixel, making the pixel more sensitive. Also, aliasing effects can become visible in the picture with a low FF (http://www.sandlotscience.com/Moire/Moire_frm.htm) 4) What are two different methods of detecting Infra Red (IR) photons? Which is most common and why? IR photons can either be detected directly through quantum electron-hole generation, or thermally (modulating) using pyroelectric materials. The pyroelectric method is most common because it is the least expensive and doesn t need special cooling like the quantum materials do. 5) How does gas-sensitive a FET work? Describe a circuit that would detect the change in the gassensitive FET. Does this make the gas-sensitive FET a direct or modulating sensor? A gas-sensitive FET has a small channel etched out of the FET s gate oxide where gas can flow. The underside of the gate is coated with a material that selectively adsorbs a gas. When the gas is adsorbed, the material s work function changes and this alters the Vt of the FET. A constant current could be passed through the FET and the voltage across it could be measured. As the Vt changes, the voltage across the source and drain would also change. This means that a gas-sensitive FET is a modulating sensor. 6) Explain how a capacitive touch screen works. What happens when more than one finger is touching the screen in different locations? A capacitive touch screen has a conductive plate with a dielectric layer on top. The conductive plate has a kHz voltage signal on it. When a finger touches the dielectric, it acts as a conductor to ground and forms a capacitor with the bottom conducting plate. The kHz voltage signal can then pass through the dielectric and through the finger to ground. The position is deduced by driving the conductive plate in each of its four corners. The amount of current that each corner needs to supply to the kHz voltage signal determines the location of the touch. If more than one finger was touching the plate, it would change the currents coming from each corner and the circuit that decides where the finger is would simply think that there was one finger touching somewhere between the two.

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