Saturday, January 16, 2016

e2v Proposes Pulsed Antiblooming Gate

e2v patent application US20160005785 "Image sensor with anti-blooming gate" by Frédéric Barbier and Frédéric Mayer gives the following explanation of the excessive dark current resulting from a positive bias of the antiblooming gate G5:

"If the potential applied to the gate G5 is 0.6 to 1.1 volts, the potential in the active layer 12 beneath the gate G5 will be positive, equal to around 0.2 volts for example. There then exists a strong local electric field beneath the gate at the surface of the silicon towards the edge of the photodiode which is maintained at 0 volts by the surface region 16. This electric field acts by lowering the forbidden band of the semiconductor and by therefore increasing the probability of electrons passing into the conduction band. This is a physical effect of band-to-band tunnelling, which creates a leakage current. Electrons are generated beneath the gate without the lighting being the cause; they will go to be stored in the photodiode with the highest potential. This current can be likened to a dark current since it exists independently of the lighting. This dark current, specifically due to the presence of a difference between the potential beneath the gate and the surface potential of the photodiode, is particularly bothersome when detection of weak lighting is desired. It can be several hundred times higher than if the potential beneath the gate was nil."

Whether this explanation is correct or not, the patent application proposes a pulsed anti-blooming bias to minimize the dark current:

1 comment:

  1. This should be already used in commercialized sensors. Since it's a common practice in 3T pixel to use pulsed AB voltage on the reset gate. It's even older in CCD use. A technique found in a commercial product is considered as a knowledge in public domain if there is no patent on it.


All comments are moderated to avoid spam.