Section 2: CCDS & APS/CMOS Flashcards
types of pixelated semiconductor devices used for digital imaging
CCDs
APS
CMOS (examples of APS)
CCDs readout image by
moving the stored charge across the image
both CCDs and APS are subject to
uneven response, thermal noise, hot pixels, electronic noise, cosmic ray hits…
CCDs/CMOS work via the
photoelectric effect in a semiconductor - electron energised by photon
semiconductors: in an isolated atom,the atomic energy levels are
well spaced out
semiconductors: in solids, atomic levels form
‘blended’ bands - the low energy bands are filled by electrons, up to fermi level
semiconductors: the last filled level is the
valence band
band gap
between valence and conduction bands
typically a few eV
for electrical conduction, electrons must be able to
move between energy levels (so the full bands cannot participate)
in conductors, the valence band
is not full and overlaps with conduction bands
electron conduction arises when
an electron moves from the valence band into a higher energy state
get an electron in conduction band and hole in valence band
how can a valence electron gain energy and jump the band gap?
- thermally (don’t like)
- photon transferring energy (ie photoelectric effect)
semiconductors: photons will produce
electron-hole pairs
doped semiconductors
adding a small amount of a different atom with more/less valence electrons
doped semiconductors are used to
improve condition and help store the charges
p type doping
eg boron into silicon
fewer valence electrons so extra hole above valence band
n type doping
eg arsenic into silicon
more valence electrons so extra electrons below conduction bands
can use p or n-type
individually or combined in a p-n junction to store the charge produced by the photons
p-type silicon held under a small bias voltage - setup
a thin insulator layer of SiO2 is placed onto the p-type Si and on top of this is a metal electrode (known as surface channel device)
p-type silicon held under a small bias voltage - depletion region
the positive holes are driven away from the small positive bias voltage near the electrode
electrons migrate to near electrode
electron hole pairs created by photons will have the electron stored
in the potential well near the electrode, at the top of p-type Si, below the SiO2
stored charge is directly proportional to the
number of photons falling on it (in IR/optical regime)
full well capacity
max no of electrons that can be held before pixel saturates
CCD readout by
applying sequences of voltages along the columns and down the rows of the CCD, transferring charge from one pixel to the next