Lecture 2 Flashcards
CCDs and CMOS detectors work
via the photoelectric effect in a semiconductor
Semiconductors
In an isolated atom, the atomic energy levels are well spaced out
in solids, atomic levels form blended bands. the low energy bands are filled by electrons up to the fermi level.
in a semiconductor the last filled level is the ‘valence’ band
the valance and higher energy conduction bands are separated in energy by the band gap
electrons must be able to move between energy levels. so full bands cannot participate
electronic conduction arises when an electron moves from the valence band into a higher energy state.
How can a valence electron gain energy and jump the band gap?
Thermally
or from the photoelectric effect
to improve a semiconductors conduction, and help store the charges from the photo-electrons, we use
doped semiconductors
- adding a small amount of a different atom to our semiconductor
p-type
fewer valence electrons so extra hole
n-type
more valence electrons so extra e-
Silicon (MOS) transistor
a thin insulator layer of SiO2 is placed onto the p-type Si and on top of this metal electrode
the positive holes are driven away from the small positive bias voltage near the electrode = depletion region
electrons migrate to near electrode
depletion region
is the region where the positive holes are driven away from the small positive bias voltage near the electrode
full well capacity
maximum number of electrons that can be held before pixel saturates
CCD readout
CCDs are readout by applying sequences of voltages along the columns and down the rows of the CCD, transferring charge from one pixel to the next.
For speed all the columns are shifted at the same time.
Analogue to digital converter:
converts voltage to data numbers (DN)
3 phase read out scheme
each pixel has 3 electrodes connected in parallel at voltages Φ1, Φ2, Φ3
voltage is varied, allowing charge to migrate but also be kept separate
contributions to CCD data numbers (formula)
Xij = Pij + Tij + Eij + Cij
Pij =
contributions from pixel charge due to photons from the astronomical source
Tij =
contribution from pixel charge due to thermal effects (dark current)
Eij =
contribution from readout process
Cij =
contribution from pixel charge due to cosmic ray hits
Thermal noise Tij or dark current arises from
thermal energy in the CCD material, leading to lattice vibrations, called phonons
energy of these vibrations can create electron-hole pairs in the absence of illumination
the electrons give a DN signal, Tij and carry a
statistical fluctuation, ΔTij which varies in time
dark frames
are exposures with no illumination falling on the CCD
dark frames are exposed for long enough to capture thermal patterns
Electronic noise Eij
each stage of the photo-electrons to DN conversion can contribute noise
electron noise can arise in
transfer of charge from pixel to pixel
amplification of readout voltage - need low-noise amplifier
measurement of amplified voltage
quantisation noise
conversion of the analogue voltage into a digital signal (DN) in the ADC introduces an additional error term
Bias frames
are exposures of zero duration, without light falling on the CCD, which capture the various sources of Eij (not thermal noise)
bias frames are needed to
quantify the effect of the ADC offset
bias frames provide
pixel-to-pixel structure in the electron noise
a series of bias frames is acquired and averaged to
reduce the SNR on the bias frame values
output of a CCD is an
analogue signal, time-varying voltage
CCD voltage is typically composed
of bias voltage and small fluctuations due to the electrons in the potential wells
pedestal voltage
ADC offset voltage
a small offset voltage is added to the CCD output voltage when the output is less than the reference voltage
