CMOS

CMOS Pixel

Contains:

• Photodiode
• Capacitor (electron trap)
• Analog Amplifier
• ADC

Comparison with CCD

CMOS advantages over CCD:

• No need for “high” voltage (only voltage needed to run the components)
• Lower consumption
• Random read
• Blooming is non-existent
• Lower manufacturing costs - uses same tech as any other chip (CPU, RAM, …)

CCD advantages over CMOS:

• Low dark current
  • CCD - 0,01 electrons per second at -20°C
  • CMOS - 1 (at best 0,5) electrons per second → CMOS chip cannot be exposed for too long
• CCD only uses one ADC
  • all pixel values are read with a single ADC (identical behaviour of the converter)
  • minimal fix-pattern noise
• Better quantum efficiency
  • due to photodiode manufacturing method
  • best CMOS today at about 90 % of [CCD]
• All CCD chips use 16-bit ADC → first CMOS chips used 10-bit ADC, nowadays 14-bits (the best ones have 16-bit, not a standard)
• Binning doesn’t make sense with CMOS, only with CCD (in CMOS chips, it only lowers the transfer bandwidth at the cost of lower resolution)

Parameters

Identical or similar in both CCD and CMOS

• Full Well
  • electron capacity of a single pixel
  • in CMOS there is only pixel saturation but no blooming
  • CMOS capacity → $10-100\cdot 10^4$ electrons
• Quantum efficiency - $Q_E$
• Dark current - electrons per second
  • good value for CMOS → 1 electron per second
• Read-out noise
• Cooling - can be cooled by 30 to 45˚C under ambient temperature

Only CMOS

• Gain - gain of the amplifier located in front of the ADC
  • increased gain lowers read-out noise but dark current is amplified
  • increased gain lowers dynamic range → pixel becomes saturated sooner
• ISO
• Thermal glow - Increase of dark current noise in a certain parts due to heat from components
  • Counteracted by a fast cache memory close to the chip

BSI CMOS - Back-side illuminated CMOS

The smaller the photodiode, the higher advantage of BSI. Phones have really small photodiodes, which was the reason for this tech being used.

Nowadays, most chips are BSI with all the advantages and disadvantages of the technology.

Shutter types

CMOS without mechanical shutter uses rolling shutter method.

• Global shutter → Picture captured at once
• Rolling shutter→ Picture read out pixel by pixel, row by row
  • Also known as “Silent shooting” in CMOS cameras

Rolling shutter can be a problem if the subject moves.

Global shutter CMOS

Every pixel contains memory, which can store the necessary number of bits (12, 14, 16). This allows for global shutter even without mechanical shutter.

Multi-layer CMOS (Stacked CMOS)

Exposure time as low as 0,001 s. Supposed to be global shutter. Do not use in a lab! Right now impossible to calibrate.