biosensors lecture 8- light sensors

light

• electrons in materials are vibrated and emit electromagnetic waves

• wave- particle duality

• light is a wave

photons

packets of energy, no mass, just pure energy

monochromatic light

light with a single wavelength

polychromatic light

light with a mixture of multiple wavelengths

• more common

for transverse electromagntic waves…

k, E, and H are mutually orthogonal to eachother

in free space, what speed do electromagnetic waves propagate at

the speed of light

when does linear polarization occur

when there is a 0 or 180 degree phase difference

when does right handed polarization occur

phase angle is negative

when does circular polarization occur

when there is a 90 degree phase difference

when does left handed polarization occur

phase angle is positive

when does ellipsoidal polarization occur

when the phase difference is 45 or 135 degrees

what is a photoresistor

photoconductive cell

• resistor that is more sensitive to light

what is a photodiode

• special type of diode that is sensitive to photons

photodiode depletion region

the width of the depletion region must be less than the light absorption length

two operation modes of a photodiode

1. photovoltaic mode

2. photoconductive mode

photovoltaic mode

w/out applied voltage

• unbiased operation

photoconductive mode

with applied voltage

• biased operation

absorption coefficient

how far into a material light of a particular wavelength can penetrate before it is absorbed

characteristics of photovoltaic mode

• simple

• low quantum efficiency

• small dynamic range

• small depletion region

• nonlinear response

characteristics of photoconductive mode

• higher quantum efficiency

• larger linear dynamic range

• creates a thicker depletion region

• more sensitive due to reverse bias

PIN photodiode

• intrinsic (I) layer inbetween the P and N semiconductors

• operates like photovoltaic mode

Solar cell

• photodiode in photovoltaic mode but with a much larger surface area

• photovoltaic mode generates electric current without applied voltage

1D photodiode array (PDA)

can be used for a spectrometer or scanner

2D photodiode array (PDA)

can be useful for digital imaging and position sensing

what is a CCD

charge coupled device

CMOS

complimentary metal oxide semiconductor

how does a CCD camera work

charge is transferred from the chip and read in a serial manner

• photons heat pixel area producing charge during exposure

• charge coupled after exposure time is over is amplified at the end

how does a CMOS work

light generated voltage is amplified in the pixel

• each pixel has own charge collection and amplifier

CCD characteristics

• high-quality, low-noise images

• greater sensitivity

• 100 times more power

• requires specialized assembly lines

• older and more developed technology

CMOS characteristics

• more susceptible to noise

• less sensitive to light

• little power consumption

• easy to manufacture

• cheaper

phototransistor characteristics

• produce much higher current than PD because of its amplifying ability

• very fast and capable of providing nearly instantaneous output

• more vulnerable to surges and spikes

semiconductor laser diode

• a device capable of producing a lasing action by applying a potential difference across a modified PN-junction

• the modified PN junction is heavily doped and contained within a cavity which provides the gain medium for the laser

LASER

• Light Amplification by Stimulated Emission of Radiation

• A cohere nt