ECE699 – 004 Sensor Device Technology Chapter 4 Optical Sensors, Radiation Sensors and temperature Sensors Fall 2018 George Mason University

1. Optical components of Sensors

Radiometry The index of regraction is a ratio of velocity of light in vacuum, c0, to that in a medium, c.

transparency

Coefficient of reflection ρ If the light flux enters from air into an object For an object with a thickness (without absorption) Transmittance:

if with absorption α and transmittance γ

Photometry Lumen where ω is the solid angle

Windows (due to refraction Fig. 4.4, a passing ray is shifted by a distance L: a silicon window

Fresnel lenses: to gain high quality of ocusing

Fiber optics To achieve total internal reflection

Can be used to detect strain

Electro-optic modulators

2. Optical sensors 2.1 Introduction The energy of a single photon The photoelectric effect

The conductivity of semiconductor The band structure

Spectral response

Compare the characteristics of difference photodetectors: NEP (noise-equivalent power) D* (detectivity) IR cutoff wavelength (λc) Maximum current Max reverse voltage Radiant responsivity Field of view (FOV) Junction capacitance (Cj)

2.2 Photodiodes

Equivalent circuit of a photodiode

Rate of production of electron (η is the probability of photon hv to produce an e) The current is Current to voltage response

PN photodiodes

The fabrication of a PN photodiodes

Connection to an amplifier (photovoltaic mode)

Use current-to-voltage converter

Photoconductive mode (better linearity)

2.3 Phototransistor

Equivalent circuit Max frequency gm is the transistor forward transconductance

2.4 Photoresistors Change in conductance

2.5 Cooled Detectors - photons with long wavelength Intrinsic noise > photoconductive signal Detectors are based on PbS, InAs, PbSe, HgCdTe, Ge Can be used for many applications: thermal imaging, water content and gas analysis

2. 6 Thermal Detectors – mid/far-infrared spectrum 2. 6 2.6 Thermal Detectors – mid/far-infrared spectrum 2.6.1 Golay Cells – for infrared detection

2.6.2 Thermopile sensors Can also be implemented by MEMS

2.6.3 Pyroelectric Sensors Heatvoltage Thermal motion detector

2.6.4 Bolometers Sensitivity ZT thermal capacity, w is frequency

2.6.5 Active Far-Infrared Sensors “active“ –surface temperature is maintained at Ts by a circuit (Ts is higher or equal to the highest temp of the targets)

2.7 Gas Flame Detectors A unique feature of the flame: a noticeable portion of its optical spectrum is located in the UV spectral range