An Introduction to mems (Micro-electromechanical Systems)

34

 

 

4.2 Radiation Transducers 

4.2.1  Radiation Sensors 

Radiation sensors cover ionising radiation as well as visible light, infra-red (IR) and ultra-

include Geiger-Müller (GM) tubes and scintillators and although they have not been realized 

using MEMS, their miniaturization is potentially feasible.  Sensors for visible, IR and UV 

radiation are generally categorized as either direct or indirect.  Direct optical sensors detect 

photons and result in an electronic signal.  Indirect sensors convert the optical signals into an 

intermediate energy form (e.g. thermal or chemical), which is then measured electrically.  

There are a wide variety of both direct and indirect sensors and only the most common will be 

described here. 

i) Photodiodes 

photoconductive effect (increase in conductivity of a semiconductor on exposure to light).  

Photodiodes are junction-based photoelectrodes which have a p-n junction.  When visible or 

near infra-red light falls on the device, additional charge carriers are generated resulting in 

increased current flow. 

ii) Charge-coupled devices 

handheld video recorders and many other consumer applications.  They consist of a metal gate 

(electrode) above a dielectric and a semiconductor substrate.  This forms a metal oxide 

semiconductor (MOS) capacitor, the charge on which arises from photogenerated carriers.  

CCDs can be linear or made up of arrays of  metal-insulator-semiconductor sensors arranged 

so that photo-generated charge can be stored and transferred between elements by an 

appropriate variation of control voltages applied to surface electrodes (memory/signal 

processing approach). 

iii) Pyroelectric sensors 

whose charge can be altered by illumination or temperature changes.  By converting incident 

light into heat, which is then measured, pyroelectric sensors have a wide range of applications 

in surveillance, military, security consumer markets etc. e.g. human motion detectors.  

Pyroelectric sensors use piezoelectric and ferroelectric materials (varying dielectric constant 

with applied voltage).  ZnO is the most common in MEMS devices.