Exam text content

12)
ASE-3036 Microsensors

Exam on Mon 13.3.2017 at 17:00-20:00 at TB103 No written material is allowed.
Exam responsible: Jukka Lekkala AII calculator types can be used.

Answer only to six (6) guestions. If you have answered to more than six guestions, only the six
answers that have given the least amount of points will be considered.

1. Sensors can be classified or categorized at least in three different way. How would you classify
the sensors shown below.

 

2. A photodiode of an optical fiber receiver is used in reverse biased mode in order to increase its
freguency band. The dark current of the diode with 10 V reverse voltage is In(-10 V) = 30 nA and
the internal shunt resistant of the diode was measured to be 10 M:n. The wavelength of the
light used in the optical communication is 900 nm. The guantum efficiency OE of the diode for
that wave length is 80 %.

a. What is the total RMS noise current in | GHz measurement bandwidth (hard stop, brick
wall) at room temperature? (2 p)

b. What is the sensitivity of the receiver at the given wavelength? (2 p)

c. What is the NEP (Noise Eguivalent Power) for the diode at room temperature when the
measurement band width is 1 GHz? (2 p)

Shot noise produced by the dark current is calculated as i =,/2ei,B and the thermal noise

S hc
current as i, =, K= . OE= x KEN - Boltzmann constant = 1.3806*10 J/K, speed of
light = 3*10* m/s, Planck constant = 4.1365 eVs. The unit of sensitivity S is A/W. NEP =.

Note! Even though we are talking about noise eguivalent power, it is usually given as power
density W/VHz.

3. a) Explain how a diode (pn-junction) can be used to measure light. (2 p)
b) What phenomena limit the useful wavelength range? (2 p)
c) Draw also an electronic circuit diagram for the measurement of a photodiode. (2 p)

at
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4. Briefly explain the terms listed below (1 p each)
a. Bulk mieromachining
b. Epitaxial silicon
c. Fused guartz
d. Polysilicon
e. Ouartz

f£ Surface micromachining

5. Explain shortly: (1 p each)

Seebeck effect

Photoconductivity and photoresistor
Pyro electricity

Magneto resistance

Hall effect

PTAT

me 800

6. Thin- and thick-film sensors. Consider e.g. following aspects: manufacturing processes,
materials, dimensions, sensor examples, properties,... (6 p)

7. Wheatstone bridge is commonly used together with V,
piezo resistive sensing elements.

a. What makes Wheatstone bridge convenient and

useful? (2 p) R-AR R+AR
b. Which kind of sensors it is commonly used
with? (1 p)
c. Derive the eguation for the output voltage Vo of Vo
J g
the Wheatstone bridge shown below. (3 p)
R+AR R-AR

8. a) What is the minimum diameter of a monocrystalline silicon wire that will support a weight of
a 65 kg person? (2 p)

b) What is the maximum strain of the monocrystalline silicon before it breaks? (1 p)
c) How long wire of some material can you hang in earth's gravity field (g=9.81 m/s?) before it

breaks under its own weight? Write your answer in terms of material properties i.e. derive an
eguation for the maximum length. (2 p)

d) What is the maximum length in the case of silicon? (1 p)

Assume 160 GPa for the Young's modulus and 6.5 GPa for the maximum tensile stress (yield
strength) of silicon. = £ EKIN , O=F/A
AL-A