mobility chapter 8 kimmo ojanperä s-69.4123, postgraduate course in electron physics i
TRANSCRIPT
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Mobility Chapter 8
Kimmo Ojanperä
S-69.4123, Postgraduate Course in Electron Physics I
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Outline•What is mobility?•Different types of mobilities•Hall mobility• Emphasis on the measurement
•MOSFET mobility
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Mobility
•Why high mobility is beneficial?
•Higher frequency response
•Higher current
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Different types of mobility
• Conductive mobility
• Hall mobility
• Magnetoresistance mobility
• Drift mobility
• MOSFET mobility
• Effective mobility
• Field-effect mobility
• Saturation mobility
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Conductive mobility
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Hall Mobility
•Hall mobility, carrier concentration and resistivity of the sample from the same measurement
•Van der Pauw measurement
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Hall Mobility – theory
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Hall Mobility – theory
http://www.nist.gov/pml/div683/hall_resistivity.cfm
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Hall Mobility – measurement
http://www.keithley.com/data?asset=55773
Carrier type:•Holes if V_H is positive•Electrons if V_H is negative
Hall effect voltage vs. van der Pauw resistance measurement configurations
Compute the Hall voltage with both positive and negative polarity current and with magnetic field both up and down, and with the two
configurations shown. Then average all voltages.
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Hall Mobility – van der Pauw measurement
http://www.keithley.com/data?asset=55773
Van der Pauw requirements•Uniform thickness•No wholes•Contacts at the edge of the sample
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Hall Mobility – sample geometry
Symmetrical sample
geometry makes it easier to verify
your measurement
results.
Use the same sample geometry for both the Hall voltage and van Pauw
measurement
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Hall Mobility – sample geometry
If you are measuring from a bar make sure it is long enough.
Should be L/W
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Hall Mobility – measurement setup
• Not too expensive
• Accurate source meter needed.
• Widely used.
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Geometric Magnetoresistance mobility
• Suitable for short and wide transistors with high mobility.• Silicon with µ = 500 – 1300 cm2/Vs is too
slow.
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Drift mobility
• Measures minority carrier mobility
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MOSFET Mobility
• Device mobility• Hall mobility is bulk material mobility
• Lower than bulk mobility because of all the additional scattering mechanisms• Coulomb scattering from oxide charges and
interface states• Surface roughness scattering
• Three types in the book• Effective mobility • Field-effect mobility• Saturation mobility
• For oxide TFTs also incremental mobility and average mobility have been defined [1]
[1] Hoffman, R. L.; , "ZnO-channel thin-film transistors: Channel mobility," Journal of Applied Physics , vol.95, no.10, pp.5813-5819, May 2004
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MOSFET Mobility – effective mobility
• For increased accuracy measure the Qn
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MOSFET Mobility – effective mobility
• The effective mobility depends on lattice scattering, ionized impurity scattering, and surface scattering. Ionized impurity and surface scattering depend on the substrate doping density and the gate voltage.
• Additional sources of error• Polysilicon gate
depletion• Gate leakage current• Series resistance • Interface trapped charge
overestimate the Qn
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MOSFET Mobility – field effect mobility
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MOSFET Mobility – field effect mobility
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MOSFET Mobility – saturation mobility
where m is the slope of the (ID,sat )1/2 versus (VGS − VT ) plot.
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Strengths and weaknesses
Conductive Mobility+ Defined straight from sample conductivity – no correction factors- Requires independent measurements for sample conductivity and carrier density
Hall effect Mobility+ Is in common use and easy to compare results.- Unknown Hall scattering factor introduces error. Normally assumed to be 1.
Magnetoresistance Mobility+ No special test structures needed for the samples that are applicable to the method- Only for high mobility semiconductors.
Drift Mobility+ Ability to measure mobility and carrier velocity at high electric fields.- Special test structures and high speed electronics needed. Rare.
MOSFET Mobility+ Operational device mobility extracted.- Different values extracted depending on the mobility definition. According to the book saturation and field-effect mobility should not be used. Almost all the papers in my field (solution processed TFTs) use them.