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Characterization of Silicon Devices
at Cryogenic Temperatures
(Thesis of Jeffrey F. Allnutt M.S.)
Kwangsik Choi
©2007 Kwangsik Choi
Outline
• Introduction
– Motivation
– Background
•
•
•
•
Cryogenic Testing
Transistor Characterization
Circuit Characterization
Conclusion
©2007 Kwangsik Choi
Motivation
• Need for low temperature
electronics
– Space exploration
– Satellite communications
– Broad temperature range
• Limited development
– Lack of simulation and modeling
capability
– Perceived Need for exotic
technologies
NASA JWST (nasa.gov)
©2007 Kwangsik Choi
Background: Semiconductor
Device Physics
• Intrinsic Silicon
– Bandgap material
– Large ionization energy
– Poor conductor
Conduction Band
Ec
Eg
Ei
• Extrinsic Silicon
– Impurity energy states
– Reduce ionization energy
• Freeze-out
– Decreased thermal energy
– May decrease carrier
concentration
Ev
Valence Band
Energy Band Diagram of Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
P
Si
q
Si
Si
Intrinsic Si
Si
Si
Si
Si
Extrinsic Si
©2007 Kwangsik Choi
Background: Low Temperature
Semiconductor Phenomena
• Increased mobility
– Reduced electron-phonon scattering
– Counteracted by impurity scattering at lower temperatures
– Improves device performance
• Incomplete Ionization
– Increased parasitic resistance
– Decreased current drive
• Impurity bands
– Heavy doping (>1018/cm3) leads to impurity band formation
– Decreased activation energy, conduction through impurity
bands
– Allow for conduction at very low temperature
– Must be accounted for in modeling
©2007 Kwangsik Choi
Cryogenic Testing: Dewar Design
• Internal component board
–
–
–
–
Thermal Diode
DIP 28/40 socket (MOSFET)
Resistive Heater
Space for other components
Zener Diode
Resistor
Thermal
Diode
MOSFET
Heater
BJT
Commercial
MOSFET
©2007 Kwangsik Choi
MOSFET Characterization
• MOSFET device was first
tested for functionality
– Small device to minimize selfheating
– AMI 0.6µm, (W/L) = (3/1)
– Biased in saturation
• Showed functionality over
entire range
– Initial increase due to
decreased electron-phonon
scattering
– Impurity band conduction
prevents roll-off
Saturation Current Vs Temperature
©2007 Kwangsik Choi
MOSFET I-V Characterization
ID-VDS Curves, T = 293K, VG = 2, 3, 4, 5V
ID-VDS Curves, T = 37K, VG = 2, 3, 4, 5V
Linear Triode
(VG=5V, VDS=2V)
Saturation
(VG=5V, VDS=4.5)
ID-VDS Curves for varying T (VG = 5V)
Linear and Saturation I Vs T (Normalized to 1 @ T = 293K)
AMI 0.6µm, (W/L) = (3/1)
©2007 Kwangsik Choi
Transistor Characterization: Self-Heating
• AMI 0.6µm, (W/L) = (200/6)
• Current decreases after saturation due to self-heating
Linear Triode
(VG=3V, VDS=1.5V)
Saturation
(VG=3V, VDS=3.7V)
Linear and Saturation I Vs T
ID-VDS Curves for varying T (VG = 3V)
©2007 Kwangsik Choi
MOSFET Comparison
• AMI 0.6µm (200/6)
• Commercial Device
• AMI 0.6µm (3/1)
• IBM 0.13µm (2/1)
Saturation I Vs T for all MOSFET devices
• Current-temperature characteristics are size and process
dependent
• Different processes require individual modeling
©2007 Kwangsik Choi
BJT I-V Characteristics
• BJT: designed with lightly
doped base
• Susceptible to freeze-out
effects
• β dropped from 140 at
room temperature to 0.1
at T=37K
• Not suitable for low
temperature applications
IC Vs VCE curves for varying T (IB=50µA)
Forward Active IC Vs T (IB=50µA, VCE=0.8V)
©2007 Kwangsik Choi
MOSFET Noise Characterization
• Used heater to maintain temperature at 20K
• Swept frequency from 10Hz to 100kHz
• Significantly reduced 1/f noise & thermal noise
Filtered Data
Unfiltered Data
MOSFET Noise Vs Frequency
©2007 Kwangsik Choi
Zener Diode Voltage Reference
• Operates in reverse
breakdown region
– Large change in current
produces very small
change in voltage
– Electrons tunnel through
potential barrier
– Conduction is insensitive
to incomplete ionization
Current
Forward
Current
Reverse
Breakdown
Zener
Voltage
Voltage
Reverse
Leakage
Current
Zener Diode I-V Characteristic
©2007 Kwangsik Choi
Zener Vs SiGe Comparison
Zener VREF Vs T near 37K
SiGe VREF Vs T near 37K
• VREF as a function of Temperature near 37K
– Zener dVREF/dT = 0.327mV/K
– SiGe dVREF/dT = 0.665mV/K
©2007 Kwangsik Choi
Ring Oscillator
• Improved device performance
 Improved ring oscillator performance?
– Oscillation frequency is proportional to drain current
f osc
1

 I INVERTER
n  t PHL  t PLH 
VDD
31-Stages
Buffer
Output
GND
©2007 Kwangsik Choi
Ring Oscillator
• Circuit:
– 31-stage oscillator, 4-stage output buffer
– AMI 1.5µm process
Oscillation Frequency Vs T
©2007 Kwangsik Choi
Conclusion
1. Standard silicon MOSFET device functionality
has been demonstrated at temperatures down to
20K.
2. MOSFET I-V characteristics have been
measured at temperatures from 300-20K.
3. Zener & SiGe structures have been presented as
a low temperature voltage reference.
4. A simple ring oscillator operation is performed at
low temperature.
©2007 Kwangsik Choi