©2007 Kwangsik Choi Characterization of Silicon Devices at Cryogenic Temperatures (Thesis of...

©2007 Kwangsik Choi

Characterization of Silicon Devices at Cryogenic Temperatures

(Thesis of Jeffrey F. Allnutt M.S.)

Kwangsik Choi


Outline

• Introduction– Motivation– Background

• Cryogenic Testing• Transistor Characterization• Circuit Characterization• Conclusion


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

technologiesNASA JWST (nasa.gov)


Background: Semiconductor Device Physics

• Intrinsic Silicon– Bandgap material– Large ionization energy– Poor conductor

• Extrinsic Silicon– Impurity energy states – Reduce ionization energy

• Freeze-out– Decreased thermal energy– May decrease carrier

concentration

Conduction Band

Valence Band

Ei

Ec

Ev

Eg

q

Si SiSi

SiSi Si

SiSiSi

Si SiSi

PSi Si

SiSiSi

Intrinsic Si Extrinsic Si

Energy Band Diagram of Si


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


Cryogenic Testing: Dewar Design

• Internal component board– Thermal Diode– DIP 28/40 socket (MOSFET)– Resistive Heater– Space for other components

MOSFET

BJT

Thermal Diode

Zener Diode

Resistor

Heater

Commercial MOSFET


MOSFET Characterization

• MOSFET device was first tested for functionality – Small device to minimize self-

heating – 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


MOSFET I-V Characterization

AMI 0.6µm, (W/L) = (3/1)

ID-VDS Curves, T = 37K, VG = 2, 3, 4, 5V ID-VDS Curves, T = 293K, VG = 2, 3, 4, 5V

ID-VDS Curves for varying T (VG = 5V) Linear and Saturation I Vs T (Normalized to 1 @ T = 293K)

Linear Triode (VG=5V, VDS=2V)

Saturation (VG=5V, VDS=4.5)


Transistor Characterization: Self-Heating

• AMI 0.6µm, (W/L) = (200/6)• Current decreases after saturation due to self-heating

ID-VDS Curves for varying T (VG = 3V) Linear and Saturation I Vs T

Linear Triode (VG=3V, VDS=1.5V)

Saturation (VG=3V, VDS=3.7V)


MOSFET Comparison

• Current-temperature characteristics are size and process dependent

• Different processes require individual modeling

• 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


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)


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


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

Reverse Leakage Current

Forward Current

Current

Voltage

Reverse Breakdown

Zener Voltage

Zener Diode I-V Characteristic


Zener Vs SiGe Comparison

• VREF as a function of Temperature near 37K– Zener dVREF/dT = 0.327mV/K

– SiGe dVREF/dT = 0.665mV/K

Zener VREF Vs T near 37K SiGe VREF Vs T near 37K


Ring Oscillator• Improved device performance

Improved ring oscillator performance?– Oscillation frequency is proportional to drain current

GND

Buffer

VDD

Output

31-Stages

INVERTERPLHPHL

osc Ittn

f

1


Ring Oscillator• Circuit:

– 31-stage oscillator, 4-stage output buffer– AMI 1.5µm process

Oscillation Frequency Vs T


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 Characterization of Silicon Devices at Cryogenic Temperatures (Thesis of...

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