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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