©2007 Kwangsik Choi 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 Extrinsic Silicon –Impurity energy states –Reduce ionization energy Freeze-out –Decreased thermal energy –May decrease carrier concentration Conduction Band Valence Band EiEi EcEc EvEv EgEg q Si P Intrinsic SiExtrinsic Si Energy Band Diagram of 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 (>10 18 /cm 3 ) 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 MOSFET BJT Thermal Diode Zener Diode Resistor Heater Commercial MOSFET

©2007 Kwangsik Choi 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

©2007 Kwangsik Choi MOSFET I-V Characterization AMI 0.6µm, (W/L) = (3/1) I D -V DS Curves, T = 37K, V G = 2, 3, 4, 5V I D -V DS Curves, T = 293K, V G = 2, 3, 4, 5V I D -V DS Curves for varying T (V G = 5V)Linear and Saturation I Vs T (Normalized to T = 293K) Linear Triode (VG=5V, VDS=2V) Saturation (VG=5V, VDS=4.5)

©2007 Kwangsik Choi Transistor Characterization: Self-Heating AMI 0.6µm, (W/L) = (200/6) Current decreases after saturation due to self-heating I D -V DS Curves for varying T (V G = 3V) Linear and Saturation I Vs T Linear Triode (VG=3V, VDS=1.5V) Saturation (VG=3V, VDS=3.7V)

©2007 Kwangsik Choi 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

©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 I C Vs V CE curves for varying T (I B =50µA) Forward Active I C Vs T (I B =50µA, V CE =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 DataUnfiltered 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 Reverse Leakage Current Forward Current Current Voltage Reverse Breakdown Zener Voltage Zener Diode I-V Characteristic

©2007 Kwangsik Choi Zener Vs SiGe Comparison V REF as a function of Temperature near 37K –Zener dV REF /dT = 0.327mV/K –SiGe dV REF /dT = 0.665mV/K Zener V REF Vs T near 37K SiGe V REF Vs T near 37K

©2007 Kwangsik Choi Ring Oscillator Improved device performance  Improved ring oscillator performance? –Oscillation frequency is proportional to drain current GND Buffer V DD Output 31-Stages

©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 K. 3.Zener & SiGe structures have been presented as a low temperature voltage reference. 4.A simple ring oscillator operation is performed at low temperature.