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SiGe Technology

SiGe Technology. Temperature Effects. 陳博文 R91943105. Outline. The Impact of Temperature on Bipolar Transistors Cryogenic Operation of SiGe HBTs Optimization of SiGe HBTs for 77K Helium Temperature Operation Nonequilibrium Base Transport High-Temperature Operation.

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SiGe Technology

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  1. SiGe Technology Temperature Effects 陳博文 R91943105

  2. Outline • The Impact of Temperature on Bipolar Transistors • Cryogenic Operation of SiGe HBTs • Optimization of SiGe HBTs for 77K • Helium Temperature Operation • Nonequilibrium Base Transport • High-Temperature Operation

  3. The Impact of Temperature on Bipolar Transistors • A modest increase in the junction turn on voltage with decreasing temperature • A strong increasing in the low-injection transconductance with cooling • A strong decrease in ß with cooling • A modest decrease in frequency response with cooling, with fT typically degrading more rapidly than fmax with decreasing temperature

  4. Current-Voltage Characteristics • For fixed bias current, VBE increase with cooling.

  5. Current-Voltage Characteristics

  6. Transconductance • We can expect an improvement in gm of roughly 3.9* in cooling from room temperature to liquid nitrogen temperature (Fig.9.1)

  7. Current Gain • Consider ideal Si BJT (constant doping profiles, metal emitter contact)

  8. Current Gain

  9. Resistances • Simulated effects of carrier freeze-out on the doping profile of a bipolar transistor at 77K

  10. Resistances • The result for realistic base profiles shows a quasi-exponential increase below about 200K and is very sensitive function of the peak base doping, particularly in strong freeze out below 77K • One can measure a base freeze activation energy ERbi

  11. Capacitance • The parasitic depletion capacitances will generally decrease (improve) with cooling, due to the increase in junction built-in voltage, since for a one-side step junction. • For the CB junction, which is the most important parasitic capacitance for switching performance due to Miller effect, CCB typically decreases by 10-20% form 300K to 77K

  12. Frequency Response • For fixed bias current, both depletion capacitances will decrease only slightly, while τband τe will both increase strongly with cooling • Unb increase only weakly with cooling since the base is heavily doped and thus cannot offset the factor of KT • In addition, enhanced carrier trapping on frozen-out acceptor sites can further degrade the base transit time

  13. Frequency Response • The strong base resistance increase at low temperatures.

  14. SiGe HBT Performance Down to 77K • Measure and calculated SiGe-to-Si current gain ratio as a function of reciprocal temperature for a comparably constructed i-p-i SiGe HBT and i-p-i Si BJT

  15. SiGe HBT Performance Down to 77K • Measure and calculated SiGe-to-Si Early voltage ratio as a function of reciprocal temperature for a comparably constructed i-p-i SiGe HBT and i-p-i Si BJT

  16. SiGe HBT Performance Down to 77K • Measure and calculated SiGe-to-Si current gain- Early voltage product ratio as a function of reciprocal temperature for a comparably constructed i-p-i SiGe HBT and i-p-i Si BJT

  17. High Temperature Operation • Percent change in peak current gain between 25°C and 125 °C for various Ge profile.

  18. 14% Ge low-noise profile

  19. High Temperature Operation • The current gain in SiGe HBTs does indeed have an opposite temperature dependence from that of a Si BJT, as expected from simple theory. • These changes in ß between 25°C and 125 °C, however, are modest at best (<25%), and clearly not cause for alarm for any realistic circuit. • The negative temperature coefficient of ß in SiGe HBTs is tunable, meaning that its temperature behavior between, say, 25°C and 125 °C can be trivially adjusted to its desired value by changing the Ge profile shape near the EB junction.

  20. High Temperature Operation • In the case of the 15% Ge triangle profile, with 0% Ge at the EB junction, ß is in fact femperature independent from 25°C to 125 °C. • It is well known that thermal-runaway in high-power Si BJT is the result of the positive temperature coefficient of ß. • The fact that SiGe HBTs naturally have a negative temperature coefficient for ß suggests that this might present interesting opportunities for power amplifiers, since emitter ballasting resistors (which degrade RF gain) could in principle be eliminated.

  21. High Temperature Operation • Gummel characteristics at 25°C and 275°C for a 14% Ge, low-noise optimized SiGe HBT

  22. High Temperature Operation • Current gain as a function of collector current at25°C and 275°C for a 14% Ge, low-noise optimized SiGe HBT

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