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Ultra Wideband DHBTs using a Graded Carbon-Doped InGaAs Base

Ultra Wideband DHBTs using a Graded Carbon-Doped InGaAs Base. Mattias Dahlström, Miguel Urteaga,Sundararajan Krishnan, Navin Parthasarathy, Mark Rodwell Department of Electrical and Computer Engineering, University of California, Santa Barbara.

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Ultra Wideband DHBTs using a Graded Carbon-Doped InGaAs Base

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  1. Ultra Wideband DHBTs using a Graded Carbon-Doped InGaAs Base Mattias Dahlström, Miguel Urteaga,Sundararajan Krishnan, Navin Parthasarathy, Mark Rodwell Department of Electrical and Computer Engineering, University of California, Santa Barbara mattias@ece.ucsb.edu 805-893-8044, 805-893-3262 fax

  2. UCSB Wideband InP/InGaAs/InPMesa DHBT Mattias Dahlström • Objectives:fast HBTs → mm-wave power, 160 Gb fiber opticsdesired: 440 GHz ft & fmax, 10 mA/mm2, Ccb/Ic<0.5 ps/Vbetter manufacturability than transferred-substrate HBTs • Approach: • narrow base mesa → moderately low Ccbvery low base contact resistance required • → carbon base doping, good base contact process • high ft through high current density, thin layers • Bandgap engineering: small device transit time with wide bandgap emitter and collector

  3. UCSB DHBT Layer Structure and Band Diagram M Dahlstrom Emitter InGaAs 3E19 Si 400 Å Collector InP 3E19 Si 800 Å InP 8E17 Si 100 Å InP 3E17 Si 300 Å InGaAs graded doping 300 Å Setback 2E16 Si 200 Å Vbe = 0.8 VVce = 1.5 V Base Grade 2E16 Si 240 Å InP 3E18 Si 30 Å InP 2E16 Si 1700 Å • 300 A doping graded base • Carbon doped 8*10195* 1019 cm-2 • 200 Å n-InGaAs setback • 240 Å InAlAs-InGaAs SL grade • Thin InGaAs in subcollector InP 1.5E19 Si 500 Å InGaAs 2E19 Si 500 Å InP 3E19 Si 2000 Å SI-InP substrate

  4. s=722 /sq ??? InP/InGaAs/InPMesa DHBTBase contact resistance UCSB Mattias Dahlström • Carbon doping 6E19 cm-3 • Pd-based p-contacts • Careful ashing and oxide etch • RTP @ 300 C, 1 minute The size of the base contacts must be minimized due to Ccb Pc is immeasurably low: below 10 –7cm-2 Critical for narrow base mesa HBT

  5. InP/InGaAs/InPMesa DHBTDevice Results UCSB Mattias Dahlström b=20-25 No evidence of current blocking or heating J=3.5 mA/um2 BVCEO=7.5 V

  6. 230 mm 230 mm UCSB Accurate Transistor MeasurementsAre Not Easy Miguel Urteaga Mattias Dahlstrom • Submicron HBTs have very low Ccb Characterization requires accurate measure of very small S12 • Standard 12-term VNA calibrations do not correct S12 background error due to probe-to-probe coupling • Solution • Embed transistors in sufficient length of transmission line to reduce coupling • Place calibration reference planes at transistor terminals • Line-Reflect-Line Calibration • Standards easily realized on-wafer • Does not require accurate characterization of reflect standards • CPW lines suffer from substrate TE, TM mode coupling: thin wafer, use Fe absorber !lateral TEM mode on CPW ground plane… present above 150 GHz , must use narrower CPW grounds Transistor in Embedded in LRL Test Structure Corrupted 75-110 GHz measurements due to excessive probe-to-probe coupling

  7. U MAG/MSG H21 InP/InGaAs/InPMesa DHBTDevice Results UCSB Mattias Dahlström • 2.7 mm base mesa, • 0.54 mm emitter junction • 0.7 mm emitter contact • Vce=1.7 V • J=3.7E5 A/cm2 ft = 282 GHz; fmax=480 GHz b = 25; BVCEO = 7.5 V

  8. InP/InGaAs/InPMesa DHBTDevice Results UCSB Mattias Dahlström Aej=3.4 um2 J=4.4 mA/m2 Vcb=0.9 V fmax measurement above 500 GHzcurrently not reliable in CPW environment • Emitter contact sizes 0.5-2.0 um, 8 um long. • Base extends 0.25-1.0 um on each side of the contact • Maximum current density >10 mA/um2 • Vce >1.5 V for best performance • Best ft found at current density of 3-5 mA/m2

  9. InP/InGaAs/InPMesa DHBTConclusions UCSB Mattias Dahlström • Doping-graded base InGaAs/InP Mesa DHBT: • High current density Operates up to 10 mA/m2 without destruction …Kirk threshold 4.4 mA/m2 at 1.5 V • ft of 280 GHz with a 220 nm collector • fmax is 450 GHz or higher • Rbb is no longer a major factor - excellent base ohmics • fmax no longer a good measure of Ccb or circuit performance • Ccb reduction a priority • 87 GHz static frequency divider circuit already demonstrated

  10. UCSB Narrow-mesa DHBT:base design Mattias Dahlström Many approximate methods for determining Ef such as Boltzmann, Joyce-Dixon are insufficient Energy (eV) Base doping (cm-3) Doping graded base: At degenerate doping levels (>1E19) the variation of the Fermi level in the base is very rapid Exponential doping roll-off not needed, linear roll-off good enough!

  11. UCSB Narrow-mesa DHBT:base design Mattias Dahlström • Base transit time calculation: • Bandgap narrowing • Fermi-Dirac statistics • doping and bandgap dependent mobility Transit time (ps) Base width (A) The exit term (electron velocity in top of collector) important for thin bases: use InGaAs, not InP, close to base

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