Multi-stage G-band (140-220 GHz) InP HBT Amplifiers M. Urteaga, D. Scott, S. Krishnan, Y. Wei, M. Dahlström, Z. Griffith, N. Parthasarathy, and M. Rodwell. Department of Electrical and Computer Engineering, University of California, Santa Barbara urteaga@ece.ucsb.edu 1-805-893-8044 GaAsIC 2002 Oct. 2002, Monterey, CA

Outline Introduction Transferred-substrate HBT technology UCSB GaAs IC 2002 Introduction Transferred-substrate HBT technology Circuit design Results Conclusion

G-band Electronics (140-220 GHz) Applications: Wideband communication systems Atmospheric sensing Automotive radar Transistor-based ICs realized through submicron device scaling State-of-the-art InP-based HEMT Amplifiers with submicron gate lengths 3-stage amplifier with 30 dB gain at 140 GHz. Pobanz et. al., IEEE JSSC, Vol. 34, No. 9, Sept. 1999. 3-stage amplifier with 12-15 dB gain from 160-190 GHz Lai et. al., 2000 IEDM, San Francisco, CA. 6-stage amplifier with 20  6 dB from 150-215 GHz. Weinreb et. al., IEEE MGWL, Vol. 9, No. 7, Sept. 1999. HBT is a vertical-transport device (vs. lateral-transport) Presents Challenges to Scaling

Transferred-Substrate HBTs Substrate transfer enables simultaneous scaling of emitter and collector widths Maximum frequency of oscillation Previously demonstrated single-stage amplifier with 6.3 dB gain at 175 GHz 2001 GaAsIC Symposium, Baltimore, MD This Work Three-stage amplifier designs: 12.0 dB gain at 170 GHz 8.5 dB gain at 195 GHz Mesa HBT Transferred-substrate HBT

Transferred-Substrate Process Flow Emitter metal Emitter etch Self-aligned base Mesa isolation Polyimide planarization Interconnect metal Silicon nitride insulation Benzocyclobutene, etch vias Electroplate gold Bond to carrier wafer with solder Remove InP substrate Collector metal Collector recess etch

Ultra-high fmax Submicron HBTs Electron beam lithography used to define submicron emitter and collector stripes InAlAs/InGaAs emitter-base heterojunction 400 Å InGaAs base with 4 x 1019 cm-3 Be base doping, 52 meV bandgap grading 3000 Å InGaAs collector, high fmax / f ratio Amplifier device dimensions: Emitter area: 0.4 x 6 m2 Collector area: 0.7 x 6.4 m2 0.3 m Emitter before polyimide planarization Submicron Collector Stripes (typical: 0.7 um collector)

On-wafer Device Measurements 230 mm Submicron HBTs have very low Ccb (< 5 fF) 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 on-wafer transmission line to reduce coupling Line-Reflect-Line calibration to place measurement reference planes at device terminals Transistor Embedded in LRL Test Structure Corrupted 75-110 GHz measurements due to excessive probe-to-probe coupling

Line-Reflect-Line Calibration LRL does not require accurate characterization of Open or Short calibration standards LRL does require single-mode propagation environment LRL does require accurate characterization of transmission line characteristic impedance Must correct for complex characteristic impedance of Line standard due to resistive losses Transferred-substrate process provides excellent wiring environment for on-wafer device measurements

RF Device Measurements RF Gains Singularity observed in Unilateral power gain measurements, cannot extrapolate fmax from U Negative resistance effects observed at moderate bias currents Maximum stable gain of 7.4 dB at 200 GHz f = 180 GHz Observation TS-HBTs have very small output conductance due to low Ccb giving rise to high transistor power gains but… Second-order transport effects in collector may lead to negative resistance phenomenon Bias Conditions: VCE = 1.25 V, IC = 3.2 mA Device dimensions: Emitter area: 0.4 x 6 m2 Collector area: 0.7 x 6.4 m2

Mesa vs. TS-HBT S-parameters S11 – red S22- blue S11 – red S22- blue Low Rbb Very low Ccb High Ccb 6-40 GHz 6-40 GHz, 75-110 GHz, 140-220 GHz Transferred-substrate HBT Device dimensions: Emitter area: 0.4 x 6 m2 Collector area: 0.7 x 6.4 m2 3000 Å InGaAs Collector Fast C-doped mesa-HBT Device dimensions: Emitter 0.5 x 7 m2 Collector area: 1.6 x 12 m2 2000 Å. InP Collector 280 GHz ft, 450+ GHz fmax Mattias Dahlstrom 2002 IPRM Conference

Ccb Cancellation by Collector Space-Charge Collector space charge screens field, Increasing voltage decreases velocity, ® modulates collector space-charge ® offsets modulation of base charge ® Ccb is reduced Derivation is limited by charge control assumption Model dynamics with uniform velocity assumption Negative Conductance Negative Capacitance at low ω

Negative Resistance Effects in Transferred-Substrate HBTs Capacitance cancellation is observed for submicron InGaAs collector HBTs Change in curvature of real (Y12) is observed with increasing current. Effect not predicted by standard transistor hybrid-pi model where at low frequencies, As of yet, we have been unable to fit dynamic capacitance cancellation model to measurements Emitter: 0.3 x 18 m2, Collector: 0.7 x 18.6 m2 Vce = 1.1 V 2 fF decrease

IC Photograph: Dimensions 1.66 x 0.59 mm2 Amplifier Designs Three cascaded common-emitter stages matched to 50 Designs based on measured transistor S-parameters Standard microstrip models and electromagnetic simulation (Agilent’s Momentum) were used to characterize matching networks Two designs at 175 GHz and 200 GHz IC Photograph: Dimensions 1.66 x 0.59 mm2

140-220 GHz VNA Measurements HP8510C VNA with Oleson Microwave Lab mmwave Extenders GGB Industries coplanar wafer probes with WR-5 waveguide connectors Full-two port T/R measurement capability Line-Reflect-Line calibration with on-wafer standards Internal bias Tee’s in probes for biasing active devices UCSB 140-220 GHz VNA Measurement Set-up

Single-stage Amplifier Design 6.3 dB peak gain at 175 GHz 2001 GaAs IC Symposium, Baltimore, MD Single stage amplifiers designs on this process run 3.5 dB gain at 175 GHz S21 Cell Dimensions: 690m x 350 m

Multi-stage Amplifiers Measurements 175 GHz Design 200 GHz Design 12.0 dB gain at 170 GHz 8.5 dB gain at 195 GHz

Simulation vs. Measurement Circuit simulations predicted 20 dB gain at 175 GHz 14.5 dB gain at 200 GHz Measured transistors show higher extrinsic emitter resistance, lower power gain than those used in design Re-simulate amplifiers using measured transistor S-parameters Good agreement with measured amplifiers confirms passive network design Measured amplifier (blue) and modeled (red) using measured transistor S-parameters

Future Work: Highly-scaled mesa-HBT Designs Mattias Dahlstrom Transferred-substrate HBTs enabled aggressive device scaling but… They are hard to yield/manufacture High Carbon base doping allows for aggressive scaling of lateral dimensions of mesa HBTs Moderate power gains have been measured in 140-220 GHz band ~ 5 dB MSG at 175 GHz Tuned circuit designs in technology appear feasible 2.7 mm base mesa, 0.54 mm emitter junction 0.7 mm emitter contact Vce=1.7 V Jc=3.7E5 A/cm2

Conclusions UCSB GaAs IC 2002 Acknowledgements Multi-stage amplifiers have been demonstrated in 140-220 GHz 12.0 dB Gain at 175 GHz 8.5 dB Gain at 200 GHz Demonstrates potential of highly-scaled InP HBTs for G-band Electronics Currently pursuing more manufacturable approaches for HBT scaling Acknowledgements This work was supported by the ONR under grant N0014-99-1-0041 And by Walsin Lihwa Corporation