Single-stage G-band HBT Amplifier with 6.3 dB Gain at 175 GHz M. Urteaga, D. Scott, T. Mathew, S. Krishnan, Y. Wei, M. Rodwell. Department of Electrical and Computer Engineering, University of California, Santa Barbara GaAsIC 2001 Oct. 2001, Baltimore, MD

Outline GaAs IC 2001 UCSB Introduction Ultra-low parasitic InP HBT technology Circuit design Results Conclusion

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  3-stage amplifier with dB gain from GHz Lai et. al., 2000 IEDM, San Francisco, CA.  6-stage amplifier with 20  6 dB from GHz. Weinreb et. al., IEEE MGWL, Vol. 9, No. 7, Sept HBT is a vertical-transport device (vs. lateral-transport) Presents Challenges to Scaling G-band Electronics ( GHz)

Transferred-Substrate HBTs Substrate transfer allows simultaneous scaling of emitter and collector widths Maximum frequency of oscillation  Submicron scaling of emitter and collector widths has resulted in record values of measured transistor power gains (U=20 dB at 110 GHz) Promising technology for ultra-high frequency tuned circuit applications This Work  Single-stage tuned amplifier with 6.3 dB gain at 175 GHz  Gain-per-stage amongst highest reported in this band Mesa HBT Transferred-substrate HBT

InGaAs 1E19 Si 1000 Å Grade 1E19 Si 200 Å InAlAs 1E19 Si 700 Å InAlAs 8E17 Si 500 Å Grade 8E17 Si 233 Å Grade 2E18 Be 67 Å InGaAs 4E19 Be 400 Å InGaAs 1E16 Si 400 Å InGaAs 1E18 Si 50 Å InGaAs 1E16 Si 2550 Å InAlAs UID 2500 Å S.I. InP Bias conditions for the band diagram V be = 0.7 V, V ce = 0.9 V InAlAs/InGaAs HBT Material System Layer StructureBand Diagram 2kT base bandgap grading

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 f max Submicron HBTs Electron beam lithography used to define submicron emitter and collector stripes Minimum feature sizes  0.2  m emitter widths  0.3  m collector widths Amplifier device dimensions:  Emitter area: 0.4 x 6  m 2  Collector area: 0.7 x 6.4  m 2 Aggressive scaling of transistor dimensions predicts progressive improvement of f max As we scale HBT to <0.4 um, f max keeps increasing, device measurements become very difficult 0.3  m Emitter before polyimide planarization Submicron Collector Stripes (typical: 0.7 um collector)

Device Measurements RF Measurements: Unilateral power gain shows peaking in DC-45 GHz band GHz measurements corrupted by excessive probe-to-probe coupling Recent device measurements have shown negative unilateral power gain in W- and G- bands (2001 DRC, Notre Dame) Second-order device physics may be important in ultra-low parasitic devices Implications Devices have extremely high power gains in GHz bands, but f max cannot be determined from 20 dB/decade extrapolation Bias Conditions: V CE = 1.2 V, I C = 4.8 mA Device dimensions:  Emitter area: 0.4 x 6  m 2  Collector area: 0.7 x 6.4  m 2 f  = 160 GHz DC properties:  = 20, BV CEO = 1.5 V RF Gains

Simple common-emitter design conjugately matched at 200 GHz Simulations predicted 6.2 dB gain Designed using hybrid-pi model derived from DC-50 GHz measurements of previous generation devices Electromagnetic simulator (Agilent’s Momentum) was used to characterize critical passive elements Shunt R-C network at output provides low frequency stabilization S21 S11, S22 Amplifier Design Schematic

Transferred-substrate technology provides low inductance microstrip wiring environment  Ideal for Mixed Signal ICs Advantages for MMIC design:  Low via inductance  Reduced fringing fields Disadvantages for MMIC design:  Increased conductor losses Resistive losses are inversely proportional to the substrate thickness for a given Z o Amplifier simulations with lossless matching network showed 2 dB more gain Possible Solutions:  Use airbridge transmission lines  Find optimum substrate thickness Design Considerations in Sub-mmwave Bands

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 GHz VNA Measurements UCSB GHz VNA Measurement Set-up

Amplifier Measurements Measured 6.3 dB peak gain at 175 GHz Device dimensions:  Emitter area: 0.4 x 6  m 2  Collector area: 0.7 x 6.4  m 2 Device bias conditions:  I c = 4.8 mA, V CE = 1.2 V Cell Dimensions: 690  m x 350  m S21

Amplifier designed for 200 GHz Peak gain measured at 175 GHz Possible sources for discrepancy:  Matching network design  Device model Simulation vs. Measurement

Breakout of matching network without active device was measured on-wafer Measurement compared to circuit simulation of passive components Simulation shows good agreement with measurement Verifies design approach of combining E-M simulation of critical passive elements with standard microstrip models Matching Network Breakout Simulation Vs. Measurement S21 S22 S11 Red- Simulation Blue- Measurement Matching Network Design

Design used a hybrid-pi device model based on DC-50 GHz measurements Measurements of individual devices in GHz band show poor agreement with model Discrepancies may be due to weakness in device model and/or measurement inaccuracies Device dimensions:  Emitter area: 0.4 x 6  m 2  Collector area: 0.7 x 6.4  m 2 Bias Conditions:  V CE = 1.2 V, I C = 4.8 mA HBT Hybrid-Pi Model Derived from DC-50 GHz Measurements Device Modeling I: Hybrid-Pi Model

Measurements and simulations of device from 6-45 GHz and GHz Large discrepancies in S11 and S22 Anomalous S12 believed to be due to excessive probe-to-probe coupling Red- Simulation Blue- Measurement Device Modeling II: Model vs. Measurement S11, S22 S21 S12

Simulated amplifier using measured device S-parameters in the GHz band Simulation shows good agreement with measured amplifier results Results point to weakness in hybrid-pi model used in the design Improved device models are necessary for better physical understanding but measured S-parameter can be used in future amplifier designs Simulation versus Measured Results Simulation Using Measured Device S-parameters Simulation vs. Measurement

Multi-stage Amplifier Design Three-stage amplifier designed using measured transistor S-parameters Simulated 20 dB gain at 175 GHz Design currently being fabricated Simulation Results Multi-stage amplifier layout

Conclusions UCSB Single-stage HBT amplifier with 6.3 dB at 175 GHz Simple design provides direction for future high frequency MMIC work in transferred-substrate process Observed anomalies in extending hybrid-pi model to higher frequencies Future Work Multi-stage amplifiers and oscillators Improved device performance for higher frequency operation Acknowledgements This work was supported by the ONR under grant N And the AFOSR under grant F GaAs IC 2001