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High Linearity and High Efficiency Power Amplifiers in GaN HEMT Technology

High Linearity and High Efficiency Power Amplifiers in GaN HEMT Technology. Shouxuan Xie Department of Electrical and Computer Engineering, University of California, Santa Barbara June 30, 2003. Outline. 1. Introduction and motivation - Why GaN HEMTs

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High Linearity and High Efficiency Power Amplifiers in GaN HEMT Technology

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  1. High Linearity and High Efficiency Power Amplifiers in GaN HEMT Technology Shouxuan Xie Department of Electrical and Computer Engineering, University of California, Santa Barbara June 30, 2003

  2. Outline 1. Introduction and motivation - Why GaN HEMTs - Objectives of the GaN HEMTs PA design • Class B for high efficiency and high linearity - Why single-ended Class B - Circuit design and measurement result • Identify and model nonlinear sources of GaN HEMTs - Nonlinear gm - Nonlinear Cgs - Nonlinear Gds • Proposed new designs to further improve linearity - Common drain Class B (to improve gm nonlinearity) - Pre-linearization diode (to improve Cgs nonlinearity) 5. Problems and future works

  3. Why GaN HEMTs Standard AlGaN/GaN HEMT structure • Advantages of GaN - High breakdown field: 3 MV/cm - High Vsat @ 2.5 x 107 cm/s - Thermal conductivity: 3x GaAs - Large channel charge: > 1x1013 cm-2 - Good electron mobility: >1200 cm2/V-s • Advantages of GaN HEMTs - High power density: 12W/mm for X-band (8-12GHz) - High Ft (50GHz) and fmax (80GHz) for 0.25um device - Linear I-V characteristics

  4. GaN HEMT process and device structure 0.25um T-gate for 50GHz ft Air-bridge for ground connection of CPW MIM capacitors SiN passivation for High RF output power SiC substrate for high heat conducting

  5. Device performance I-V Curve for 600m SG device Linear Id-Vgs characteristic on SiC Idss = 1 A/mm @Vgs=0V RF Performance 150m DG device Device performance summary: • Lg ~ 0.25um, • Idss ~ 1A/mm • ft ~ 55GHz (50GHz for DG) • Vbr ~ 40V (55V for DG)

  6. Objectives of GaN HEMT PA design Design RF MMIC power amplifier in GaN HEMT technology to achieve: 1. High linearity (low IMD3 distortion) 2. High efficiency 3. High output power 4. Broad bandwidth (High linearity and high efficiency are primarily concerned here) • Class A: Very high linearity and wide bandwidth; but very low efficiency (Ideal PAE 50%, feasible PAE 20-30%). • Switch mode Amplifiers (Class D, E): Very high efficiency (Ideal PAE 100%, feasible PAE 60-70 %); but poor linearity and poor bandwidth. • Class B: Good efficiency (Ideal PAE 78.6%; feasible PAE 40-50% ) and good bandwidth, and potentially low distortion.

  7. Push-pull Class B • Even harmonics are suppressed by symmetry=> wide bandwidth • Half-sinusoidal current is needed at each drain. This requires an even-harmonic short. It can be achieved at HF/VHF frequencies with transformers or bandpass filters. However, • Most wideband microwave baluns can not provide effective short for even-mode. Efficiency is then poor. • They occupy a lot of expensive die area on MMIC.

  8. ID1 ID1 Vin +Vin 0 0 + -Vin ID2 -ID2 180 180 -Vin ID = ID Zero Z at 2f0 Vin Single-ended = push-pull Push-pull Class B Even harmonics suppressed by symmetry Single-ended Class B with bandpass filter Even harmonics suppressed by filter Conclusion: From linearity point of view, push-pull and single-ended Class B with bandpass filter B are equivalent – same transfer function. Bandwidth restriction < 2:1

  9. ID1 ID1 ID1 Vin Vin Vin + Vp + + Vp Vp ID2 ID2 ID2 Vin Vin Vin = ID = ID = ID Vin Vin Vin Class B bias for high linearity Ideal Class B Bias too low: Class C Bias too high: Class AB

  10. Single-ended Class B Power Amplifier  - section lowpass filter Lossy input matching • Dual gate device is used since it has higher Vbr, higher MSG (smaller S12) and higher output resistance Rds • Lossy input matching network to widen the bandwidth • Cds is absorbed into output matching network (Low pass filter)

  11. Measurement setup Measurements: • Single tone from 4 GHz to 12 GHz; • Two-tone measurement at f1 = 8 GHz, f2 = 8.001 GHz; • Bias sweep: Class A (Vgs = -3.1V), Class B (Vgs = -5.1V), Class C (Vgs = - 5.5 V) and AB (Vgs = -4.5 V).

  12. Class B PA measurement results Gain and bandwidth Class AB Class B 3 dB bandwidth for Class B: 7GHz - 10GHz

  13. Class B bias @Vgs = - 5.1V Single tone performance @ f0 = 8GHz: Saturated output power 36 dBm PAE (saturated) ~ 34% f1,f2 Two tone performance @ f1=8GHz, f2=8.001GHz : 2f1-f2, 2f2-f1 • Good IM3 performance: • 40dBc at Pin = 15 dBm • > 35 dBc for Pin < 17.5 dBm

  14. Class A bias @Vgs = - 3.1V Single tone performance @ f0 = 8GHz: Saturated output power 36 dBm PAE (saturated) ~ 34% Two tone performance @ f1=8GHz, and f2=8.001GHz : f1,f2 2f1-f2, 2f2-f1 Good IM3 performance at low power level but becomes bad rapidly at high power levels

  15. Class B Class A Class AB Psat Class C IM3 suppressions of all Classes • Low output power levels (Pout < 24 dBm), Class A and Class B both exhibit good linearity (Class B > 36 dBc, Class A > 45 dBc). • Higher output power levels, Class A behaves almost the same as Class B. • Class AB and C exhibit more distortion compared to Class A and B.

  16. Class B vs. Class A PAE of single tone IM3 suppression and PAE of two-tone Class A Class B Class B Class A Maintaining good IM3 suppression, Class B can get 10% PAE improvement over Class A during low distortion operation.

  17. Nonlinear sources of GaN HEMT 1. gm vs. Vgs of 600um SG device Goal: Try to investigate nonlinear sources of the GaN HEMT device and understand how they affect the linearity on circuit Three major sources have been investigated: 1. Nonlinear gm ( or Ids -Vgs characteristic) 2. Nonlinear Cgs 3. Nonlinear Gds 3. Gds vs. Vgs and Vds of 600um SG device 2. Cgs vs. Vgs of SG device Vds=20V Vds=15V Vds=10V

  18. Nonlinear sources of GaN HEMT Input MN (linear, Zs) Cgs Cds Gds RL

  19. Nonlinear gm Input MN (linear, Zs) Cgs Cds Gds RL

  20. Nonlinear gm Modeled as: This term creates IM3 distortion Dominate at high output power levels – more interesting Vp Dominate at low output power levels

  21. Nonlinear Cgs Directly effect of Cgs Input MN (linear, Zs) Cgs - Cds + Gds RL Q(Vgs)

  22. Nonlinear Cgs Cgs vs Vgs of GaN HEMTs on SiC If modeled as: direct Anti-symmetric about V=Vc then should be no distortion Vc  Vp Therefore even order component of Cgs(Vgs) creates IM3 distortion This term creates IM3 distortion

  23. Nonlinear Cgs – Indirect effect Input MN (linear, Zs) Cgs - + Cds Q(Vgs) Gds RL

  24. Nonlinear Cgs – Indirect effect Input MN (linear, Zs) Cgs - + Cds Q(Vgs) Gds RL

  25. Nonlinear Cgs + nonlinear gm Input MN (linear, Zs) Cgs Cds Gds direct Indirect

  26. Nonlinear Gds Input MN (linear, Zs) Cgs Cds Rds RL Gds vs. Vgs of 600um SG device Vds=20V Vds=15V Vds=10V

  27. Nonlinear Gds DC I-V curve of 600um device on SiC Short channel effect Vgs = 0 V Vgs = -7V Vds = 8V Vgs = -7 V Vds = 15V Current through GaN buffer, need more gate voltage to pinch off

  28. Vp shift due to short channel effect 1.2mm SG device DC I-V curve at different drain bias Vds=20V Vds=15V Vp shift Vds=10V

  29. C C C g s g s C g s g s C C C 0 0 C 0 0 Vc Vc Vc V V V i n i n V i n Vc i n Vb Vb Vb Vb C C C C g s g s g s g s C C C C 0 0 0 0 V V V V i n i n i n i n 2 C 0 2 C 2 C 0 0 2 C 0 C C g s C g s g s Vc Vc Vc V V V i n i n i n Vc V -C i n g s Vb Vb Vb Vb Nonlinear Cgs + Vp shift Cgs(Vin) Cgs(-Vin) DC Even order component Vb=Vp=Vc Vb<Vc Vb>Vc Vb>>Vc

  30. Paidi’s nonlinear model Nonlinear Gds currently is modeled by shift in Vp; Cgs is ideal tanH I-V characteristic currently is linear

  31. Further improve linearity 1. Common drain Class B to improve gm linearity CD circuit schematic Linearizationfactor RL also functions as series-series feedback resistor, which increase gm linearity. RL Disadvantage -- Stability problem: Since the MSG is less, the circuit is not unconditionally stable in order to keep reasonable high efficiency. Therefore, extra requirement for the source and load impedance is needed.

  32. Simulation result of CD @5GHz Pout and PAE in single tone Pout ~ 38dBm Pout PAE PAE(sat) ~ 38%

  33. Simulation result of CD vs. CS – cont. Two-tone simulation result of CD vs. CS Common Drain 10 dB 12 dB Common Source: with 37.6dBm Pout and 42% PAE(sat)

  34. Common Drain vs. Common Source – cont. Simulation result of IM3 suppression at 1W total output power as a function of bias point Class C Common Drain Class AB Common Source Class B Class A

  35. Further improve linearity – cont. 2. Pre-linearization diode to improve Cgs linearity C_total Cgs C_pd Vc

  36. Pre-linearization diode Vb1=Vp=-4V 0.25umx100umx12 Vb1=2*Vp=-8V Can be very easily implemented on chip and occupy very small area Gate length can be varied and optimum value can be found since write using E-beam-lithography 0.75umx100umx4

  37. Simulation result of PD IM3 simulation result the designed dual gate CS Class B with pre-linearization diode @10GHz At least 4dBc improvement in IMD3 With PD Without PD

  38. Problems and future works !! Problem: Short channel effect for 0.25um device !! 0.25umx100um device on Sapphire 0.75umx100um device on Sapphire Vgs=0V Vgs=0V Vgs=-10V Vds=16V Vgs=-7V • Nonlinear Gds will affect linearity performance directly; • It creates Vp shift, hence generate nonlinear Cgs distortion; • Increases DC bias current, hence decreases PAE; • Decreases breakdown voltage, hence decreases the output power and also PAE …

  39. Short channel effect Currently dual gate device is used: - Nearly no Vp shift - Lower Gds (higher Rds) - Higher maximum stable gain (MSG) I-V curve of 600um DG device Gds of 600um devices at Vds=20V Vds =15V Single gate Vds =20V Dual gate - Number of gates get doubled, hard to yield all - Little bit lower ft, and higher Vknee, hence lower PAE - Not easy to model the nonlinear effect

  40. Layouts of the new designed circuits CD SG Class B @5GHz CS SG Class B @5GHz CS DG Class B @10GHz CS DG Class B @10GHz with PD

  41. New device structures to improve linearity Improve short channel effect by: - Make the Fe doping layer closer to the channel - Gate recess to increase aspect ratio Add Fe doping layer to decrease leakage current through the buffer ??? Question: How about decrease Al% in AlGaN -Increase breakdown and decrease gm? How about P-type doping GaN buffer layer? ??? Other ideas to increase breakdown???

  42. Summary • Class B bias is good for high linearity and high efficiency; • Three main nonlinear sources of the GaN HEMT device have been investigated with a new idea of nonlinear model; • According to simulation, common drain class B can improve linearity by 10dB over CS, and pre-linearization diode can improve linearity by 4dB. Four more circuits are designed and being fabricated to prove them; • Short channel effect for 0.25um device has been observed. New device structure is proposed to solve the problem and better linearity performance is expected.

  43. Proposed future works 1. Fabricate and measure the new designed circuits (CD and PD) - Need to stabilize the PECVD passivation process 2. Complete the new model to understand all the nonlinear effects - Add gm nonlinearity - More accurate model for dual gate device 3. Further improve linearity by new device structures - Work with Mishra’s group to improve the short channel effect 4. Publish paper and write thesis 5. New ideas on device structure and model to further increase linearity and efficiency summer summer Fall Fall

  44. Publications and references Publications: • Vamsi Paidi, Shouxuan Xie, R. Coffie, U. Mishra, M J W Rodwell, S. Long, “Simulations of High linearity and high efficiency of Class B Power Amplifiers in GaN HEMT Technology.”  Lester Eastman Conference, Aug. 2002 • Shouxuan Xie, Vamsi Paidi, R. Coffie, S. Keller, S. Heikman, A. Chini, U. Mishra, S. Long, M. Rodwell, “High Linearity Class B Power Amplifiers in GaN HEMT Technology.” Topical Workshop on Power Amplifiers, Sept. 2002 • Shouxuan Xie, Vamsi Paidi, R. Coffie, S. Keller, S. Heikman, A. Chini, U. Mishra, S. Long, M.J.W. Rodwell, “High linearity of Class B Power Amplifiers in GaN HEMT technology.” Microwave and Wireless Components Letters, to be published • Vamsi Paidi, Shouxuan Xie, R. Coffie, B. Moran, S. Heikman, S. Keller, A. Chini, S. P. DenBaars, U. K. Mishra, S. Long and M. J.W. Rodwell, “High Linearity and High Efficiency of Class B Power Amplifiers in GaN HEMT Technology.”  IEEE Transactions on Microwave Theory and Techniques, Vol. 51, No. 2, Feb. 2003 Other references: • K. Krishnamurthy, R. Vetury, S. Keller, U. Mishra, M. J. W. Rodwell and S. I. Long, “ Broadband GaAs MESFET and GaN HEMT Resistive Feedback Power Amplifiers.” IEEE Journal of Solid State Circuits, Vol. 35, No. 9, Sept. 2000. • K. Krishnamurthy, S. Keller, C. Chen, R. Coffie, M. Rodwell, U. K. Mishra, “Dual-gate AlGaN/GaN Modulation-doped Field-effect Transistors with Cut-Off Frequencies ƒT >60 GHz”, IEEE Electron Device Letters, Vol. 21, No. 12, Dec. 2000

  45. Publications and references- cont. • Solid State Radio Engineering, Herbert L. Krauss, W. Bostian, Frederick H. Raab/ Wiley, John & Sons, Nov. 1980 • Raab, F.H. Maximum efficiency and output of class-F power amplifiers. IEEE Transactions on Microwave Theory and Techniques, vol.49, (no.6, pt.2), IEEE, June 2001. p.1162-6. • Kobayashi, H.; Hinrichs, J.M.; Asbeck, P.M. “Current-mode class-D power amplifiers for high-efficiency RF applications”. IEEE Transactions on Microwave Theory and Techniques, vol.49, (no.12), IEEE, Dec. 2001. p.2480-5. • Eastman, L.F.; Green, B.; Smart, J.; Tilak, V.; Chumbes, E.; Hyungtak Kim; Prunty, T.; Weimann, N.; Dimitrov, R.; Ambacher, O.; Schaff, W.J.; Shealy, J.R.Power limits of polarization-induced AlGaN/GaN HEMT's. Proceedings 2000 IEEE/ Cornell Conference on High Performance Devices, Piscataway, NJ, USA: IEEE, 2000. p.242-6. 274 pp.. • Wu, Y.-F.; Kapolnek, D.; Ibbetson, J.; Zhang, N.-Q.; Parikh, P.; Keller, B.P.; Mishra, U.K. “High Al-content AlGaN/GaN HEMTs on SiC substrates with very high power performance”. International Electron Devices Meeting 1999, Piscataway, NJ, USA: IEEE, 1999. p.925-7. 943 pp. • Joseph, J.Teaching design while constructing a 100-watt audio amplifier. Proceedings. Frontiers in Education 1997, 27th Annual Conference (vol.1)Pittsburgh, PA, USA, 5-8 Nov. 1997.) Champaign, IL, USA: Stipes Publishing, 1997. p.170-2 vol.1. 3 vol. xxxvi+1624 pp. 3 • Shealey, V.; Tilak, V.; Prunty, T.; Smart, J.A.; Green, B.; Eastman, L.F.” An AlGaN/GaN high-electron-mobility transistor with an AlN sub-buffer layer”. Journal of Physics: Condensed Matter, vol.14, (no.13), IOP Publishing, 8 April 2002. p.3499-509. • W. R. Curtice and M. Ettenberg, "A nonlinear GaAsFET model for use in the design of output circuits for power amplifiers," IEEE Trans of Microwave Theory Tech, vol. MTT-33, pp. 1383-1394, Dec. 1985.

  46. Thank you!

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