AlGaN Polarization Graded Field Effect Transistors for High Linearity Microwave Applications

Transcription

1 AlGaN Polarization Graded Field Effect Transistors for High Linearity Microwave Applications Shahadat H. Sohel, Hao Xue, Towhidur Razzak, Sanyam Bajaj, Yuewei Zhang, Wu Lu, Siddharth Rajan Department of Electrical and Computer Engineering The Ohio State University, Columbus, OH, USA Jason A. Roussos, David J. Meyer Naval Research Laboratory, Washington, DC, USA Andy Xie, Edward Beam, Yu Cao, Cathy Lee Qorvo, Inc., Richardson, TX, USA Funding: Office of Naval Research: Grant ONR N (Dr. Paul Maki)

2 2 Outline Motivation Polarization engineering DC/RF characteristics Large signal characteristics

3 3 Outline Motivation Polarization engineering DC/RF characteristics Large signal characteristics

4 4 Importance of Linearity Power amplifier Adjacent channel interference Desired Signal Adjacent channel Signal Signal strength Ch. 3 Ch. 1 Ch. 2 Ch. 4 Ch. 5 Bandwidth Frequency Non-linearity can cause distortion in adjacent channel The biggest source for non-linearity in the system is power amplifier!

5 5 Non-linearity in Amplifiers ii dddd = gg mm vv gggg = gg mmm vv gggg gg mmm vv 2 gggg gg mmm vv 3 gggg gg mmm vv 4 gggg. Output Non-linear component vv gggg = aa cos(ωω 1 tt) bb cccccc(ωω 2 tt) Fundamental signal 2ωω 1 ωω 2 Output vv gggg 2 = aa 2 cos(2ωω 1 tt) bb 2 cccccc(2ωω 2 tt) aaaa cccccc((ωω 2 ωω 1 )tt) aaaa cccccc((ωω 2 ωω 1 )tt) 2ωω 2 ωω 1 Second order distortion product ωω 2 ωω 1 vv 3 gggg = aa 3 cos(3ωω 1 tt) bb 3 cccccc(3ωω 2 tt) 3aa2 bb cos((2ωω 4 1 ωω 2 )tt) 3aaaa2 cos((2ωω 4 2 ωω 1 )tt) Third order distortion product 2ωω 1 ωω 2 2ωω 2 ωω 1 ωω 2 ωω 1 ωω 1 ωω 2 2ωω 1 2ωω 2 3ωω 1 3ωω 2 Frequency Ratio of fundamental frequency power to the third order harmonic power C/I3 or IMD3 of the amplifier

6 GaN transistors Advantages of GaN HEMTs for RF power amplification High Power Density/Unit Area High Frequency Operation Harsh Environment Applications High Linearity Applications Space Communications Mobile/Wireless Communications Imaging Military 6

7 7 Outline Motivation Polarization engineering DC/RF characteristics Large signal characteristics

8 8 GaN HEMT basics - Polarization Strained AlGaN P SP P PE AlN has a larger spontaneous polarization (5x10 13 q/cm 2 ) Strain leads to a piezoelectric polarization GaN P SP GaN has a large spontaneous polarization (1.8x10 13 q/cm 2 )

9 9 Polarization in Heterostructures Strained Al x Ga 1-x N GaN Charge - P P Spontaneous polarization difference Piezoelectric polarization Surface E C Energy AlGaN GaN buffer E V

10 10 High electron mobility transistor - HEMT Strained Al x Ga 1-x N GaN Surface donor P Charge - P 2DEG Surface 2DEG E C Energy AlGaN GaN buffer E V

11 Non-linearity sources in GaN HEMT Major sources: 1. Nonlinear g m 2. Nonlinear C gs Gate R G C GS C GD g m R DS Drain C gs (pf/cm 2 ) V gs (Volts) g m (S/mm) 1. Khurgin et al., Appl. Phys. Express 9, (2016) 2. Sanyam et al., IEEE TED, VOL. 64, NO. 8 (2017) Lg= 250 nm Vd= 10 V V gs (Volts) Source v Sat (x 10 7 cm/s) g m drop in HEMT is a result of sheet charge density dependent saturation velocity 1,2 2.0 Measurement Optical Phonon Model n S (x /cm 2 ) R S

12 12 Linear polarization grading Polarization charge in Al x Ga 1-x N: Increases with Al composition! Linearly graded AlGaN Unit cell c NN DDDD Positive unbalanced charge Fixed Polarization charges σσ ππ Neutralizing charges σσ ss Mobile electrons DDDDDD No doping involved High electron mobility No carrier freeze-out

13 Polarization Graded FET: PolFET Surface Donors N Dππ 3DEG z GaN Graded AlGaN GaN E C E F E V Graded Al X Ga 1-X N NN DDππ = [xx zz ] Rajanet al., APL 88, (2006) Bajaj et al., APL 109 (13), (2017) Polarization grading creates a 3D electron gas (3DEG) Change of bias only changes the depletion width 2D electron density, and hence, saturation velocity remains same 13

14 14 Comparison of large signal simulation CC\II3 dddddd = 10 log PP ff1 PP 2ff1 ff 2, PP ff1 = Power output at fundamental ff 1 frequency and PP 2ff1 ff 2 = Power output at third harmonic 2ff 1 ff 2 C\I3 (dbc) PolFET HEMT Frequency = 10 GHz Device Width = 1 mm Keysight ADS large signal simulation P out (dbm) PolFETs could provide ~10 dbc higher third harmonic compression than standard HEMT for a wide output power range

15 15 Outline Motivation Polarization engineering DC/RF characteristics Large signal characteristics

16 16 S Device Structure and DC performance L G = 0.7 μm G MOCVD grown 20 nm graded AlGaN channel 40% Al 0% Al 1.7 μm GaN on SiC Alloyed ohmic S-D contact D I D (A/mm) V GS =1 V V GS =-0.5 V V DS (V) g m I DS_MAX ~ 720 ma/mm at 1 V Contact resistance ~ 0.6 Ω-mm g m_max ~ 300 ms/mm g m stays almost constant as compared to AlGaN/GaN HEMT I D V DS = 10 V

17 17 3-Terminal Breakdown Voltage The device was passivated using 150 nm plasma-enhanced chemical vapor deposition (PECVD) SiNx at 300 C S Pinched off -6 V G MOCVD grown 20 nm graded AlGaN channel SiNx 40% Al 1.7 μm GaN on SiC Variable Drain bias V DS 0% Al D I D, I G, I S (µa/mm) 1200 VGS 1000 = -6 V L 800 GD = 0.5 µm I G (compliance) I D I S V DS (V) Three-terminal breakdown voltage (V BR ) of 50 V was measured - average break down field of more than 1.1 MV/cm We can theoretically achieve a maximum output power = (VV BBBB VV KKKKKKKK ) II DD,mmmmmm = W/mm 8 =

18 18 Pulsed I-V S V DS V DS,Q = 10V G MOCVD grown 20 nm graded AlGaN channel SiNx 40% Al 1.7 μm GaN on SiC Pulse time 5 μμμμ 0% Al D I D (A/mm) V GS =1 V V GS =-0.5 V V knee DC IV Pulsed IV 5 µs pulse Duty cycle 0.1% V DS (V) V GS V GS,Q = - 5V Period = 5 ms Time Time Pulsed I-V results show dispersion in the form of knee walk-out Dispersion will result in lower output power need more optimized passivation

19 Small signal characteristics DC bias DUT Vector Network analyzer S-parameter RF gain RF gain (db) f T = 23 GHz f MAX = 65 GHz h21 U MSG VGS = -0.5 V VDS = 10 V Frequency (Hz) f T or f max (GHz) ft fmax 20 V DS = 10 V V GS (V) Maximum current gain cut-off frequency, f T = 23 GHz and Maximum oscillation frequency, f max = 65 GHz suitable for X-band (8-12 GHz) application f T -L G product of 16.2 GHz-μm (state of the art f T -L G value for GaN HEMT is 16.8 GHz- μm 1 f T and f max remain almost flat throughout the gate bias range suggestive of improved linearity performance of the PolFETs Nidhi et al., IEEE International Electron Device Meeting (2009)

20 20 Small signal characteristics Typical GaN HEMT f T or f max (GHz) PolFET ft fmax 20 V DS = 10 V V GS (V) Maximum current gain cut-off frequency, f T = 23 GHz and Maximum oscillation frequency, f max = 65 GHz suitable for X-band (8-12 GHz) application f T -L G product of 16.2 GHz-μm, for GaN HEMT the highest reported value is 16.8 GHz- μm f T and f max remain almost flat throughout the gate bias range suggestive of improved linearity performance of the PolFETs

21 21 Outline Motivation Polarization Engineering DC/RF Characteristics Large Signal characteristics

22 22 10 GHz Load-pull results Signal source P IN Source Tuner Bias conditions: V DS = 30 V, V GS = -0.6 V I DS = 38 ma (close to Class A) Width = 150 μm Power meter PAE (%) V GHz DS = 30 V Class A P IN (dbm) Maximum output power of 2 W/mm was measured Maximum power added efficiency ~ 29% - likely due to the knee walkout at large signal and higher contact resistance from alloyed contact High small signal gain at 10 GHz ~16 db DUT P OUT Load Tuner Computer / Spectrum analyzer POUT (dbm), Gain (db)

23 23 Two-tone results i/p ff 1 = 10 GGGGGG ff 2 = GGGGGG Non-linear transistor o/p IMD3 C/I3 P f0 IM3 P fo (dbm), IM3 (dbm), IMD3 (dbc) OIP3 = 33 dbm f o = 10 GHz V DS = 30 V P in (dbm) Gate length (μm) Gate width (μm) ff 1 ff 2 2ff 1 ff 2 2ff 2 ff 1 OIP3 (dbm) OIP3/P DC (db) Type Ref x MMIC Ref x MMIC Ref. 3-2x50-12 Transistor Ref MMIC This work 0.7 2x Transistor OIP3 (intersection between fundamental and third harmonic) was calculated to be 33 dbm Linearity figure of merit, OIP3/P DC was found to be 3.4 db 1. Chang et al., Microwave and Optical Tech. Let. Vol. 56, Issue 1 (2014) 3. Arias et al., IEEE Compound Semiconductor Integrated Circuit Symposium (2017) 2. Liero et al., IEEE Trans. MicowaveTheory and Tech, Vol. 58, No. 4 (2010) 4. Moon et al., IEEE Conf. on RF/Microwave Power Amplifiers for Radio and Wireless Applications (2017)

24 24 Summary Polarization engineering is a promising technique which gives flexibility in designing high linearity transistors X-band power and linearity performance of PolFETs are reported for the first time Reasonably good power and linearity performance from PolFETs have been demonstrated g m I D PAE (%) V GHz DS = 30 V Class A P IN (dbm) POUT (dbm), Gain (db) P fo (dbm), IM3 (dbm), IMD3 (dbc) OIP3 = 33 dbm f o = 10 GHz V DS = 30 V P in (dbm)

25 25 Backup slides

26 26 Adjacent channel power ratio, ACPR = 10 log 10 PP aaaaaa PP rrrrrr Better ACPR means higher bandwidth and more efficient system

27 27 Charge from C-V Polarization charge in Al x Ga 1-x N: PP xx = 2xx xx xx (in cm -2 ) C (µf/cm 2 ) V (Volts) Charge density (cm -3 ) 1E20 Nd 1E19 1E18 1E17 1E16 1E15 1E14 1E13 1E12 1E Distance (nm) Hall measurement using Van der Pauw structure Mobility Sheet charge density 1160 cm 2 /V-s cm -2

28 28 RF gain calculation