IEEE EDS Distinguished Lecture Boston Chapter, July 6 2005 Novel III-Nitride HEMTs Professor Kei May Lau Department of Electrical and Electronic Engineering Hong Kong University of Science and Technology Clear Water Bay, Hong Kong Contributors: Prof. Kevin J. Chen, Dr. Shujun Cai, Dr. Yong Cai, Dr. Yugang Zhou, Jie Liu, Shuo Jia, Zhihong Feng, Ronming Chu
Founded in 1991 Introduction to The Electrical and Electronic Engineering Department Hong Kong University of Science and Technology, HK.
Hong Kong University of Science and Technology New Territories Kowloon Lantau Island HK Island HKUST
Students, Faculty, and Staff Undergraduate ~400 Undergraduate (CPEG joint with CS ) ~350 Postgraduate ~180 (FTE) PhD ~100 (FTE) MPhil ~70 (FTE) MSc (part-time) ~10 (FTE) UG + PG ~930 Post-doc./visiting scholars/ft-ra ~40 Regular Faculty 37 UG/Faculty = 15 Visiting Faculty (> 1 year ) 3 PG/Faculty = 4.6 (Counting PhD and MPhil only) Demonstrators/Instructional Assistants 8 Technicians Clerical and non-faculty Staff English Tutors 2 20 (only those supported by UGC) 11 (only those supported by UGC)
III-nitride HEMT Projects Composite Channel HEMTs Enhancement-mode HEMTs HEMTs with In doping HEMTs grown on Si substrates HEMTs on patterned sapphire substrates HEMTs with In-notch
Composite-Channel HEMT Design SiN Ti/Al/Ni/Au Ni/Au S G D 2 nm i-al 0.3 Ga 0.7 N 10 nm n-doped Al 0.3 Ga 0.7 N 6 nm i-al 0.05 Ga 0.95 N 3 nm i-al 0.3 Ga 0.7 N 2.5 µm i-gan sapphire J. Liu, Y. Zhou, R. Chu, Y. Cai, K. J. Chen, and K. M. Lau, Highly Linear Al0.3Ga0.7N/Al0.05Ga0.95N/GaN Composite-Channel HEMTs, IEEE Electron Device Lett., vol. 26, pp.145-147, April 2005.
DC characteristics of CC-HEMT 1000 800 (b) 150 150 I DS (ma/mm) 600 400 100 50 G m (ms/mm) G m (ms/mm) 100 50 200 V DS = 8V 0-8 -6-4 -2 0 V GS (V) 0 0 5 V DS (V) 10 15-6 -4-2 V GS (V) 0 Nearly flat g m from low to high V GS (I DS )
Small signal characteristics of CC-HEMT 40 f T, f max (GHz) 30 20 10 0 f T f max V DS = 10V -2.5-2.0-1.5-1.0-0.5 0.0 0.5 1.0 V GS (V) Current cutoff frequency (f T ) and Power gain cutoff frequency (f max ) vs gate bias, source-to-drain voltage V DS fixed at 10 V
Power performance at 2 GHz (100 µm wide CC-HEMT) P OUT (dbm), G T (db) 30 28 26 24 22 20 18 16 14 12 10 8 P OUT = 3.38W/mm P OUT G T PAE -10-5 0 5 10 0 15 P IN (dbm) 50 40 30 20 10 PAE (%)
Why? Self-Aligned Enhancement-mode AlGaN/GaN HEMTs Using Fluoridebased Plasma Treatment RF/microwave applications Simplify circuit configuration and lower the cost Negative polarity voltage supply can be saved Digital ICs Monolithic integration of E-mode and D-mode for direct-coupled FET logic (DCFL) Presented at the DRC June 2005, UCSB
Major Challenge for E-mode HEMT S G D AlGaN Access Modulated Access region region region Depleted GaN Maintain the low-resistance in access regions while depleting the channel at zero gate bias Substrate Need a selfaligned technique
Previous work Year and reporters 2000. X. Hu et al: (Lg=10um) 2002. J.S. Moon et al: (Lg=0.2um) and 2003, V. Kumar et al. (Lg= 1um) 2004. A. Endoh et al: (Lg=0.12um) Technologies Regrown P-type GaN Cap layer in the gate region Recessed gate by Cl-based RIE followed by annealing (at temperature higher than 500 C) Thin AlGaN barrier (10nm) with Pt- or Mo-gate with larger work function Concerns Large access resistance Damages induced by ICP- RIE Non-self-aligned gate Limited range for V GS (<1 V) Start with a D-mode HEMT with V th already close to zero Limited range for V GS (<1 V)
A New V th Control Technique: Fluoridebased Plasma Treatment Source Immobile negative charges ( F - ) Al x Ga 1-x N Gate ----- ----- Un-doped GaN Substrate Drain 2DEG density under the gate modulated by the amount of F- Incorporating the fluorine ions, which have strong electronegativity, into the AlGaN barrier F-ions can deplete 2DEG in the channel and shift V th positively Low damage (or recoverable) and stability are the key!
Structure of starting sample 3nm un-doped Al 0.3 Ga 0.7 N 15nm n-doped Al 0.3 Ga 0.7 N Si: 2.5 10 18 cm -3 2nm un-doped Al 0.3 Ga 0.7 N 2.5μm un-doped GaN Sapphire substrate Room temp. : N s ~ 1.3 10 13 cm -2, μ e ~ 1000 cm 2 /Vs
Device Fabrication Mesa etching Formation of S/D Ohmic contacts CF 4 Plasma 150W, 150s Gate windows opening Self-aligned CF 4 plasma treatment E-beam gate evaporation and lift-off Photoresist Source AlGaN Gate CF 4 plasma treated GaN Drain Post-gate annealing at 400 o C for 10 min. Sapphire
Vds = 6V Device characteristics (DC) Lg = 1 μm, Lgs = 1 μm, Lgd = 2 μm Plasma treatment: 150W, 150 sec Ids (A/mm) 0.7 0.6 0.5 0.4 E-mode HEMT (before RTA) E-mode HEMT (after RTA) Conventional D-mode HEMT 0.18 0.16 0.14 0.12 0.10 0.08 0.3 0.06 0.2 0.04-4.0V 0.1 0.882V 0.02 0.0 0.00-6 -4-2 0 2 4 V gs (V) g m (S/mm) V th E-mode 0.9 V V th D-mode = -4.0 V g m E-mode, max = 148mS/mm g m D-mode, max = 151mS/mm @ Vgs = 0V, Vds = 6 V g m E-mode = 0 ms/mm I ds,e-mode = 25 µa/mm True E-mode
Effect of post-gate annealing 0.30 0.25 E-mode HEMT (before RTA) E-mode HEMT (after RTA) Vgs= 2.5V~0V, step= -0.5V 0.03 Vgs = 0 V I ds (A/mm) 0.20 0.15 0.10 Ids (A/mm) 0.02 0.01 65V @1mA/mm 0.05 0.00 0 2 4 6 8 10 12 14 16 Vds (V) Output characteristics Low on-resistance and low knee-voltage 0.00 0 20 40 60 80 100 Vds (V) Off-state drain current No degradation after plasma treatment and annealing
Small-signal RF characteristics Bias point: Vds = 12V, Vgs = -3V for D-mode HEMT Vds = 12V, Vgs = 1.9V for E-mode HEMT. 40 30 Current gain of E-mode AlGaN/GaN HEMT Current gain of D-mode AlGaN/GaN HEMT MSG/MAG of E-mode AlGaN/GaN HEMT MSG/MAG of D-mode AlGaN/GaN HEMT f t, D-mode = 13.1GHz f max, E-mode = 34.3GHz Gain (db) 20 10 0 f t, E-mode = 10.1GHz 10 8 10 9 10 10 Frequency (Hz) f max, D-mode = 37.1GHz
Mechanism of the V th shift Fluorine ions incorporation in the AlGaN barrier SIMS measurement of fluorine atomic concentration F Atom Counts 10 5 10 4 10 3 AlGaN GaN CF 4 treatment: 600W 1min RTA 400C 10min 10 2 0 20 40 60 80 100 120 Depth (nm)
C-V results from Schottky diodes C-V curves (measured at f = 100KHz) 2DEG concentration extracted from C-V results C (F/m 2 ) 5.0x10-3 4.0x10-3 3.0x10-3 2.0x10-3 1.0x10-3 0.0 Freq=10K -6-4 -2 0 F - ions Normal AlGaN/GaN CF 4 150W 60s & RTA 400C 10mins CF 4 150W 150s & RTA 400C 10mins V (V) n e ( cm -3 ) 8.0x10 19 AlGaN GaN 6.0x10 19 4.0x10 19 2.0x10 19 Shift of pinch-off voltage 0.0 20 22 24 26 28 30 Distance from surface (nm) Reduction of 2DEG concentration No significant change in AlGaN thickness Normal AlGaN/GaN RTA 400C 10min CF 4 150W 60s RTA 400C 10min CF 4 150W 150s RTA 400C 10min
Effect of Plasma Treatment on Gate Leakage Current I gs (A/mm) 10-1 10-3 10-5 10-7 10-9 10-11 Gate leakage current E-mode HEMT (before RTA) E-mode HEMT (after RTA) Conventional D-mode HEMT -20-15 -10-5 0 5 V gs (V) Negative-chargeinduced upward bending (Φ F ) in the conduction band Reduction in gate current in both reverse and forward bias --- allow wider gate bias operating range and higher drain current density
Preliminary Reliability Test 0.30 Before Stress After Stress (Vds=20V, Vg= 2V, Id=126mA/mm) for 40hrs 0.15 0.8 Before Stress After Stress (Vds=20V, Vg= -3.2V, Id=126mA/mm) for 32hrs 0.20 I d (A/mm) 0.25 0.20 0.15 0.10 0.05 0.00 0.10 0.05 0.00-4 -2 0 2 4 V gs (V) G m (S) I d (A/mm) 0.6 0.4 0.2 0.0 0.15 0.10 0.05 0.00-6 -4-2 0 2 V gs (V) G m (S) E-mode D-mode
GaN-based HEMT on patterned silicon AlGaN GaN buffer GaN buffer silicon ridge Silicon Substrate GaN buffer Source Drain Polyimide GaN buffer GaN buffer GaN buffer Source Gate Drain GaN buffer GaN buffer GaN buffer AlGaN GaN buffer Source GaN buffer Drain GaN buffer S. Jia, Y. Dikme, D. Wang, K. J. Chen, K. M. Lau, and M. Heuken, AlGaN-GaN HEMTs on Patterned Silicon (111) Substrates, IEEE Electron Device Lett., vol. 26, pp.130-132, 2005
Output characteristics and transfer characteristics of HEMT grown on the ridges of patterned Si 1.2 1.0 V GS from 1 V to -8V step -1 V 180 160 140 V DS =8V 1.2 1.0 0.8 120 0.8 I DS [A/mm] 0.6 0.4 Gm (ms/mm) 100 80 60 0.6 0.4 I (A/mm) DS 0.2 40 20 0.2 0.0 0 3 6 9 12 15 V DS [V] 0-8 -6-4 -2 0 V GS [V] 0.0 I max = 1.05 A/mm, Peak g m = 150 ms/mm
40 30 H21: patterned H21: un-patterned MAG/MSG: patterned MAG/MSG: un-patterned Gain (db) 20 10 0 10 8 10 9 10 10 Frequency (Hz) Current gain (H21), maximum available gain (MAG) and maximum stable gain (MSG) for devices on the ridges (patterned) and below the ridges (un-patterned) f t = 9.7 GHz, f max = 20.5 GHz on patterned Si
AlGaN/GaN HEMT with indium-surfactant-doping Ti/Al/Ni/Au S 1µm Ni/Au G D 2 nm In-doped Al 0.3 Ga 0.7 N 15 nm Si-doped Al 0.3 Ga 0.7 N 3 nm In-doped Al 0.3 Ga 0.7 N 20 nm In-doped GaN 2 µm i-gan sapphire substrate
I DS (A/mm) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 conventional In-doped V GS =1~-4V 0.0 0 2 4 6 8 10 12 14 V DS (V) step=-1v Current-voltage (I DS -V DS ) characteristics of conventional and In-doped AlGaN/GaN HEMTs
10-1 Ig (A/mm) 10-2 10-3 10-4 10-5 10-6 conventional In-doped 10-7 10-8 10-9 -20-15 -10-5 0 Vg (V) Reverse gate leakage currents as a function of the gate bias
I DS (A/mm) 1.0 0.8 0.6 0.4 I DS (A/mm) 10-1 conventional In-doped 10-2 10-3 10-4 -8-7 -6-5 -4-3 V GS (V) 0.25 0.20 0.15 0.10 Gm (S/mm) 0.2 0.05 0.0 0.00-8 -7-6 -5-4 -3-2 -1 0 1 V GS (V) DC transfer (I DS -V GS, and G m ) characteristics of conventional and In-doped AlGaN/GaN HEMTs Inset : off-state drain leakage currents
I DS (A/mm) 1.2 1.0 0.8 0.6 0.4 V GS =-6V, V DS =15V 1 ms/0.2 µs static pulse 0.2 0.0 0 2 4 6 8 10 12 14 V DS (V) Pulsed and static IV characteristics of In-doped AlGaN/GaN HEMTs In-doped device shows negligible current collapse
Gain (db) 45 40 35 30 25 20 15 10 5 0 f t =12.3 GHz f t =19.1 GHz h21 (conventional) U (conventional) h21 (In-doped) U (In-doped) f max =34.5 GHz f max =56.3 GHz 1G 10G 100G Frequency (Hz) Small signal rf characteristics of conventional and In-doped 1 100 µm AlGaN/GaN HEMTs
Pout (dbm), Gain (db) 30 28 26 24 22 20 18 16 14 12 10 8 6 3.01 W/mm Pout Ga PAE -10-5 0 5 10 15 Pin (dbm) 45 40 35 30 25 20 15 10 5 0 PAE (%) On-wafer output power (P OUT ), gain and power-added efficiency (PAE) of 1 100 µm In-doped AlGaN/GaN unpassivated HEMTs at 2 GHz.
P OUT (dbm), Gain (db) 24 20 16 12 8 4 2.26 W/mm Pout Ga PAE -10-5 0 5 10 15 40 35 30 25 20 15 10 5 0 PAE (%) Pin (dbm) On-wafer output power (P OUT ), gain and power-added efficiency (PAE) of 1 100 µm In-doped AlGaN/GaN unpassivated HEMTs at 4 GHz.
AlGaN-GaN HEMTs on patterned sapphire substrates Ti/Al/Ni/Au Ni/Au Ti/Al/Ni/Au i-al 0.3 Ga 0.7 N 2 nm n-al 0.3 Ga 0.7 N 15 nm 100 i-al 0.3 Ga 0.7 N 3 nm 2 µm i-gan 2 µm 4 µm 60 nm grooved sapphire substrate
DC characteristics of AlGaN-GaN HEMTs on planar and grooved sapphire substrates I DS (A/mm) 0.9 (a) grooved substrate 0.8 V GS :1~-4V planar substrate 0.7 step:-1v 0.6 0.5 0.4 0.3 0.2 0.1 I DS (A/mm) 0.8 0.6 0.4 0.2 (b) planar substrate grooved substrate 0.25 0.20 0.15 0.10 0.05 G m (S/mm) 0.0 0 2 4 6 8 10 12 14 V DS (V) 0.0-6 -4-2 0 V GS (V) 0.00
AlGaN-GaN HEMTs on planar and grooved sapphire substrates I G (A/mm) 10-1 planar substrate 10-2 10-3 10-4 10-5 10-6 10-7 10-8 grooved substrate I DS (A/mm) 0.1 0.01 1E-3 Ids (A/mm) 2.0 1.5 1.0 0.5 off-state breakdown 0.0 0 10 20 30 40 50 60 70 80 Vds (V) planar substrate grooved substrate 10-9 10-10 (a) -20-18 -16-14 -12-10 -8-6 -4-2 0 V G (V) (b) 1E-4-8 -7-6 -5-4 -3 V GS (V) Reverse gate leakage currents as a function of the gate bias Off-state drain leakage currents Inset : off-state source-drain breakdown characteristics
Small signal characteristics AlGaN-GaN HEMT (1.4 100 µm) on grooved sapphire substrates Gain (db) 45 40 35 30 25 20 15 10 5 0 (a) f t =15.1 GHz Current Gain Power Gain f max =49.1 GHz 1G 10G 100G Frequency (Hz) current gain cutoff frequency (ft) and power gain cutoff frequency (fmax) measured at a drain bias of 10 V and a gate bias of 2 V
Large signal characteristics of AlGaN-GaN HEMT (1.4 100 µm) on grooved sapphire substrates P OUT (dbm), Gain (db) 28 26 24 22 20 18 16 14 12 10 (b) 4 GHz P sat =3.26 W/mm P OUT G a PAE -10-5 0 5 10 P IN (dbm) 70 60 50 40 30 20 10 PAE (%) on-wafer output power (P OUT ), gain and power-added efficiency (PAE) measured at 4 GHz
MOCVD System -AIXTRON 2000 HT for III-nitride Epitaxy
AIX2000HT MOCVD Reaction Chamber
AIX200/4 MOCVD Reactor for As/P/Sb/dilute Nitride Growth
Inductively Coupled Plasma (ICP) System for Nitridebased Material Dry-Etching
Electron-Beam Evaporator for Contact Metal Deposition PECVD for Oxide and Nitride Deposition
Rapid Thermal Annealing System for Contact Metal Annealing Resistivity Mapping System
Device Characterization S-parameter (up to 40 GHz) measurement and Model extraction HP8510C network analyzer High-frequency probe station Agilent IC-CAP device characterization and modeling suite HP8730C noise meter HP4142B semiconductor parameter analyzer (for DC biasing and characterization)
Load-pull measurement for large-signal characterization and model validation Maury s microwave automatic tuning systems (for finding optimum matching impedance) Capable of noise and linearity characterizations
Close-up View of the Load-Pull System
MOCVD Growth of AlGaN/GaN HEMT 2 nm undoped Alx Ga1-xN cap n-alx Ga1-xN Al 0.3 Ga 0.7 N undoped Alx Ga1-xN spacer ~ 2.5 µ m G an buffer GaN Sapphire Standard HEMT structure TEM cross-section of HEMT with 20 nm Al 0.3 Ga 0.7 N on GaN buffer
Optimization of the Surface Morphology 2x2 µm 2 area, Surface roughness ~ 0.203 nm.
Fabrication AlGaN AlGaN AlGaN Die photo GaN GaN GaN Sapphire Substrate Sapphire Substrate Sapphire Substrate Cross-sectional views showing the process flow. Photo of a fabricated HEMT for highfrequency testing. W x L = 100 x 1 µm.
Typical Device Characteristics DC RF Ids (ma/mm) 1200 1000 800 600 400 200 Vgs starts from 1V, step -1V Gain (db) 50 40 30 20 10 #110 @ VGS=-3.5V, VDS=10V f T =15.5GHz h21 2 U f max =39.3GHz 0 0 2 4 6 8 10 12 14 Vgs (V) 0 100M 1G 10G 100G Frequency (Hz)
Double-Heterostructure AlGaN/GaN/AlGaN HEMT Enhanced carrier confinement Pronounced nonlinear device behavior Nonlinear applications: frequency multipliers and mixers 1400 1200 Vgs = 1 ~ -9V Step: -1V 10000 1000 Vds = 6V 250 200 Ids (ma/mm) 1000 800 600 400 Ids (ma/mm) 100 10 150 100 Gm (ms/mm) 200 1 50 0 0 2 4 6 8 10 12 14 0.1 Vds (V) -14-12 -10-8 -6-4 -2 0 2 4 Vgs (V) 0
RF Characterization of III-N HEMTs Single-heterostructure HEMT double-heterostructure HEMT Frequency (GHz) 40 30 20 10 f T f max #110 Frequency (GHz) 40 30 20 10 f T fmax #140 0-5 -4-3 -2-1 0 1 V GS (V) 0-7 -6-5 -4-3 -2-1 0 1 V GS (V) DH-HEMT reaches higher cutoff frequencies at high current operation (high V GS )
Airbridge for Large-Periphery HEMTs for Power Applications Succeeded on dummy samples Device fabrication underway