Vertically scaled 5 nm GaN channel Enhancement-mode N-polar
GaN MOS-HFET with 560 mS/mm gm and 0.76 W-mm Ron
Uttam Singisetti*, Man Hoi Wong, Jim Speck, and Umesh Mishra
ECE and Materials Departments
University of California, Santa Barbara, CA
2011 Device Research Conference
Santa Barbara, CA, USA
*uttam@ece.ucsb.edu
1
Outline
• Next generation GaN electronic devices
• N-polar GaN HEMTs
• Vertically scaled channel devices
• Results and Conclusion
2
Next-generation mm-wave GaN devices
John Albrecht, DARPA
1.3 W at 75 GHz
Fujitsu, CSIC 2010
3 W at 87 GHz
Caltech, HRL
ISSSTT, 2011
W-band GaN power amplifiers
− GaN HEMTs: Power-switching, microwave, W-band power amplifiers
−Future GaN devices for beyond mm-wave and to sub-mm-wave bands
−Higher operating voltages than traditional III-Vs and Si  robust and rugged
mixed signal ICs
3
Device goals and structure
−Aggressive dimensional scaling (Lg and Lsd )
−Vertical scaling with back-barrier and high-k dielectric
− Parasitic resistances and capacitances scaling
−Maintain high breakdown voltage
4
Ultra-scaled N-polar HEMTs
No electron barrier
N-polar inverted HEMT
N-polar GaN
− No barrier to electron on top of 2-DEG grading to narrowgap InN 
low resistance contacts (0.027 W-mm)1
− AlGaN back  confinement of 2-DEG, control short channel effects2
− Record high gm = 1105 mS/mm demonstrated4 in D-mode
− E-mode devices
1. S.Dasgupta, APL 2010,
2. S. Rajan, IEEE TED 2011
3. NIdhi, DRC 2011
5
E-mode device structure and design
AlN
4
AlN
GaN
0
Al2O3
4
0
3.0
0
EF
-1
GaN
2.0
1.5
1.0
-2 E
V
15
20
25
30
35
AlN
3
40
2
1
GaN:Si
n+ Graded InGaN
(In: 0% to 65%) GaN
InN
channel
0
EF
-1
-2
-3
-4
-3
0.5
0
20
40
60
80
100
Depth (nm)
-4
20
n (x10 cm )
1
GaN
-3
Energy (eV)
2 EC
10
Under gate
2.5
GaN:Si
5
Depth (nm)
19
SiNx
3
Top AlN depletes 2-DEG under gate
EV
-6
AlN
n(x)
EF
-2
-4
4
AlxGa1-xN:Si
0.05<x<0.25
Energy (eV)
Energy (eV)
2
EC
40
60
Depth (nm)
80
0.0
100
Under S/D
contacts*
Under sidewall
AlN removed under sidewall
* S.Dasgupta, APL 2010
6
Short channel effect and vertical scaling
8 nm GaN channel2
20 nm GaN channel1
Vt roll-off with gate length
Poor saturation at sub-100nm Lg
• Vth roll off with gate length
• Vertical scaling needed to maintain E-mode at sub-50 nm gate lengths
• Vertical scaling for high Rds at sub-50-nm gate lengths
Need 5 nm GaN channel for sub-50 nm devices
1. U.Singisetti, EDL 2010,
2. U.Singisetti, APEX 2011
7
Ultra-thin channel challenges: Mobility
QW thickness flutuations
GaN
• Need 5 nm thick GaN channel for sub-50 nm devices
• Mobility drops with decreasing GaN channel thickness
• Interface roughness, surface roughness scattering increases*
* U.Singisetti, ISCS 2011
8
Mobility dependence on Si doping
Si : 5 e18 cm-3
Si : 2 e 19 cm-3
• Low mobility in high-3D Si density samples
• High Si density may lead to rougher interface
9
Ultra-thin channel challenges: surface depletion
• Surface depletion increases in thin channels
• Lower charge in the access regions lead to higher source resistance
10
5nm-GaN channel device design
• Graded back-barrier high mobility and t reduce the effect of trap*
• 4.5 nm of Al2O3 gate dielectric
• 1.6×1013 cm-2 in the sidewall access regions after top-AlN etch
* M -H Wong, DRC 2011.
11
Device fabrication process*
* U.Singisetti, EDL 2010.
12
DC characteristics
• Reduced short channel effects due to vertical scaling and graded barrier1
• Peak gm = 560 mS/mm, peak Id = 1.3 A/mm
• Positive threshold voltage of 1.3 V
* M -H Wong, DRC 2011.
13
DC characteristics: Ron and Rs
Gate
InN
InN
Gate
No InN
• Record low Ron = 0.61 W-mm* for Lg = 115 nm
• InN growth optimization for complete coverage near the gate
• Regrowth sheet resistance = 100 W/sq, rc = 5 W-mm
14
RF performance: peak ft
• peak ft = 115 GHz at Vds = 4.5 V and Vgs = 2.5 V
• low fmax = 30 GHz due to thin W gate ( ~ 1500 W/sq)
15
RF performance : small-signal model
Measured (circles)
Modeled (line)
S21/5
S12*3
S11
S22
• Equivalent circuit model
16
RF performance: bias dependence
• Vgs corrosponding to peak ft is 2.5 V
• Absence of drain delay
17
Conclusions and future work
• Demonstrated vertically scaled 5-nm GaN channel MOS-HFET devices
• E-mode with Vth = 1.3 V, peak gm = 560 mS/mm, peak Id = 1.3 A/mm
• Record low Ron = 0.61 W-mm, for 115 nm E-mode GaN HEMTs
• peak ft = 115 GHz for 120 nm gate length device
Future work
• Scale the gate length to 50 nm
• Top gate for fmax
• Scale the gate dielectric (HfO2, ZrO2)
This work was supported by DARPA NEXT program
18
New measurements post DRC
• peak ft = 122 GHz at Vds = 5.5 V and
Vgs = 2.5 V
• ft-Lg product of 14 GHz-mm
19
New measurements post DRC
• maximum Ion/Ioff ratio ~ 2×105
• Breakdown voltage 8. 6 V
• dielectric breakdown
20