device and monte carlo simulation of gan material and...
TRANSCRIPT
Device and Monte Carlo Simulation of
GaN material and devices
Presenter: Ziyang Xiao
Advisor: Prof. Neil Goldsman
University of Maryland
OUTLINE - GaN
Introduction and Background
Device Simulation (Lateral vs Vertical)
Monte Carlo Simulation for bulk GaN and 2DEG Electron Transport
2/23
GaN Application Advantages
Superior Material Properties
Large Bandgap
High saturation velocity
High carrier density and high electron
mobility
Technical Advantages
Improved transient characteristics and
switching speed
Power System Reduction in system volume and weight
High Frequency RF power
3/23
GaN Electron Transport
n+ GaN Drain
UID GaN
AlGaNS contact S contact
Gate
Figure: Sketch of Current Aperture Vertical Electron Transistor (CAVET)
p-GaN (CBL) p-GaN (CBL)Aperture
n- GaN Drift Region
D contact
4/23
OUTLINE - GaN
Introduction and Background
Device Simulation (Lateral vs Vertical)
Monte Carlo Simulation for bulk GaN and 2DEG Electron Transport
5/23
Lateral vs. Vertical
n+ GaN Drain
UID GaN
AlGaNS contact S contact
Gate
Figure: Sketch of Current Aperture Vertical Electron Transistor (CAVET)
p-GaN (CBL) p-GaN (CBL)Aperture
n- GaN Drift Region
D contact
๐ฐ
๐ฐ
6/23
Lateral vs. Vertical
Lateral:
โข Low parasitic capacitance thus low conduction loss and low switching losses
โข Relatively simpler fabrication process
โข Easier to obtain bi-directional switch.
โข Increase of breakdown voltage increases the chip sizes
โข Current flows near the device surface. Thus current collapse phenomenon and increase dynamic on-resistance is more serious
Vertical:
โข Require high quality native substrate (GaNsubstrate)
โข More complex fabrication process
โข The increase of breakdown results in the increase of the thickness of the device, thus expecting to achieve a higher power density.
โข Current flows through the bulk region away from the surface, expecting to have less current collapse.
7/23
Simulated Devices (Lateral)
Sou
rce D
rain
AlGaN
๐ โ ๐๐๐๐: ๐๐๐๐๐๐โ๐
GaN
๐ โ ๐๐๐๐: ๐๐๐๐๐๐โ๐
๐ โ ๐๐๐๐: ๐๐๐๐๐๐โ๐
+++++++++++++++++++++
Gate
8/23
Gate Sweep (Lateral)
Figure: Drain current with gate sweeping of the simulated lateral device. ๐๐ = 0.02๐, ๐๐ = 0๐.
Figure: Sheet electron density at the interface vs. applied gate voltage. ๐๐ = 0๐, ๐๐ = 0๐.
๐๐(๐)
๐ผ ๐(๐ด
)
๐๐(๐)
9/23
IV characters (lateral)
Figure: I-V character curve of simulated lateral device
๐๐ = 0,โ2,โ4,โ6,โ8,โ10๐
Figure: Zoom in on the I-V character curve in the 0-10V range
๐๐ = 0,โ2,โ4,โ6,โ8,โ10๐
๐๐๐ (๐)
๐ผ ๐(๐ด
)
๐ผ ๐(๐ด
)
๐๐๐ (๐)
10/23
Electron Concentration
Figure: Animation of how electron concentration changes w.r.t. changing drain voltage at Vg = 0V
Figure: Animation of how electron concentration changes w.r.t. changing drain voltage at Vg = -6V
11/23
Simulated Devices (Vertical)
๐ โ ๐ฎ๐๐ต:๐ ร ๐๐๐๐๐๐โ๐
๐ โ ๐ฎ๐๐ต:๐ ร ๐๐๐๐๐๐โ๐
๐ โ ๐ฎ๐๐ต:๐ ร ๐๐๐๐๐๐โ๐
๐ช๐ฉ๐ณ
๐ โ ๐ฎ๐๐ต:๐ ร ๐๐๐๐๐๐โ๐
๐ช๐ฉ๐ณ
Source Source
Gate
๐ โ ๐จ๐๐ฎ๐๐ต: ๐ ร ๐๐๐๐๐๐โ๐
๐ โ ๐ฎ๐๐ต: ๐ ร ๐๐๐๐๐๐โ๐
12/23
Gate Sweep (Vertical vs. Lateral)
Figure: Sheet electron density comparison( between lateral and vertical device) at the interface vs. applied gate voltage. ๐๐ = 0๐, ๐๐ = 0๐.
Figure: Drain current comparison( between lateral and vertical device) with gate sweeping of the simulated lateral device. ๐๐ = 0.02๐, ๐๐ = 0๐.
๐๐(๐) ๐๐(๐)
๐ผ ๐(๐ด
)
13/23
IV character (Vertical)
๐๐ = 0๐
๐๐ = 3๐
๐๐ = 4๐
๐๐ = 5๐
๐๐ = 6๐
Figure: I-V character of the simulated vertical device Figure: I-V character of the simulated lateral device
๐๐ = 0๐
๐๐ = 2๐
๐๐ = 4๐
๐๐ = 6๐๐๐ = 8,10๐
๐๐๐ (๐)
๐ผ ๐(๐ด
)
๐๐๐ (๐)
๐ผ ๐(๐ด
)14/23
Electron Concentration (Vertical)
Figure: Animation of how electron concentration w.r.t. changing drain voltage at Vg = 0V
Figure: Animation of how electron concentration w.r.t. changing drain voltage at Vg = -6V
15/23
OUTLINE - GaN
Introduction and Background
Device Simulation (Lateral vs Vertical)
Monte Carlo Simulation for bulk GaN and 2DEG Electron Transport
16/23
Bulk GaN Monte Carlo Simulation
โข The GaN bulk Monte Carlo is based on a three-valley model (ฮ1 valley, ฮ3 valley and U valley), among which ฮ1 valley handles mostly low electrical field scattering events, while the ฮ3 valley and U valley will participate in the high field scattering.
Figure: EPM calculated conduction band structure with illustration of included valleys for Monte Carlo simulation
๐ด ๐ฟ ๐ ฮ ๐ด
Ener
gy (
eV)
ฮ1
ฮ3๐
Three-valley model parameters
Valley OffsetEffective
massNonparabolicity
ฮ1 0 eV 0.2๐0 0.189 ๐๐โ1
ฮ3 1.9 eV ๐0 0.065 ๐๐โ1
๐(๐ฟ โ ๐) 2.1 eV ๐0 0.029 ๐๐โ1
โข The included scattering types are: acoustic phonon scattering, piezoelectric scattering, impurity scattering, polar optical scattering, inter-valley scattering.
17/23
MC Results (Velocity and Valley Occupancy)
Figure: Valley occupation vs. electric field (full range: 0 โ450 kV/cm) with impurity concentration of 1017 ๐๐โ3. The insert is part of the conduction band structure of GaN and the approximated three valley model used in the simulation
๐ด ๐ฟ ๐ ฮ ๐ด
Ener
gy (
eV)
ฮ1
ฮ3
๐
Figure: Average drift velocity vs. electric field (full range: 0 -450kV/cm) with impurity concentration of 1017๐๐โ3. The inserts are the distribution of the drift velocity at selected electrical field.
18/23
MC Results (Mobility)
Figure: Bulk low field mobility vs. Impurity concentration extracted from MC simulation. The experimental data sets Data.1-4 are mobility values taken from references [1], [2], [3] and [4], respectively.
Reference:
[1] Rode el ta. 1995, Applied Physics Letters 66
[2] Tompkins el ta. 2015, ARL-TR-7209
[3] Tang el ta. 1999, Applied Physics Letters 74
[4] Redwing el ta. 1996, Applied Physics Letters 69
Exp. Data
This Work
19/23
2DEG Monte Carlo Simulation
Figure: The approximated wave function ฮจ 2 for two triangular potential wells. The Potential well is also shown together with the wavefunctions. The parameters are the two potential wells are: (a) ๐น๐๐๐ก=0.057V/nm, ๐ธ๐ก=0.45eV; (b) ๐น๐๐๐ก=0.116V/nm, ๐ธ๐ก=0.75eV.
1. ๐น๐๐๐ก determines where the subbands are located inside the potential well;
๐ธ๐ =ั2
2๐โ
13 3๐
2
๐๐น๐๐๐ก
๐ โ14
23
ฯ๐ ๐ง = ๐ด โ ๐ด๐
2๐โ๐๐น๐๐๐ก
ั2
13
๐ง โ๐ธ๐
๐๐น๐๐๐ก
1. ๐ธ๐ก determines how many subbands are included in the 2D Monte Carlo simulation
2. If the electron energy is below ๐ธ๐ก, it will be considered under 2D scattering.
3. If the electron energy is above ๐ธ๐ก, it is regarded as being in 3D scattering realm.
๐ธ๐ก
20/23
2D MC Results (Drift Velocity)
Figure: (Left) Average drift velocity vs. full range electrical field; (Right) Zoom-in onto the low electrical field range of the left graph. Curves labeled "Case(a)" and "Case(b)" are 2D Monte Carlo simulation results with potential well parameters listed in the table on the right. Curve labeled "3D" is the bulk Monte Carlo simulation result with impurity concentration of 1017๐๐โ3.
๐ธ๐ก(๐๐) ๐น๐๐๐ก(๐ฝ/๐๐)
Case(a) 0.45 0.057
Case(b) 0.75 0.115
Table: Parameters for triangular potential wells labeled Case(a) and Case(b) in the figure on the left implemented in 2D Monte Carlo simulation
21/23
2D MC Results (Mobility)
Figure: collections of experimental data for 2DEG mobility and the results of 2D MC simulation from this work. The experimental data sets Data.1-8 are mobility values taken from references [1]-[8], respectively.
Reference:[1] Gaska el ta. 1998, Applied Physics Letters 72[2] Wu el ta. 1996, Applied Physics Letters 69[3] Redwing el ta. 1996, Applied Physics Letters 69[4] Recht el ta. 2006, IEEE Electron Device Letters 27, 205โ207[5] Tang el ta. 1999, Applied Physics Letters 74[6] Tompkins el ta. 2015, ARL-TR-7209[7] Acar el ta. 2008, Thin Solid Films 516, 2041 โ2044[8] Katz el ta. 2003, IEEE Transactions on Electron Devices 50, 2002โ2008
Exp. Data
This work
22/23
Conclusion
1. Both lateral and vertical devices simulated are normally-ON devices due to the presents
of the polarization induced charges at the interface.
2. The conductivity of the both vertical and lateral devices are mainly dominated by the
channel of 2DEG at the interface of GaN/AlGaN.
3. Pinch-off in lateral device happens under the gate edge near the drain side, while in
vertical device, the pinch-off happens in the aperture region.
4. More scattering mechanisms needs to be included to account for the discrepancies for
bulk MC simulation while not for 2DEG MC simulation.
23/23
Thank you!Any Questions?
Backup Slides
Figure: Average electron energy vs. electric field (full range: 0 โ 450 kV/cm) with impurity concentration of 1017๐๐โ3. The inserts are the distribution of the electron energy at selected electric field
25/23
2D MC Analysis
1. 2DEG shows higher mobility that 3D bulk
โข Possibly because of the absence of impurity
scattering
โข The quantized energy levels possibly lower
the crossover between the original state and
possible final states to be scattered into:
๐๐โ๐โฒ = ๐ด ๐|โ๐| ๐โฒ
2. 2DEG mobility differs from one another with
different quantum well structure (i.e. different
๐น๐๐๐ก and ๐ธ๐ก)
โข Future work is needed to reveal the
relationship between the mobility and the
quantum well structural parameter
Electron Energy (eV) Electron Energy (eV) Electron Energy (eV)
Scat
teri
ng
rate
(s^
-1)
Scat
teri
ng
rate
(s^
-1)
Scat
teri
ng
rate
(s^
-1)
(a) (b) (c) Aco
ustic
Polar O
ptical
Emissio
nPo
lar Op
ticalA
bso
rptio
n
Figure: Scattering rate comparison between 3D scattering (blue) and 2D scattering (Orange) with electrons starting from 1st subband (a), 2nd subband (b) and 3rd
subband, respectively. The structural parameter for the calculation is from Case(a) mentioned in the previous slide
26/23