Device and Monte Carlo Simulation of GaN material and devices. Presenter: Ziyang Xiao Advisor: Prof. Neil Goldsman University of Maryland

Transcription

1 Device and Monte Carlo Simulation of GaN material and devices Presenter: Ziyang Xiao Advisor: Prof. Neil Goldsman University of Maryland

2 2/23 OUTLINE - GaN Introduction and Background Device Simulation (Lateral vs Vertical) Monte Carlo Simulation for bulk GaN and 2DEG Electron Transport

3 3/23 GaN Application Advantages Superior Material Properties Technical Advantages Large Bandgap Improved transient characteristics and switching speed High saturation velocity Power System Reduction in system volume and weight High carrier density and high electron mobility High Frequency RF power

4 4/23 GaN Electron Transport S contact p-gan (CBL) Gate AlGaN UID GaN Aperture S contact p-gan (CBL) n- GaN Drift Region n+ GaN Drain D contact Figure: Sketch of Current Aperture Vertical Electron Transistor (CAVET)

5 5/23 OUTLINE - GaN Introduction and Background Device Simulation (Lateral vs Vertical) Monte Carlo Simulation for bulk GaN and 2DEG Electron Transport

6 6/23 Lateral vs. Vertical Gate S contact AlGaN S contact I p-gan (CBL) UID GaN Aperture I n- GaN Drift Region p-gan (CBL) n+ GaN Drain D contact Figure: Sketch of Current Aperture Vertical Electron Transistor (CAVET)

7 7/23 Lateral vs. Vertical Lateral: Low parasitic capacitance thus low conduction loss and low switching losses Relatively simpler fabrication process Easier to obtain bi-directional switch. Increase of breakdown voltage increases the chip sizes Current flows near the device surface. Thus current collapse phenomenon and increase dynamic on-resistance is more serious Vertical: Require high quality native substrate (GaN substrate) More complex fabrication process The increase of breakdown results in the increase of the thickness of the device, thus expecting to achieve a higher power density. Current flows through the bulk region away from the surface, expecting to have less current collapse.

8 Source 8/23 Simulated Devices (Lateral) Gate AlGaN n type: cm 3 Drain GaN n type: cm 3 p type: cm 3

9 I d (A) Gate Sweep (Lateral) V g (V) Figure: Sheet electron density at the interface vs. applied gate voltage. V d = 0V, V s = 0V. V g (V) Figure: Drain current with gate sweeping of the simulated lateral device. V d = 0.02V, V s = 0V. 9/23

10 I d (A) I d (A) IV characters (lateral) V g = 0, 2, 4, 6, 8, 10V V g = 0, 2, 4, 6, 8, 10V V ds (V) Figure: I-V character curve of simulated lateral device V ds (V) Figure: Zoom in on the I-V character curve in the 0-10V range 10/23

11 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

12 12/23 Simulated Devices (Vertical) Source Gate n AlGaN: cm 3 Source n GaN: cm 3 p GaN: cm 3 CBL n GaN: cm 3 p GaN: cm 3 CBL n GaN: cm 3

13 I d (A) Gate Sweep (Vertical vs. Lateral) V g (V) Figure: Sheet electron density comparison( between lateral and vertical device) at the interface vs. applied gate voltage. V d = 0V, V s = 0V. V g (V) Figure: Drain current comparison( between lateral and vertical device) with gate sweeping of the simulated lateral device. V d = 0.02V, V s = 0V. 13/23

14 I d (A) I d (A) IV character (Vertical) V g = 0V V g = 3V V g = 0V V g = 4V V g = 2V V g = 5V V g = 6V V g = 4V V g = 6V V g = 8,10V V ds (V) Figure: I-V character of the simulated vertical device V ds (V) Figure: I-V character of the simulated lateral device 14/23

15 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

16 16/23 OUTLINE - GaN Introduction and Background Device Simulation (Lateral vs Vertical) Monte Carlo Simulation for bulk GaN and 2DEG Electron Transport

17 Energy (ev) 17/23 Bulk GaN Monte Carlo Simulation The GaN bulk Monte Carlo is based on a three-valley model (Γ 1 valley, Γ 3 valley and U valley), among which Γ 1 valley handles mostly low electrical field scattering events, while the Γ 3 valley and U valley will participate in the high field scattering. The included scattering types are: acoustic phonon scattering, piezoelectric scattering, impurity scattering, polar optical scattering, inter-valley scattering. Valley Three-valley model parameters Offset Effective mass Nonparabolicity Γ 1 0 ev 0.2m ev 1 Γ ev m ev 1 U(L M) 2.1 ev m ev 1 U A L M Γ A Figure: EPM calculated conduction band structure with illustration of included valleys for Monte Carlo simulation Γ 3 Γ 1

18 Energy (ev) MC Results (Velocity and Valley Occupancy) U A L M Γ A Γ 1 Γ 3 Figure: Average drift velocity vs. electric field (full range: 0-450kV/cm) with impurity concentration of cm 3. The inserts are the distribution of the drift velocity at selected electrical field. Figure: Valley occupation vs. electric field (full range: kv/cm) with impurity concentration of cm 3. The insert is part of the conduction band structure of GaN and the approximated three valley model used in the simulation 18/23

19 MC Results (Mobility) This Work Exp. Data 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 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. 19/23

20 20/23 2DEG Monte Carlo Simulation 1. F int determines where the subbands are located inside the potential well; E n = ћ2 2m 1 3 3π 2 qf int n φ n z = A A i 2m qf int ћ z E n qf int E t 1. E t determines how many subbands are included in the 2D Monte Carlo simulation 2. If the electron energy is below E t, it will be considered under 2D scattering. Figure: The approximated wave function Ψ 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) F int =0.057V/nm, E t =0.45eV; (b) F int =0.116V/nm, E t =0.75eV. 3. If the electron energy is above E t, it is regarded as being in 3D scattering realm.

21 21/23 2D MC Results (Drift Velocity) Table: Parameters for triangular potential wells labeled Case(a) and Case(b) in the figure on the left implemented in 2D Monte Carlo simulation E t (ev) F int (V/nm) Case(a) Case(b) 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 cm 3.

22 2D MC Results (Mobility) Exp. Data This work 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, [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, [8] Katz el ta. 2003, IEEE Transactions on Electron Devices 50, 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. 22/23

23 23/23 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.

24 Thank you! Any Questions?

25 25/23 Backup Slides Figure: Average electron energy vs. electric field (full range: kv/cm) with impurity concentration of cm 3. The inserts are the distribution of the electron energy at selected electric field

26 Scattering rate (s^-1) Scattering rate (s^-1) Scattering rate (s^-1) 2D MC Analysis 1. 2DEG shows higher mobility that 3D bulk Possibly because of the absence of impurity scattering The quantized energy levels possibly lower the crossover between the original state and possible final states to be scattered into: S k k = A k V k 2. 2DEG mobility differs from one another with different quantum well structure (i.e. different F int and E t ) Future work is needed to reveal the relationship between the mobility and the quantum well structural parameter (a) (b) (c) Electron Energy (ev) Electron Energy (ev) Electron Energy (ev) Acoustic Polar Optical Emission Polar Optical Absorption Figure: Scattering rate comparison between 3D scattering (blue) and 2D scattering (Orange) with electrons starting from 1 st subband (a), 2 nd subband (b) and 3 rd subband, respectively. The structural parameter for the calculation is from Case(a) mentioned in the previous slide 26/23