High Voltage GaN Devices for Photovoltaics and High Frequency Switched Power Supplies

Transcription

1 HIPER Lab Harris Integrative Photonics and Electronics Research Laboratory High Voltage GaN Devices for Photovoltaics and High Frequency Switched Power Supplies H. Rusty Harris Texas A&M University Depts. Of Physics and ECE SLAC Advanced Instrumentation Seminars July 7, 11

2 Outline Motivation GaN design for 5kV Additional device and integration needs Summary

3 Students and Collaborators PhD Students Iman Rezanejhad Derek Johnson Mary Coan Jung Hwan Woo Collaborators Mark Holtz and Sergey Nikishin Texas Tech University Eddie Piner Texas State University

4 Outline Motivation GaN design for 5kV Additional device and integration needs Summary

5 Area Needs for advanced HV switches Photovoltaics RF and microwave communications High Voltage drives

6 PV Progress We re getting there

7 Switched power supplies in PV Higher voltage in the arrays will reduce resistive losses Residential: 6V Commercial: 1V

8 Higher Voltage requires more components A E B E C D Minority carriers in large drift regions cause reverse current after switching E E A B C D

9 Medium Voltage energy advantage Clear Advantage in terms of scaling for MV applications 6 MW LV (69 V) full-power converter system for wind turbine 6 MW MV (33 V) full-power converter for wind turbine

10 Microwave Switches- RF Switches

11 Outline Motivation GaN design for 5kV Additional device and integration needs Summary

12 On-State Resistance (milli Ohms.Cm^) Current Status of Alternate switching technology Matsushit Toshiba [9] Sanke US [8] [7] [6] GaN- Limit Kansai Power Si- Electronics & CREE Limit Mitsubishi Siemens SICE Siemens CREE Huang SICE D CELEGY D Furukawa Denso Purdue SiC- Limit Breakdown Voltage (V) GaN SiC

13 Benefits of GaN HEMT in HV applications AlGaN/GaN forms a high density D Electron Gas Low on state resistance! We can grow high quality GaN on a Si platform Up to 3mm possible!

14 Our Goal Use the HEMT architecture and all its benefits, but still retain high voltage blocking characteristics

15 Spontaneous and Piezoelectric Polarizations j ij p i e P,,,],, [ 3 1 ^ 31 ^ e e 31 e 33 e e 14 e 15 ] [,, ^ ^ e e e P p ) ( c c e e a a a e e P P AlGaN AlGaN GaN p z PE

16 Degree of Relaxation, r(x) Degree of Relaxation Any strained epitaxial films will have partial, spatially dependant relaxation below the critical thickness (T c ) Above T c, full relaxation occurs and the only polarization left is P SP 1.8 P PE a [1 r( x)] a r( x) a GaN r( x) min[3.5 ( x a a AlGaN AlGaN AlGaN, strained AlGaN, relaxed x 1 ),1] ( e a a 31 GaN GaN e 33 c c x x x 1 1 x 1 ).6 r(x) = 3.5 (x -.35) Al Mole Fraction, x

17 Parametric numerical modeling of polarization Any field above Ecrit will result in device failure Collapse to universal polarization above T c E crit, AlGaN 8.4x 3.3( MV / Cm)

18 DEG Sheet Carrier Concentration (1/Cm) Resulting Sheet Carrier Density Consistently greater than 1 1 cm -3 sheet charge density We now have a parametric design template for concentration in terms of Al fraction and thickness 1 13 d = 6nm d = 1nm d = nm d = 3nm d = 4nm d = 5nm n s e AlGaN ) [ eb E de ( P SP, AlGaN PPE, AlGaN PSP, GaN e b E C 1.3x.84( ev ) 1.x.7x ( ev ) 9e n s 3 E F [ ] / * * 8 m AlGaN m 8 F E n s C ] Al Mole Fraction, x

19 DEG Sheet Resistivity (Ohms) And (almost) finally DEG resistivity Now we can design our device DEG 1 en s s d = 6nm d = 1nm d = nm d = 3nm d = 4nm d = 5nm Al Mole Fraction, x

20 Device consideration Our goal is to use the DEG sheet resistance to create a device that uniformly drops 5kV without any formation of electric field above E crit E V / L x gd gd E AlGaN E P E x E E AlGaN = E crit,algan crit, AlGaN EP Ex EP ( Vgd / Lgd) EP ( VBR / Lgd) L gd V BR / Ecrit, AlGaN EP

21 Breakdown in GaN Peizoelectric field is negligible in GaN Smaller subset of previous Lgd parameters due to substrate breakdown E GaN E E ' x E ' SP V ( L V dg gd ) P ( P SP, GaN GaN BR SP, GaN crit, GaN ( ) ( ) Lgd GaN )

22 Drain Resistance (milli Ohms.Cm) The result A full set of design parameters that allow HV design with ultralow on-state resistance AlGaN Thickness AlGaN concentration Gate-drain distance Need corroboration! Al Mole Fraction, x d = 6nm d = 1nm d = nm d = 3nm d = 4nm d = 5nm

23 Outline Motivation GaN design for 5kV Additional device and integration needs Summary

24 Caveat Emptor Other consideration that we are researching Enhancement mode operation GaN breakdown to substrate Alternate electric field smoothing techniques Thermal management

25 Depletion HEMT to Enhancement MOSHFET HEMT has Negative Threshold Voltage! Application of a dielectric and high work function metal lifts the Fermi level, removing the equilibrium DEG Φ~5.1eV DEG ΔP InAlN-InGaN Al y Ga 1-y N GaN Metal Al y Ga 1-y N GaN

26 Alternate Field Smoothing Source Gate Dielectric Gate Gate Field Plate (FP) Passivation Dielectric (translucent) Drain Field Plate 6 July 11

27 Summary Pushing high voltage design of low Ron,sp GaN HEMT devices opens up miniaturization, integration and efficiency options A first-time complete design spec outlined for HV GaN HEMTs Alternate means being developed to eliminate additional device design constraints