Wide Bandgap Semiconductors
for Microwave Power Devices
Dr. C. E. Weitzel
Consultant: Wide Bandgap Power Devices
Presentation Outline
•  Why Wide Bandgap Semiconductors? •  SiC Microwave Power Devices •  What are FET Field‐Plates? • AlGaN/GaN Microwave Power Devices The Rest of the Wide Bandgap story
•  SiC produced the first commercial BLUE LED’s •  GaN produced first high brightness BLUE LED’s •  GaN based materials enabled and dominate the LED commercial market •  Low intrinsic carrier concentraKon enables significantly higher operaKng temperatures •  GaN‐on‐Si looks promising for Switching Power Devices to compete with silicon IGBT’s, silicon MOSFET’s, and SiC MOSFET’s. Why Wide Bandgap Semiconductors?
RelaKve Cost $ $$ $$$$$ $$$$$ $$$$ $$$$$$$$$$ B. Ozpineci and L.M. Tolbert, “Comparison of Wide‐Bandgap Semiconductors for Power Electronic ApplicaKons,” Oak Ridge NaKonal Laboratory, December 12, 2003 Diode Breakdown Voltages
•  Wide bandgap enables high breakdown voltages •  This enables a tradeoff between operaKng voltage and current, but in either case boosts power handling capability B. Ozpineci and L.M. Tolbert, “Comparison of Wide‐Bandgap Semiconductors for Power Electronic ApplicaKons,” Oak Ridge NaKonal Laboratory, December 12, 2003 Diode Drift Region Resistance
•  Wide bandgap enables lower device resistance for a given voltage •  Lower resistance enables higher operaKng current B. Ozpineci and L.M. Tolbert, “Comparison of Wide‐Bandgap Semiconductors for Power Electronic ApplicaKons,” Oak Ridge NaKonal Laboratory, December 12, 2003 RF Power Density / Operating Voltage
48V Micropipes Result from Screw Dislocation
Reduction of Micropipe Density / Wafer Size
Very Low Micropipe Density SiC Wafer
SiC MESFET --the first wide bandgap
microwave power transistor
SiC MESFET’s, SiC MOSFET’s & SiC SIT’s Probably Highest SiC MESFET power density ever reported I would certainly like to know more details about this device High power 4H‐SiC staKc inducKon transistors (SITs) SEM photo of a SIT device. The mesa fingers are 1 µm wide and 100 µm long. The total mesa length is 1 cm (100 fingers). Measured staKc I‐V characterisKcs of a SIT Microsemi introduced a 2.2kW SiC SIT in Sept. 2010
What are Field-Plates?
aka - Step-Gates, Faraday Shields,
Source connected Field-Plates
Field‐plate is a metal structure added to a FET between the gate and drain that increases the microwave gain and/or breakdown voltage (Si, GaAs, SiC, GaN). Microwave Field-Plates
•  First demonstraKon of field‐plate on microwave transistor (GaAs) •  Source connected field‐plate sits on top of a dielectric •  RF gain is increased by making the device more unilateral Source •  CGD is reduced at the expense of CGS and CDS Gate Field Plate Dielectric Drain •  Can also increase breakdown voltage if properly designed Field‐Plate increases small signal gain approx. 3 dB 1.7 W/mm 1.5 GHz Vds = 35V 2003 Field‐Plate increases CW gain 2 dB AlGaN–GaN Field Effect Transistor
‐ the SiC MESFET “KILLER” Basic AlGaN‐GaN HeterojuncKon FET +++++++++++++++++++++ Substrate: Sapphire, SiC, or Silicon Spontaneous and Piezoelectric PolarizaKon cause +++ charge in the AlGaN. The band structure forms a quantum well at the AlGaN‐GaN interface. Electrons fill the quantum well to a very high density > 1E7 cm‐2. There is no intenKonal doping. Small Signal GaN HFET reported at SiC Conference in Kyoto, Japan 12.6 mm AlGaN/GaN HFET 48 V 2.14 GHz 74 W 5.9 W/mm No Field Plate IMS 2006 AlGaN/GaN HFET 30 V 15 GHz 60 W ~ 3 W/mm IMS 2010 Wide Bandgap Summary
•  Material properKes: ‐ high operaKng voltages ‐ high thermal conducKvity (SiC substrate) •  SiC MESFET first wide bandgap microwave power transistor •  Field Plates: ‐ increased RF gain ‐ even higher operaKng voltages •  AlGaN/GaN heterojuncKon achieves current density > 1 x 1013 A/cm2 (SiC MESFET “killer”) •  Heavily supported for US defense applicaKons, but is there a “killer” commercial applicaKon??