Wide Band-Gap Power Device 1 Contents Revisit silicon power MOSFETs Silicon limitation Silicon solution Wide Band-Gap material Characteristic of SiC Power Device Characteristic of GaN Power Device 2 1
Power semiconductor Voltage Range 3 MOSFET 4 2
DMOS in the forward blocking state 5 On-state Losses In the On-state the resistance R DS(ON) is a key figure of merit (FOM) for a MOSFET The value naturally depends on the voltage rating of a device since the epi-layer will have a higher resistance for higher voltage devices For high values of gate voltage the value of R DS(ON) is independent of drain current, but depends on gate voltage and so is usually given for R DS(ON) =10 V or above The value can range from 1mΩ (20V device) to 10Ω This figure alone is not fair comparison, a better FOM is the product of area and R DS(ON) the specific on-state resistance 6 3
R DS(ON) measurement 7 Where does R DS(ON) come from? 8 4
Limit for high voltage devices When R DS(ON) is dominated by the resistance of the epitaxial layer R drift then there is a physical limit beyond which it is not possible to exceed, given by This increase rapidly and makes the use of MOSFETs above 700V impractical R on specific 4V 2 BR = W epi = qμ n N 3 D ε S μ n E = 8.3 10 9 V BR C 2.5 Silicon 9 The Power MOSFET The table below shows the approximate contribution of each of these resistances for two extreme devices, one designed for a 30 V and one designed for 600 V device. 10 5
Solutions for Silicon Device Trench MOS technology derives from VLSI memory technology The sidewalls are made using dry etching techniques The main advantage is that more cells per unit area can be packed in thus drastically lowering the specific R DS(ON) or package size Another advantage is that the input capacitance can be reduced giving higher switching speeds Finally the Ruggedness of the device is improved; making these devices suitable for harsh environments *VLSI is shorthand for Very Large Scale Integration 11 Switching Characteristic 12 6
Trench MOSFET 13 Trench MOSFET 14 7
CoolMOS with super junction technology 15 The trench CoolMOS The ultimate power MOS structure combine both superjunction technology and trench technology The trench variant of the CoolMOS could possibly offer even lower onstate voltage drops (especially for low voltage ratings) due to the reduction in the channel resistance and the 1D natural flow of the current 16 8
IGBT Punch-through (PT) and Non-punch-through (NPT) IGBTs Higher switching rate Negative temperature coefficient Lower on-state voltage drop 17 The Field Stop (or Soft Punch-Through) 18 9
The Field Stop, PT and NPT comparison 19 The ultimate IGBT device--superjunction Trench IGBT Superjunction Trench IGBT 20 10
Alternatives to Silicon Technology Wide Band Gap Semiconductors Stronger Atomic Bonds Larger breakdown voltage Lower intrinsic carrier concentration SiC Relatively mature technology, native oxide, blue light GaN Poor thermal conductivity, no native oxide, high frequency C (diamond) No established technology at present Other WBG materials like diamond and III/V nitrides suffer from the lack of suitable substrates for epitaxial growth. 21 Physical Properties of Various Semiconductors for Power Devices 22 11
Properties of SiC compared to Si 23 Band Gap Electron energy level 24 12
Theoretical Limits 25 Power Electronics for Grid Storage 26 13
SiC Material What do they look like? 27 What is Silicon Carbide? 28 14
History of SiC First observed in 1824 by Jons Jacob Berzelius Acheson process produced an abrasive material LED was made from SiC in 1907 In 1955, Lely presented a new concept of growing high quality crystals 1978, discovery of seeded sublimation growth process by Tairov and Tsvetkov to produce substrates 1987, Cree Research was founded Cree produces 85% of material New producer now entering market 29 SiC Key Technology Problems 30 15
SiC Power Devices 31 Example-MOSFET Drift Region 32 16
Commercial SiC Diodes 33 Reverse Recovery Comparison 34 17
Commercial application 35 SiC PiN Diodes 36 18
JBS 37 State of the art SiC Diodes 38 19
SiC IGBT Problems of MOSFETs (Channel mobility, reliability) +Problems of Bipolar (current gain, degradation) +Problems of Highly doped P substrate growth September 2006: Cree 10kV P-channel IGBT 3V+20 mω x cm² V F =3.9 V at 10A instead of 4.4V for the VDMOS Improvement of channel mobility and conductivity modulation possible 39 SiC JFETs 40 20
SiC JFET state-of-the-art 41 SiC MOSFETs Recent advances in the SiC gate oxidation and device design have enabled the MOSFET to emerge as an attractive transistor for high power switching applications Previous attempts to fabricate 4H-SiC MOSFETs have suffered from poor quality of the silicon dioxide/4h-sic interface This leads to very low inversion channel mobility and large shifts in threshold voltage at elevated temperatures 42 21
SiC MOSFETs Unipolar limit 43 Physical Properties: comparison 44 22
Applications for GaN-based Devices 45 GaN High Electron Mobility Transistor (HEMT) 46 23
GaN HEMTs---Field Plates 47 Heterojunctions: band diagram 48 24
Physics of group III-Nitride Piezoelectric Polarization 49 Physics of group III-Nitride Piezoelectric Polarization 50 25
2DEG Formation 51 GaN HEMTs---Passivation Layers 52 26
GaN HEMTs---AlGaN/GaN heterostructure 53 GaN HEMTs---Buffer 54 27
GaN HEMTs---Substrate 55 GaN HEMT Operation mode 56 28
HEMT Operation mode: Blocking mode 57 E-Mode HEMT (Enhancement mode) 58 29
E-Mode HEMT (Enhancement mode) MISFET stand for Metal Insulation Semiconductor Field Effect Transistor 59 30