A New Generation of Power Semiconductor Devices

A New Generation of Power Semiconductor Devices José Millán Centro Nacional de Microelectrónica, CNM CNM-CSIC, Campus Universitad Autónoma de Barcelona, 08193 Bellaterra, Barcelona, Spain

Outline Introduction Si Power Devices Si IGBTs Si Super-junctions SiC Power Devices SiC Power Rectifiers SiC Power Switches GaN Power Devices WBG Future Trends

Introduction Power Electronics is: efficient processing of electrical energy through means of electronic switching devices 40% of Energy consumed as electricity

Introduction Energy Distribution Communications Traction/Automotive

Power Devices Classification of High Voltage Devices

Si Power Devices Si Power Devices

GTO, Power MOSFET and Cool MOS Voltage Range Power MOSFET Si Power Devices Cool MOS Power supplies Electric cars Motor control Traction & HVDC GTO Thyristor

Si IGBTs IGBT Structure & Output Characteristics Structure of DMOS IGBT Static Characteristics Current x10 compared with power MOSFET

Si IGBTs IGBT OFF-state The p -base/n-base junction blocks the voltage while the device is in the off-state

Si IGBTs - IGBT ON-state When the device is in the on-state the electron current at the cathode flows through the channel like in a MOSFET and acts as the base current for the pnp transistor formed between the p+ anode-(emitter), n- base & n+ buffer (base) and p-base (collector). Due to high level of injection in the on-state the entire n-base is modulated by mobile carriers in equilibrium with an effective charge of few orders of magnitude higher than the original doping

Si IGBTs The IGBT Equivalent Circuit The IGBT has within its structure three MOS- bipolar devices: (i) The cascade MOSFET - PIN diode (ii) MOS base current controlled - wide base PNP transistor (iii) Parasitic MOS turn-on thyristor - must be always suppressed Source/Cathode Gate Source/Cathode n + p well p well n + p+ p+ p + Anode n- drift region

Si IGBTs IGBT turn-off Characteristics (1) (2) (3) (4) Examples of measured IGBT turn-off characteristics in inductive conditions. The characteristics are plotted for different rail voltages. There are three distinctive regions (1) voltage rise (2) electron current fall, (3) removal of main charge stored in the drift region (4) current tail through recombination

Si IGBTs Three concepts that led to major advancements in IGBTs from one generation to another Trench and thin wafer technologies led to ~30 % cut in the on-state voltage drop PIN diode effect Enhanced injection of electrons at the top side (channel side) of the drift region led to a further 20% decrease in the on-state voltage drop Field stop (Soft Punch Through) technology led to ~20% cut in the turn-off losses and 10-20% decrease in the on-state voltage drop

Si IGBTs PT & NPT IGBT Structures E cr E cr Punch-Through (PT IGBT) Safety distance Non Punch-Through (NPT IGBT)

Si IGBTs Trench IGBT Cross Sections 5μm 4μm Schematic SEM

Si IGBTs Breakdown vs on-state in DMOS IGBT & Trench IGBT

Si IGBTs The ability to engineer the PIN diode section in the TIGBT can be used to optimise its performance Cathode Channel p+ n+ n+ p+ p -well Gate Electron injector Cathode p -well The heavily charged accumulation layer serves as an electron injector forming a PIN diode with n-drift region and p-anode There are two paths for the current flow: (i) the double sided injection path of the PIN diode with increased plasma at both injection ends (anode and cathode end), and (ii) the pnp path with increased plasma only at the IGBT anode end. n- drift region n buffer P anode Anode Increasing the PIN diode contribution over that of the pnp transistor is the key to enhance the device performance This is equivalent to suppressing the collection of holes by the p well to the cathode short

Si IGBTs On-state Characteristics of a TIGBT

The Field Stop (or Soft Punch-Through), PT and NPT structures Source/Cath PT - IGBT NPT -IGBT SPT -IGBT Gate Source/Cath Gate Source/Cath Gate n + p well n + p well Si IGBTs n + p well 120μ m 15μm n- drift region N-buffer 200μm 120μ m 1-2 μm 1 μm n- drift region n- buffer field stop P transparent anode 250μ m p+ (substrate) 1μm P transparent anode

Si IGBTs The Field Stop (or Soft Punch-Through), PT and NPT comparison Structure PT -IGBT NPT -IGBT SPT - IGBT Drift layer thickness thin thick thin Wafer type (for 600 V Epitaxial Float zone (FZ) Float Zone (FZ) and 1.2 kv) Buffer Layer Thick and highly doped N/A Thin and lowly doped P+ anode injector Thick and highly doped Thin and relatively Thin and relatively (whole substrate) lowly doped lowly doped Bipolar gain control Lifetime killing Injection efficiency Injection efficiency On-state losses low medium low Switching losses high medium low Turn-off tail short long short Voltage overshoot (in high low low some applications) Temperature coefficient negative (mostly) positive positive SCSOA (short circuit conditions) RBSOA (reverse bias conditions) medium large large narrow large large

1.2 kv IGBTs. SPT has a better carrier profile than the PT and NPT structures with the Trench SPT showing the most favorable result. Si IGBTs

Si IGBTs The trade-off between on-state voltage and turn-off energy losses for 1.2 kv DMOS PT IGBT, the Trench IGBT and the Trench SPT IGBT

Si IGBTs The Reverse Conducting IGBT n+ n+ M. Rahimo, 3.3 kv RC IGBT using SPT+ technology (ISPSD 2008) H. Takahashi, 1.2 kv Reverse Conducting IGBT (ISPSD 2004), Mitsubishi

Si IGBTs The Reverse Blocking IGBT 600V RB-IGBT designed and fabricated at CNM Additional protection of IGBT periphery: trench isolation (patent pending) Applications: Current inverters, resonant converters, Matrix converters, BDS P + - Epitaxy -N Substrate-P+ Al SiO 2 Poly Si N Body-P Junction supporting forward bias Junction supporting reverse bias + Substrate-P I C (ma) 1,25 1,00 0,75 0,50 0,25 0,00-0,25-0,50-0,75-1,00-1,25 3328-RBI Wafer 11 Bidirectional Blocking Capability RB-IGBT (G-E short) -800-600 -400-200 0 200 400 600 800 V CE (V)

Si Super-junctions Super-Junction MOSFETS COOLMOS Rectangular e-field distribution allows increasing Nepi doping. RonxA below Si limit

WBG Semiconductors WBG Power Devices

WBG Semiconductors Why WBG Semiconductors? Si devices are limited to operation at junction temperatures lower than 200 ºC Si power devices not suitable at very high frequencies SiC and GaN offer the potential to overcome both the temperature, frequency and power management limitations of Si.

WBG Semiconductors Physical properties of WBG for Power Devices Material E g (ev) @300K μ n (cm²/vs) μ p (cm²/vs) V sat (cm/s) E c (V/cm ) λ (W/cm.ºK) ε r Si 1.12 1450 450 10 7 3 10 5 1.3 11.7 4H - SiC 3.2 950 115 2 10 7 3 10 6 5 10 GaN 3.39 1000 350 2 10 7 5 10 6 1.3 8.9 Diamond 5.6 2200 1800 3 10 7 5 10 7 20 5.7

WBG Technology GaN & SiC process technologies are more mature At present, SiC is considered to have the best trade-off between properties and commercial maturity GaN can offer better HF and HV performances, but the lack of good quality large area substrates is a disadvantage for vertical devices GaN presents a lower thermal conductivity than SiC GaN allows forming 2DEG heterojunctions (InAlGaN alloys) grown on SiC or Si substrates Currently, it is a sort of competition SiC vs GaN, in a battle of performance versus cost There is not a clear winner at the moment. They will find their respective application niches with a tremendous potential market

SiC Power Devices SiC Power Devices

SiC Power Diodes SiC Power Rectifiers Schottky barrier diodes (SBD): extremely high switching speed but lower blocking voltage and high leakage current. PiN diodes: high-voltage operation and low leakage current, reverse recovery charging during switching. Junction Barrier Schottky (JBS) diodes: Schottky-like on-state and switching characteristics, and PiN-like off-state characteristics.

SiC Power Diodes State-of-the-Art SiC rectifiers Schottky and now JBS diodes are commercially available up to 1.2 kv: CREE, Infineon basically. PiN diodes will be only relevant for BV over 3 kv. - Need to overcome its reliability problem (forward voltage drift) before commercialisation

SiC Power Switches

SiC Power Switches (unipolar) x Main problem: Normally on (?) Very low R on Rugged Gate-structure Excellent short-circuit capability High temperature possible

SiC Power Switches (unipolar) Hybrid Si/SiC cascode electric switch Compared to a COOLMOS based converter, the SiCbased one offers the highest efficiency (90%) All SiC sparse matix converters CoolMOS + SiC efficiency higher than 96% All SiC sparse matrix converter: 100 KHz 1.5 kw efficiency 94% 1300V 4 A SiCED Cascodes + 1200 V 5 A CREE Schottky diodes 3 phase PWM rectifier 10kW 500KHz 480V CoolMOS + SiC Schottky diodes : efficiency higher than 96%

SiC Power Switches (unipolar) Trench/DiMOSFET MOSFET Advantages Simple planar structrure Voltage gate control Extensively used in Si technology Normally-off x MOSFET main problems Low channel mobility in SiC High temperature operation? Gate reliability? Lateral DMOFET

SiC Power Switches (unipolar) CREE: 2.3KV-5A R on =0.48 Ω (25ºC) 13.5mΩ.cm 2, Ir=200uA. Cin=380pF, Cout=100pF, reverse transfer C=19pF (Vgs=0,Vds=25V, 1MHz) Infineon: 1200V-10A, Ron=0.27 Ω (25ºC) 12mΩ.cm 2 Denso: 1200V-10A, 5 mω.cm 2 (25ºC), 8.5mΩ.cm 2 (150ºC)

SiC Power Switches (unipolar) [M. Das et al. at ISPSD 2008, pp. 253-259] 10 kv MOSFET (Cree)

SiC Power Switches (bipolar) State-of-the-art [S. Krishnaswami et al., ISPSD 2006, pp. 289-292] 3500 V - 6500 V range 4 kv, 10 A BJT βmax = 34 chip area = 4.24 mm 4.24 mm IR =50 µa @ 4.7 kv turn-on time = 168 ns @ RT turn-off time = 106 ns @ RT Unlike Si BJT, SiC BJT does not suffer from a secondary breakdown ß is reduced (50%) under bias stress (stacking faults base-emitter region)

SiC Power Switches (bipolar) SiC IGBT? Problems of MOSFETS (Channel mobility, reliability) Problems of Bipolar (current gain degradation, stacking faults) Problems of highly doped P substrate growth May 2008 (ISPSD 2008): CREE 10kV n-channel IGBT 3V knee, 14.3 mωcm 2 At 200ºC the n-igbt operates at 2 the current density of the n-mosfet

GaN Power Devices GaN Power Devices

GaN Power Diodes GaN Power Rectifiers Until recently, because of the lack of electrically conducting GaN substrates, GaN Schottky diodes were either lateral or quasi-vertical Breakdown voltages of lateral GaN rectifiers on Sapphire substrates as high as 9.7 kv have been reported Zhang et al. IEEE T-ED,48, 407, 2001 SBD PiN

GaN Power HEMTs GaN Power HEMTs GaN HEMTs have attracted most attention with impressive trade-off between Ron vs BV Power densities 1.1 W/mm in 1996 initially to microwave power HEMTs with high output power capability as high as 40 W/mm recently A major obstacle trapping effects though drain-current collapse Several solutions : (1) surface-charge-controlled n-gan-cap structure (2) the recessed gate and field-modulating plate structure (3) passivation of surface states via silicon nitride or other dielectric.

GaN Power HEMTs High voltage AlGaN/GaN HEMTs over 1 kv were reported in 2006 S. YOSHIDA et al. ISPSD 2006 It has been also demonstrated a GaN power switch for kw power conversion. The switch shows a speed grater than 2 MHz with rise- and fall-time of less than 25 ns, and turn-on/turn-off switching losses of 11 µj with a resistive load. Switching at 100 V/11 A and 40 V/23 A was achieved with resistive and inductive loads, respectively.

GaN Power HEMTs Via-holes through sapphire at the drain electrodes enable very efficient layout of the lateral HFET array as well as better heat dissipation 8.3 kv HEMT (Panasonic) Y. Uemoto et al. IEDM 2007

GaN Power HEMTs GaN Power HEMTs The state-of-the-art AlGaN/GaN HEMT [T. Nomura et al., ISPSD 2006, pp. 313-316] Process technology based on a tri-metal Ti/AlSi/Mo layer very low contact resistance and an excellent surface morphology. Mo (barrier metal) to improve the surface morphology AlSi results more efficient for a low contact resistance than Al. Low stress, high-refractive index SiN x layer Gate leakage current as low as 10-7 A/mm. R on = 6.3 mω.cm 2, V BR = 750 V. Turn-on time: 7.2 ns (1/10 of Si MOSFET). Switching operation no significant degraded at 225ºC.

GaN Power HEMTs GaN Power normally-off AlGaN/GaN HEMTs The state-of-the-art normally-off AlGaN/GaN HEMT [N. Kaneko et al., ISPSD 2009, pp. 25-28] Recess gate electrode and NiOx as gate electrode (NiOx operates as a p-type) W gate = 157 mm, V th = +0.8 V R on A = 6.3 mω.cm 2 R on = 72 mω V BR > 800 V I Dmax > 20 A Thegateleakagecurrentfourordersofmagnitudesmallerthan the conventional normally-on HFETs.

GaN Power MOSFETs Lateral GaN MOSFETs Lateral MOSFETs have been fabricated on p-gan epilayer (MOCVD) on sapphire substrates [W. Huang et al., ISPSD 2008, pp. 291]. - High quality SiO 2 /GaN interface - 2.5 kv breakdown voltage - High channel mobility (170 cm 2 /V.s) Lateral GaN MOSFETs can compete with SiC MOSFETs and GaN HEMTs? Reduction of source/drain resistance is crucial to further reduce the device on-resistance.

WBG Future Trends WBG Future Trends SiC Switches Successful demonstration of the cascode pair (a highvoltage, normally-on SiC JFET + a low-voltage Si MOSFET). An industrial normally-off SiC switch is expected. It could be the SiC MOSFET (<5kV) or the SiC IGBT (>5kV). BJTs/Darlingtons are promising, they also suffer from reliability problems. A normally-off SiC power transistor in the BV range of 600V-1200V available within next two years.

GaN Power HEMTs GaN Power Devices WBG Future Trends GaN is already commercialised in optoelectronics. Its applications in power switching still require further work in materials, processing and device design. GaN HEMT (5-10 A, 600-1200 V normally-off) It will be interesting to see if GaN power devices, especially low cost Schottky diode, can overtake or displace SiC diodes.

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