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


• Introduction

• Si Power Devices• Si IGBTs• Si Super-junctions

• SiC Power Devices

• SiC Power Rectifiers

• SiC Power Switches

• GaN Power Devices

• WBG Future Trends

Outline


“efficient processing of electrical energy through means of electronic switching devices ”

Power Electronics is:

40% of Energy consumed as electricity

Introduction


Traction/AutomotiveCommunicationsEnergy Distribution

Introduction


Classification of High Voltage Devices

Power Devices


Si Power Devices

Si Power Devices


GTO, Power MOSFET and Cool MOS Voltage Range

Power MOSFET

Cool MOS

GTO Thyristor

Power supplies

Electric cars

Motor control

Traction & HVDC

Si Power Devices


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 pnptransistor 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


p+

n- drift region

Source/Cathode

Gate

Source/Cathode

Anode

n+

n+

p well p well

p+p+

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

The IGBT Equivalent Circuit

Si IGBTs


IGBT turn-off Characteristics

(2)

(3) (4)

(1)

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

Punch-Through (PT IGBT)

Non Punch-Through (NPT IGBT)

Ecr Ecr

Safety distance

Si IGBTs


Trench IGBT Cross Sections

SchematicSchematic SEMSEM

4μm5μm

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

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.

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

n buffer

n- drift region

Cathode

Anode

n+ n+

p -well p -wellGate

Channel

Cathode

P anode

p+ p+

Electron injector

Si IGBTs


On-state Characteristics of a TIGBT

Si IGBTs


The Field Stop (or Soft Punch-Through),PT and NPT structures

n- buffer – field stop

P transparent anode

p+ (substrate)

n- drift region

GateSource/Cath

n+p well

250μm

120μm

N-buffer15μm

P transparent anode

n+p well

200μm

1μm

n- drift region

n+p well

120μm

1- 2 μm

1 μm

PT - IGBT NPT - IGBT SPT - IGBTSource/Cath Gate GateSource/Cath

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 and 1.2 kV)

Epitaxial Float zone (FZ) Float Zone (FZ)

Buffer Layer Thick and highly doped N/A Thin and lowly doped

P+ anode injector Thick and highly doped (whole substrate)

Thin and relatively 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 some applications)

high low low

Temperature coefficient negative (mostly) positive positive

SCSOA (short circuit conditions)

medium large large

RBSOA (reverse bias conditions)

narrow large large

Structure PT -IGBT NPT -IGBT SPT - IGBT

Drift layer thickness thin thick thin

Wafer type (for 600 V and 1.2 kV)

Epitaxial Float zone (FZ) Float Zone (FZ)

Buffer Layer Thick and highly doped N/A Thin and lowly doped

P+ anode injector Thick and highly doped (whole substrate)



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 some applications)

high low low

Temperature coefficient negative (mostly) positive positive

SCSOA (short circuit conditions)

medium large large

RBSOA (reverse bias conditions)

narrow large large

Si IGBTs


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


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


n+ n+

H. Takahashi, 1.2 kV Reverse Conducting IGBT (ISPSD 2004), Mitsubishi

M. Rahimo, 3.3 kV RC IGBT using SPT+ technology (ISPSD 2008)

The Reverse Conducting IGBT

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

N

Al

SiO2

Poly Si

P +

Junction supportingforward bias

Body-P

Epitaxy -N

Substrate-P

Substrate-P

+

+

-

Junction supportingreverse bias

-800 -600 -400 -200 0 200 400 600 800-1,25-1,00-0,75-0,50-0,250,000,250,500,751,001,25

I C (m

A)

VCE (V)

RB-IGBT(G-E short)

3328-RBI Wafer 11 Bidirectional Blocking Capability

Si IGBTs


Super-Junction MOSFETS

COOLMOSRectangular e-field distributionallows increasing Nepi doping.

RonxA below Si limit

Si Super-junctions


WBG Power Devices

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.

Why WBG Semiconductors?

WBG Semiconductors


Physical properties of WBG for Power Devices

Material Eg (eV)@300K

μn(cm²/Vs)

μp(cm²/Vs)

Vsat(cm/s)

Ec(V/cm )

λ(W/cm.ºK)

εr

Si 1.12 1450 450 107 3×105 1.3 11.7

4H - SiC 3.2 950 115 2 × 107 3 × 106 5 10

GaN 3.39 1000 350 2 × 107 5 × 106 1.3 8.9

Diamond 5.6 2200 1800 3 × 107 5 × 107 20 5.7

WBG Semiconductors


• 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 (InAlGaNalloys) 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

WBG Technology


SiC Power Devices

SiC Power Devices


• 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 Diodes

A New Generation of Power Semiconductor Devices SiC Power Switches


• Very low Ron

• Rugged Gate-structure• Excellent short-circuit

capability• High temperature possibleMain problem: Normally on (?)x

SiC Power Switches (unipolar)


• Compared to a COOLMOS –based converter, the SiC-based one offers the highest efficiency (90%)

• All SiC sparse matixconverters

• CoolMOS + SiC efficiency higher than 96%

Hybrid Si/SiC cascode electric switch

• All SiC sparse matrix converter: 100 KHz – 1.5 kW – efficiency 94% 1300V 4 A SiCED Cascodes + 1200 V 5 A CREE Schottkydiodes

• 3 phase PWM rectifier 10kW – 500KHz – 480V CoolMOS + SiCSchottky diodes : efficiency higher than 96%



• Simple planar structrure• Voltage gate control• Extensively used in Si

technology• Normally-off

• Low channel mobility in SiC• High temperature operation ?• Gate reliability ?

MOSFET main problemsx

MOSFET Advantages

Trench/DiMOSFET

Lateral DMOFET



• CREE: 2.3KV-5A Ron=0.48 Ω (25ºC) 13.5mΩ.cm2, Ir=200uA. Cin=380pF, Cout=100pF, reverse transfer C=19pF (Vgs=0,Vds=25V, 1MHz)

• Infineon: 1200V-10A, Ron=0.27 Ω (25ºC) 12mΩ.cm2

• Denso: 1200V-10A, 5 mΩ.cm2 (25ºC), 8.5mΩ.cm2

(150ºC)



10 kV MOSFET (Cree)

[M. Das et al. at ISPSD’2008, pp. 253-259]



• 3500 V - 6500 V range

• Unlike Si BJT, SiC BJT does not suffer from a secondary breakdown

• ß is reduced (50%) under bias stress (stacking faults base-emitter region)

• 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

State-of-the-art

[S. Krishnaswami et al., ISPSD’2006, pp. 289-292]

SiC Power Switches (bipolar)


• Problems of MOSFETS (Channel mobility, reliability)

• Problems of Bipolar (current gain degradation, stacking faults)

• Problems of highly doped P substrate growth

SiC IGBT?

• May 2008 (ISPSD 2008): CREE 10kV n-channel IGBT

• 3V knee, 14.3 mΩcm2

• At 200ºC the n-IGBT operates at ×2 the current density of the n-MOSFET

SiC Power Switches (bipolar)


GaN Power Devices

GaN Power Devices


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 Diodes


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

• 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.

S. YOSHIDA et al. ISPSD 2006

GaN Power HEMTs


Y. Uemoto et al. IEDM 2007

8.3 kV HEMT (Panasonic)

Via-holes through sapphire at the drain electrodes enable very efficient layout of the lateral HFET array as well as better heat dissipation

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 SiNx layer →Gate leakage current as low as 10-7 A/mm. • Ron = 6.3 mΩ.cm2, VBR = 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 HEMTs


The state-of-the-art normally-off AlGaN/GaN HEMT[N. Kaneko et al. , ISPSD 2009, pp. 25-28]

• Recess gate electrode and NiOxas gate electrode(NiOx operates as a p-type)

• Wgate= 157 mm, Vth = +0.8 V• Ron ×A = 6.3 mΩ.cm2

• Ron = 72 mΩ• VBR > 800 V• IDmax > 20 AThe gate leakage current four orders of magnitude smaller thanthe conventional normally-on HFETs.

GaN Power HEMTs

GaN Power normally-off AlGaN/GaN HEMTs


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 SiO2/GaN interface

- 2.5 kV breakdown voltage

- High channel mobility (170 cm2/V.s)

• Lateral GaN MOSFETs can compete with SiC MOSFETs and GaNHEMTs?

• Reduction of source/drain resistance is crucial to further reduce the device on-resistance.

GaN Power MOSFETs


WBG Future Trends

SiC Switches

• Successful demonstration of the cascode pair (a high-voltage, 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.

WBG Future Trends


GaN Power Devices

• 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.

GaN Power HEMTs

WBG Future Trends


Thanks for your attention

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A New Generation of Power Semiconductor Devices