A New Generation of Power Semiconductor Devices - SAAEI

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


Introduction

Power Electronics is: “efficient processing of electrical energy through means of electronic switching devices ”

40% of Energy consumed as electricity


Energy Distribution Communications Traction/Automotive

Introduction


Power Devices

Classification of High Voltage Devices


Si Power Devices

Si Power Devices


Si Power Devices

GTO, Power MOSFET and Cool MOS Voltage Range Power MOSFET Power supplies

Motor control GTO Thyristor

Cool MOS

Electric cars

Traction & HVDC


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), nbase & 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 Gate Source/Cathode

Source/Cathode

n + p well

n

+ p well p+

p+

n- drift region p +

Anode


Si IGBTs

IGBT turn-off Characteristics

(2) (1) (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

Ecr

Ecr

Punch-Through (PT IGBT)

Safety distance

Non Punch-Through (NPT IGBT)


Si IGBTs

Trench IGBT Cross Sections

4μm 5μm

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 Channel

Electron injector Cathode

Cathode p+

n+ p -well

n+

p+

p -well

Gate

n- drift region n buffer P anode

Anode

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


Si IGBTs

On-state Characteristics of a TIGBT


Si IGBTs

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

Source/Cath

n + p well

120μ

NPT - IGBT Source/Cath Gate

n +p well

n + p well

120μ m

n- drift region

m 15μm N-buffer

SPT - IGBT Gate Source/Cath

200μm

n- drift region

1- 2 μm

1 μm

250μ m p+ (substrate)

1μm

P transparent anode

n- buffer – field stop P transparent anode


Si IGBTs

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

PT -IGBT

NPT -IGBT

SPT - IGBT

thin

thick

thin

Epitaxial

Float zone (FZ)

Float Zone (FZ)

Thick and highly doped

N/A

Thin and lowly doped

Thick and highly doped (whole substrate)

Thin and relatively lowly doped

Thin and relatively lowly doped

Lifetime killing

Injection efficiency

Injection efficiency

On-state losses

low

medium

low

Switching losses

high

medium

low

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

Drift layer thickness Wafer type (for 600 V and 1.2 kV) Buffer Layer P+ anode injector Bipolar gain control

Turn-off tail


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

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 1,25

Al SiO2

1,00

Poly Si N Body-P

0,75

+

Epitaxy -N

0,50 -

Substrate-P+

Junction supporting forward bias

Junction supporting reverse bias

IC (mA)

P

3328-RBI Wafer 11 Bidirectional Blocking Capability

0,25 0,00 -0,25 -0,50 -0,75

RB-IGBT (G-E short)

-1,00 + Substrate-P

-1,25 -800 -600 -400 -200

0

200 400 600 800

VCE (V)


Si Super-junctions

Super-Junction MOSFETS

COOLMOS

Rectangular e-field distribution allows increasing Nepi doping. RonxA below Si limit

A New Generation of Power Semiconductor Devices WBG Semiconductors

WBG Power Devices


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.


Physical properties of WBG for Power Devices

Material Si 4H - SiC GaN Diamond

Eg

μn

μp

Vsat

Ec

λ

(eV) @300K

(cm²/Vs)

(cm²/Vs)

(cm/s)

(V/cm )

(W/cm.ºK)

εr

1.12

1450

450

107

3×105

1.3

11.7

3.2

950

115

2 × 107

3 × 106

5

10

3.39

1000

350

2 × 107

5 × 106

1.3

8.9

5.6

2200

1800

3 × 107

5 × 107

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

A New Generation of Power Semiconductor Devices SiC Power Switches (unipolar) • • •

Main problem: Normally on (?)

•

Very low Ron Rugged Gate-structure Excellent short-circuit capability High temperature possible

x

A New Generation of Power Semiconductor Devices 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%


MOSFET Advantages

Trench/DiMOSFET

• Simple planar structrure • Voltage gate control • Extensively used in Si technology • Normally-off

MOSFET main problems

x

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

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)


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

10 kV MOSFET (Cree)

A New Generation of Power Semiconductor Devices SiC Power Switches (bipolar)

State-of-the-art [S. Krishnaswami et al., ISPSD’2006, pp. 289-292]

• • •

• 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

A New Generation of Power Semiconductor Devices 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 mΩcm2 • 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 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 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) • Wgate= 157 mm, Vth = +0.8 V • Ron ×A = 6.3 mΩ.cm2 • Ron = 72 mΩ • VBR > 800 V • IDmax > 20 A The gate leakage current four orders of magnitude smaller than 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 SiO2/GaN interface - 2.5 kV breakdown voltage - High channel mobility (170 cm2/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). • 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

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.


Thanks for your attention