Power Manufacturing

Commercially Available Cree Silicon Carbide Power Devices:

Historical Success of JBS Diodes and Future Power Switch Prospects

Mrinal K. Das

Cree, Inc., 4600 Silicon Dr., Durham, NC 27703, abpta9@r.postjobfree.com, 919-***-****

Keywords: SiC, JBS, Schottky, MOS, MOSFET, Power Devices

Abstract

JUNCTION BARRIER SCHOTTKY DIODES

Silicon Carbide has begun to fulfill its promise of

The Cree SiC JBS diode is a relatively simple device that

delivering next generation power devices to the power

combines the conduction benefits of a Schottky diode and

electronics market. The SiC JBS diode has witnessed

the blocking benefits of a pn junction. The key to its 600 V

explosive growth recently thanks to the demand for

and 1200 V market acceptance has been the elimination of

higher efficiency systems. While accumulating over 150

reverse recovery current resulting in higher efficiency

billion field hours, the SiC JBS failure rate has remained

switch-mode power supplies, AC motor drives and solar

an order of magnitude below the competing Si PiN diode

inverters. Since its introduction in 2004, the Cree Zero-

products. After decades of research, Cree has finally

Recovery JBS diode has accumulated over 150 billion

released the first commercial SiC MOSFET which has

hours in the field with a failure rate of less than 0.5 failures

overcome traditional gate oxide issues. The combination

per billion device hours which is an order of magnitude

of SiC MOSFETs and SiC JBS diodes allows system

lower than competing Si PiN diodes. In addition to

designers to provide the highest efficiency systems to the

improving reliability, the cost has also been reduced. R&D

marketplace. Despite the great potential for GaN power

innovations have resulted in smaller chip sizes and larger

devices, careful examination of material and device

defect free material while manufacturing experience has

qualification standards indicates that SiC power devices

reduced fabrication induced defects and lowered the cycle

should remain attractive for 600 V and higher

time. The end result is a 60% reduction in the ASP since

applications.

product introduction thereby dramatically accelerating

INTRODUCTION growth to achieve a CAGR of 68%. In FY2010, Cree

Silicon Carbide (SiC) has become the material of choice maintained its leadership in the high-efficiency power

for next generation power semiconductor devices to supplant electronics market by shipping over 70 billion Volt-Amperes

existing silicon (Si) technology. The wider bandgap, higher of SiC JBS diodes (Fig. 1) while introducing the latest

generation of Z-Rec devices that include 1700 V diodes.

thermal conductivity, and larger critical electric field allow

SiC devices to operate at higher temperatures, higher current

density, and higher blocking voltages than Si power devices.

From an application standpoint, these advantages translate

into SiC devices that allow the high speed majority carrier

devices (Schottky diodes and MOSFETs) to be used at much

higher voltage levels where typically Si minority carrier

devices (PiN diodes and IGBTs) currently exist. Hence,

system designers have the luxury of reducing conduction

and switching losses by incorporating SiC power devices to

produce higher efficiency systems. From a manufacturing

standpoint, SiC has become a mature material system that is

approaching defect-free quality with ever increasing wafer

diameters currently demonstrated at 150 mm [1]. This

combination of performance and manufacturability has

Fig. 1. Explosive growth of Cree SiC sales resulting in over 70 Giga-Volt-

resulted in the successful commercial adoption of SiC Amperes of power devices sold in FY2010.

junction barrier Schottky (JBS) diodes and the recent

commercial release of Cree s first power switch product Z- MOSFETS

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FET the 1200 V, 80 m SiC MOSFET, the first The SiC JBS diode provides an interim solution for

commercially available device of its kind.

b

system designers where the turn-on losses of the Si IGBT

are significantly reduced. Ultimately, the turn-off losses of

the switch can also be reduced by replacing the Si IGBT

CS MANTECH Conference, May 16th-19th, 2011, Palm Springs, California, USA 315

with a Cree Z-FET the first commercially released SiC

MOSFET for high volume manufacturing. Comparing total

losses of the 1200V SiC MOSFET against a commercially

available 1200 V Si IGBT, the SiC MOSFET outperforms

the Si IGBT in both conduction and switching losses

resulting in an 80% reduction in power losses at 10 kHz or

facilitating a 10x increase in the operating frequency at a 60

W power loss (Fig. 2). With the commercial release of the

Cree Z-FET, system designers finally have the

opportunity to investigate potential ultra high efficiency

systems based on all-SiC power device components.

Fig. 3. Gate oxide lifetime extrapolated from high field TDDB

measurements predicting adequately long lifetime for the SiC MOSFET at

175oC.

The other half of gate oxide reliability is the stability of

the gate electrical properties. SiC MOSFETs, like its Si

counterpart, have an inherent amount of threshold voltage

(VT) shift ~0.2 V due to border traps [4]. The careful design

of post-oxidation processing steps to minimize any

subsequent oxide trap generation has resulted in VT shift of

~1 V after HTGB stressing (Fig. 4). Although this may

appear to be a concern, it should be noted that the HTGB is a

Fig. 2. Total power loss vs. operating frequency plot showing the marked

constant gate bias applied at 150oC for 1000 hr which results

advantage of 1200 V Cree Z-FET over existing Si IGBTs.

in significant electron trapping. In the field, the MOSFET

In the course of developing the Cree Z-FET, will actually be switched on and off thereby receiving gate

considerable resources have been invested in overcoming biases of 20 V on and 0 V off. The off-state relaxes the

traditional SiC MOSFET issues like the quality of the MOS oxide field thereby allowing the trapped electrons to emit.

interface, the reliability of the gate oxide, and the stability of To properly gauge field stability, the SiC MOSFET has been

the device electrical properties. Nitridation annealing of the subject to a new gate stress high temperature gate

SiC MOS interface has been shown to effectively raise the switching (HTGS) where the gate voltage is switched

electron channel mobility to modest values [2]. Although between 20 V and 0 V at 20 kHz and 50% duty cycle to

further improvements are being investigated in R&D, the simulate a more realistic operating condition. In HTGS

quality of the current MOS interface has enabled SiC stressing, the SiC MOSFET has demonstrated excellent gate

MOSFETs with 75% reduction in the conduction losses over stability with very minimal changes in the relevant device

a Si MOSFET and a 55% reduction over a Si IGBT. properties, as shown in Fig. 5.

Nitridation, coupled with careful device design, has also

improved the gate oxide reliability. Dielectric strength

measurements confirm a uniform distribution centered

around 10 MV/cm consistent with theoretical values

predicted for the SiO2 gate dielectric. Time-dependent-

dielectric-breakdown (TDDB) measurements performed on

small area MOSFETs (identical cellular structure as the Z-

FET with ~10,000x fewer cells and no termination for high

voltage) predict SiC gate oxide lifetime to be sufficiently

long at device operating conditions and comparable to, if not

better than, Si MOSFET TDDB [3] at 175oC (Fig. 3). The

improved SiC gate oxide lifetimes have been independently

confirmed by Dr. John Suehle at NIST and Dr. Robert

Kaplar at Sandia National Laboratory.

Fig. 4. Optimized processing reduces VT shift from 5 V to 1 V during

HTGB stressing.

316 CS MANTECH Conference, May 16th-19th, 2011, Palm Springs, California, USA

availability of SiC power switches and rectifiers will usher a

new generation of compact, ultra high efficient systems to

meet the ever increasing demands for energy efficiency.

ACKNOWLEDGEMENTS

The author would like to thank the Cree Power R&D and

Product Development teams for their valuable assistance and

discussions. The MOSFET development work has been

greatly assisted by government support from Army Research

Laboratory (Charles J. Scozzie), Air Force Research

Laboratory (James D. Scofield), and Defense Advanced

Research Projects Agency (Sharon Beerman-Curtin).

Fig. 5. HTGS stressing shows a 0.25 V increase in VT resulting in only a

10% increase in VDS,ON.

COMMERCIALIZING WIDE BANDGAP POWER DEVICES

In the evolution of SiC power devices from invention to

commercial release, several high voltage derating steps

occur. At invention, the device is typically specified by its

avalanche breakdown voltage which was 1700 V for the SiC

MOSFET. After the successful fabrication of several

thousand MOSFETs, an avalanche breakdown distribution

demonstrates that the MOSFET design generated a family of

parts ranging from 1400 V to 1900 V which effectively

derates the device to 1400 V. A final 200 V derating occurs Fig. 6. Comparison of actual SiC MOSFET (Cree and Rohm [6]) and GaN

to make the novel device technology robust enough to pass power switch (IR [5]) performance against their ideal values. The more

technologically mature SiC power switch technology shows only a small

stringent JEDEC/AEC qualification standards. Hence, for

amount of derating (difference between Actual and Ideal lines) while the

commercial purposes, the SiC MOSFET is rated for 1200 V novel GaN power switches suffer from a large derating factor due to

even though it has a 1700 V median avalanche breakdown material defects and design issues.

(30% derating). The amount of derating naturally reduces as

the device technology matures from the accumulation of REFERENCES

both manufacturing and field data. It is expected that power [1] http://www.cree.com/about/milestones.asp

[2] G.Y.Chung, et al., IEEE Electron Dev. Lett. 22, p. 176 (2001).

devices fabricated on competing wide bandgap materials like

[3] D.J. Dumin, Oxide Reliability: A Summary of Silicon Oxide Wearout,

Gallium Nitride (GaN) will have additional derating due to Breakdown, and Reliability, (World Scientific, London, 2002) ISBN

the orders of magnitude higher defect densities and the high 978-***-**-****-0.

surface fields associated with a lateral device structure. For [4] M.K. Das, et al., Journal of Electron. Mater. 27, p. 353 (1998).

example, GaNPowIR 600 V devices on high defect [5] http://www.irf.com/product-info/ganpowir/GaNGeneral.pdf

[6] T. Nakamura, et al., Physica Status Solidi, 206, pp. 2403-2416 (2009).

density GaN-on-Si epitaxy have been demonstrated to have

an on-resistance that predicts 4 kV breakdown from the ideal

ACRONYMS

GaN material properties (85% derating as shown in Fig. 6)

AC: Alternating Current

[5]. This indicates that GaN power devices may ultimately

AEC: Automotive Electronics Council

be a viable solution for applications below 600 V where they

ASP: Average Selling Price

provide a substantial advantage over Si devices. SiC power

CAGR: Compound Annual Growth Rate

devices should remain the attractive choice at the higher

FY2010: Fiscal Year 2010

voltage levels.

HTGB: High Temperature Gate Bias

IGBT: Insulated Gate Bipolar Transistor

CONCLUSIONS

IR: International Rectifier

While SiC JBS diodes have flourished in the field with

JEDEC: Joint Electron Device Engineering Council

12

steady growth and industry leading field reliability statistics,

MOSFET: Metal Oxide Semiconductor Field Effect

the SiC MOSFET power switch device has been

b

Transistor

successfully developed into a commercially released product

NIST: National Institute of Standards & Technology

by overcoming historical issues with the gate dielectric. The

SiO2: Silicon Dioxide

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