Technical Review

Vol.4

Evolution of Wide-Bandgap

Semiconductors for Power Devices

Expanding Fields of Application

Wide-bandgap semiconductors including silicon carbide (SiC) and gallium nitride (GaN) are

currently attracting attention for use in next-generation power devices in view of their excellent

characteristics oering higher energy eciency.

In developing key technologies to improve the performance of SiC power devices, Toshiba

Electronic Devices & Storage Corporation has focused on the reduction of wafer thickness and

cell miniaturization, and has been continuously releasing a wide variety of SiC power devices. The

application of SiC hybrid modules to traction inverters for rolling stock, for example, is contributing

to reductions in the size and weight of such inverters. We are also developing GaN power devices

capable of performing high-speed switching operations, including a quasi-normally-off GaN high-

electron-mobility transistor (HEMT) and a GaN metal-oxide-semiconductor field-effect transistor

(MOSFET).

Power devices have achieved high eciencies in recent decades because of improvement in the performance of power

semiconductors, which are made of silicon (Si). However, Si power devices are approaching their theoretical limits of

performance, and wide-bandgap semiconductors are expected to break through these limits.

In 2013, Toshiba Electronic Devices & Storage Corporation developed Schottky barrier diodes (SBD) made from silicon

carbide (SiC), which is a wide-bandgap semiconductor. In 2014, we announced the launch of a SiC hybrid module, a

device that assembles silicon injection-enhanced gate transistors (IEGTs) and SiC SBDs in a single integrated package.

By developing SiC hybrid modules and applying them to inverters for electric railways, improved power conversion

eciencies and space savings are achieved

(1)

. Another promising wide-bandgap semiconductor, gallium nitride (GaN),

excels in high-speed switching performance, leading the way toward more efficient solutions and miniaturization of

power supply equipment.

Here, we describe the latest advances associated with these two new semiconductor materials and their future trends.

1. Introduction

Unlike Si devices, SiC devices can achieve high breakdown voltage and low energy loss simultaneously. However, the

materials are expensive and dicult to process, the need to minimize the defects in the crystals poses a challenge, and

moreover, the manufacturing methods are quite dierent from those for traditional Si. These issues have hampered the

widespread adoption of SiC.

2.1 SiC discrete products

In recent years, the progress of discrete products consisting of one chip in one package has been remarkable. As the

quality of wafers improves and their diameter increases, these devices are finding uses not only for small industrial

2. Expansion of applicable eld of SiC devices

SiC SBD

Short-circuit current

by Cj of D1

Short-circuit current

by Cj of D1

Q1 On current D1 Forward current

Junction capacitance C

Short-circuit current

current

Forward

current

(Figure 2) : Application of SiC SBD to power factor correction (PFC) circuit and

current waveforms

DPAK TO-220-2L TO-220F-2L TO-247 TO-3P (N)

Insulation type

Surface mount

type

Through-hole type

(Figure 1) : Examples of packages for SiC Schottky barrier diodes (SBDs)

(Table 1) : Evolution of cross-sectional structure and performance of SiC SBDs

Generation Basic Study 1 Basic Study 2

1st-gen.

（Current）

2nd-gen.

（New）

Structure

SBD

Structure

MPS

Structure

JBS

Structure

Improved

JBS Structure

Wafer Thickness

(Relative Value)

1 1 1 1/3

Sectional Structure

Performance

Leak current

Improvement：Low

Eect: Thermal

runaway suppression

Eciency

performance

（

・

）

Improvement: low

Eect: Low loss

Surge current

FSM

Improvement: High

Eect: High tolerance

: Very good　　 : Good　　 : Fair　　 : Poor

MPS: Merged PN Schottky　　N: N-type semiconductor　　P: P-type semiconductor

JBS Structure → Low

Thinner Wafer → Low

any current

Metal Electrode

N+

Metal Electrode

N+

Metal Electrode

N+

P P P P

Metal Electrode

P+

P P

N+

Improved JBS Structure → Low

at high current

Technical Review

Vol.4

apparatus (eg., power supplies for information and communication equipment, electric vehicle (EV) charging

stations, solar light inverters, commercial air conditioners) but also for high-end consumer products (eg., organic

electroluminescent TVs, audio-visual ampliers).

The following is a description of the features of our company's SiC-based discrete SBD and MOSFET products.

2.1.1 SiC SBD discrete products

Figure 1 shows examples of packages of our current products. These products have a breakdown voltage of either 650

V or 1,200 V, and the current rating can be selected within the range of 2 A to 24 A. They are used for power factor

correction (PFC) circuits from the alternating current (AC) power supply.

Figure 2 shows a PFC circuit using a SiC SBD, together with a schematic diagram and a sample waveform. In the PFC

circuit, the turn-on of D1 (SiC SBD) is controlled by the switching device Q1. The energy loss of Q1 is aected by the

characteristics of D1. When Q1 turns on, short-circuit current ows according to the total charge Q

determined by the

junction capacitance Cj of D1 and the reverse voltage V

. This short-circuit current increases the turn-on loss of Q1, such

that the smaller the Q

, the smaller the turn-on loss.

On the other hand, the loss of D1 is due to charging/discharging of Q

and on-loss from the forward voltage V

. Using

the same design rules, there is a trade-o between Q

and V

, in that the smaller the eciency performance index (V

), expressed as the product of V

and Q

, the higher the eciency and the lower the loss.

Mass production of the rst-generation SBDs began in 2013, and transitioned to the second-generation SBDs in 2017.

The second-generation design combines an improved Schottky structure with a thin wafer. The eciency performance

index required for the PFC circuit is reduced to about two-thirds of that of the rst generation, enabling a device with

lower loss as well as an improved surge current tolerance I

FSM

1.7 times larger than that of the rst generation (Table 1).