Wideband Gap Materials

8/3/2019 Wideband Gap Materials

1/15

WIDE/ LARGE BAND GAP MATERIALS

What are the Wide Band Gap (WBG)Materials?Band gap energy is the amount of energyrequired for an electron to jump fromthe conduction band to valance band.WBG materials have high band gapenergy (typically 3-6eV).A high band gap energy gives a high

breakdown voltage, which in turn leads tohigh power operation.

What are the advantages of Wide BandGap Materials?

High Power Density

High Voltage/ low current operation

High output impedance leading to simplematching

High temperature operation

Good thermal properties Good pulsed gain and Phase stability

Example of Wide Band Gap Materials

SiCGaN

are mostly used and easily available,apart from the several others.


2/15

Wide Band Gap Materials

1. SiC demonstrates upto 5 times thepowerDensity and 7 times the thermal

conductivityof GaAs.2. SiC MESFETs are suitable for lowfrequency (


3/15


If you need high power output in smallarea

If you need good thermal

dissipation.If you need low currentoperation.

If you need good gain and phase stability..

Then ,

Wide Band Gap are the Materials ofChoice.

Dr. Om Prakash Sinha, AINT, NOIDA 3

3


4/15


WIDE BANDGAP SEMICONDUCTORS FOR

UTILITY APPLICATIONS

With an increasing percentage of the electricity generated inthe future being processed by power electronic converters inutility applications such as distributed energy resourceinterfaces, medium voltage motor drives, flexible ACtransmission systems (FACTS), and high voltage DC (HYDC)systems, the efficiency and reliability of these converters isof utmost importance.

Several of these applications require voltage-blockingcapabilities in the tens and hundreds of kV and thus need a

series connection of many silicon-based power electronicsdevices to achieve the necessary voltage rating.

In the near future, power electronic converters will processgigawatts of power at some point between where it isgenerated and where it is ultimately utilized this emphasizesthe need for highly efficient power electronic converters andsystems in these utility applications.

Most present commercial power electronics devices (diodes,thyristors, IGSTs, MOSFETs, etc.) are silicon based devices.

The performance of these systems is approaching thetheoretical limits of the Si fundamental material properties.

The emergence of new power electronics devices based onwide band gap semiconductor materials will likely result insubstantial improvements in the performance of powerelectronics converter systems in terms of higher blockingvoltages, efficiency, and reliability as well as reducedthermal requirements.

Recent development advances have allowed silicon (Si)semiconductor technology to approach the theoreticallimits of the Si material; however, power devicerequirements for many utility applications of powerelectronics arc at a point that the present Si-based powerdevices cannot handle.


4


5/15


The requirements include higher blocking voltages, switchingfrequencies, efficiency, and reliability.

To overcome this limitation, new semiconductor materialsfor power device applications are needed.

Wide band gap semiconductors like silicon carbide (SiC),gallium nitride (GaN) and diamond, with their superiorelectrical properties are likely candidates to replace Si in thenear future for these high power requirements.

Among these, SiC is the forerunner as the only wide bandgap semiconductor with several commercially availablepower devices.

PROPERTIES OF WIDE BANDGAP

SEMICONDUCTORS

Wide band gap semiconductor materials have superiorelectrical characteristics compared with Si. Some of thesecharacteristics are tabulated for the most popular widebandgap semiconductors and Si in Table I.


5


6/15


Among all these semiconductors, diamond has the widestbandgap; consequently it also has the highest electricbreakdown field. SiC polytypes and GaN have similarbandgap and electric field values which are significantlyhigher than Si and GaAs.

Semiconductors with wider band gaps can operate at higher


6


7/15


temperatures; therefore, diamond power devices have thecapability to operate at higher ambient temperatures thanthe other materials.

In addition, higher electric breakdown field results in power

devices with higher breakdown voltages. For example. thebreakdown voltage of a diode is expressed in [I] as follows:

(1)

where q is the charge of an electron and Nd is thedoping density

Using (I), the breakdown voltages of diodes made of thematerials in Table I are calculated assuming the samedoping density, and the results arc plotted in Fig. Inormalized to the breakdown voltage of a Si diode.

As seen in this figure, the theoretical breakdown voltage of adiamond diode is 514 times more than that of a Si diode.

This number for 6H-SiC, 4H-SiC, and GaN is 56,46, and 34times that of a Si diode, respectively.


7


8/15


Note that with higher electric breakdown field, more dopingcan be applied to the material which will further increase thegapbetween the upper breakdown voltage limits of the widebandgap semiconductors compared to Si.

Another consequence of the higher electric breakdown fieldand higher doping density is the reduction of the width of thedrift region in devices. The required width of the drift regioncan be expressed as [2]:

(2)

The width of the drift region is calculated for all thesemiconductors in Table 1, and the results are plotted in Fig.2 for a breakdown voltage range of 100 to 1O,000V.Diamond, as expected, requires the minimum width, while6H-SiC, 4H-SiC, GaN follow diamond in the order ofincreasing widths. Compared to these, Si requires around tentimes thicker drift region.


8


9/15


The last device parameter to be calculated from the

properties in Table I is the on-resistance of the drift regionfor unipolar devices, which is given by the equation below[3]:

( )( )

ncs

Bspon

E

vR

3

2

,

4=

(3)

The calculation results for on-resistance are plotted in Fig. 3with respect to the breakdown voltage of the device. Again,

diamond shows the best performance with 4HSiC, GaN, and6H-SiC following in increasing resistance order.


9


10/15


11/15


lower than Si.

For a better comparison of the possible power electronicsperformances of these materials, some commonly knownfigures of merit are listed in Table II. In this table, the

numbers have been normalized with respect to Si and thelarger the number, the better the material's performance inthe corresponding category.

The figure of merit values for diamond arc at least 40-50times more than any other semiconductor in the table. SiCpolytypes and GaN have similar figures of merit, whichimplies similar performances.

The performances of Si and GaAs have the poorestperformance among the semiconductor materials listed in

Dr. Om Prakash Sinha, AINT, NOIDA11

11


12/15


Table I and II, and diamond has the best electricalcharacteristics. Much of the present power device research isfocused on SiC.

Now, Diamond, GaN, and SiC will be compared with respect

to their advantages and disadvantages with respect to eachother.

SiC SiC technology is the most mature one among the other

wide bandgap semiconductors. It has advanced greatly since1987 with the foundation of CREE Inc., which is the majorsupplier of SiC wafers.

Pending material processing problems like micropipes and

screw dislocations limit the die size, but these problemshave not stopped the commercialization of the first SiCpower devices, Schottky diodes with twice the blockingvoltage (600V) of Si Schottky diodes (300V).

Apart from the commercial devices, many other SiC powerdevices in the kV range with reduced on-resistances arebeing investigated such as 4H-SiC and 6HSiC PiN diodes,Schottky diodes, IGBTs, Thyristors, BJTs, various MOSFETs,GTOs, MCTs, and MTOs. However, except for some of the

diodes, the reported devices arc all experimental deviceswith very low current ratings.

The use of SiC power electronics instead of Si devices willresult in system level benefits like reduced losses, increasedefficiency, and reduced size and volume. When SiC powerdevices replace Si power devices, the traction driveefficiency in a hybrid electric vehicle (HEY) increases by 10percentage points, and the required heat sink for the drivereduces to one-third of the original size..

However, a dc power supply is considered where in additionto the increase in efficiency, decrease in losses, size andvolume of the heat sink, the effects of increasing switchingfrequency are also considered.

The results have shown that as the switching frequency isincreased, the sizes of the passive components, which


12


13/15


14/15


In the literature, up to 2000V GaN Schottky diodes and upto 6000V GaN p-n diodes have already been demonstrated;however, 4.9 kV SiC Schottky diodes and 19.2 kV p-n diodeshave also been demonstrated. These figures show how

advanced SiC technology is at this point compared to GaNtechnology.

GaN has some disadvantages compared to SiC. The firstone is that it does not have a native oxide, which is requiredfor MOS devices. SiC uses the same oxide as Si, Si02.ForGaN, more studies are underway to find a suitable oxide;without it, GaN MOS devices are not possible.

The second important problem is that with the presenttechnology, GaN boules cannot be grown; therefore, pure

GaN wafers are not available; instead GaN wafers are grownon sapphire or SiC. Even then, thick GaN substrates are notcommercially availablc. As a consequence, GaN wafers aremore expensive than SiC wafers.

An additional disadvantage of GaN compared to SiC is itsthermal conductivity, which is almost one-fourth of that ofSiC. This property is especially important in high power hightemperature operation because the heat generated insidethe device needs to be dissipated as quickly as possible. The

higher the thermal conductivity is, the quicker the heat isdissipated. Growing GaN on SiC wafers increases the overallthermal conductivity but still does not reach theperformance of SiC.

DIAMOND

Diamond shows the best theoretical performance asshown in Section II, with several times improvement in everycategory compared with every other wide bandgap

semiconductor. However, its processing problems have notbeen solved yet. After several years of research, SiC still hasprocessing issues because of the high temperatures requiredin the process; diamond is a harder material and needs evenhigher temperatures for processing, and not as muchresearch has been done on its processing yet.


14


15/15


In the literature, diamond is used in sensors and fieldemission devices. There are no power devices available yet.

FORECASTING THE FUTURE EVALUATION

OF THE RESULTS

With further development, wide bandgap semiconductorshave the opportunity to meet demanding utilityrequirements.

While diamond has the best electrical properties,research on applying it for high power applications is only inits preliminary stages.

Its processing problems are more difficult to solve than

any of the other materials; however, it likely will be animportant material for power devices in 20 to 50 years. Inthe meantime, there needs to be a transition material.

GaN and SiC power devices show similar advantages overSi power devices. GaN's intrinsic properties are slightlybetter than SiC; however, no pure GaN wafers are available,and thus GaN needs to be grown on SiC wafers.

SiC power device technology is much more advancedthan GaN technology and is leading in research andcommercialization efforts. The slight improvement GaNprovides over SiC might not be sufficient to change gearsand use GaN instead of SiC. SiC isthe best suitable transitionmaterial for future power devices.


15

Wideband Gap Materials

Documents