Prospects for the application of GaN power devices in hybrid electric vehicle drive systems This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2013 Semicond. Sci. Technol. 28 074012 (http://iopscience.iop.org/0268-1242/28/7/074012) Download details: IP Address: 131.215.225.9 The article was downloaded on 25/06/2013 at 07:03 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Prospects for the application of GaN power devices in hybrid electric vehicle drive systems
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2013 Semicond. Sci. Technol. 28 074012
(http://iopscience.iop.org/0268-1242/28/7/074012)
Download details:
IP Address: 131.215.225.9
The article was downloaded on 25/06/2013 at 07:03
Please note that terms and conditions apply.
View the table of contents for this issue, or go to the journal homepage for more
Home Search Collections Journals About Contact us My IOPscience
Prospects for the application of GaNpower devices in hybrid electric vehicledrive systemsMing Su1, Chingchi Chen1 and Siddharth Rajan2
1 Research and Innovation Center, Ford Motor Company, Dearborn, MI 48124, USA2 Department of Electrical and Computer Engineering, Ohio State University, OH 43210, USA
Received 29 November 2012, in final form 8 January 2013Published 21 June 2013Online at stacks.iop.org/SST/28/074012
AbstractGaN, a wide bandgap semiconductor successfully implemented in optical and high-speedelectronic devices, has gained momentum in recent years for power electronics applications.Along with rapid progress in material and device processing technologies, high-voltagetransistors over 600 V have been reported by a number of teams worldwide. These advancesmake GaN highly attractive for the growing market of electrified vehicles, which currentlyemploy bipolar silicon devices in the 600–1200 V class for the traction inverter. However, tocapture this billion-dollar power market, GaN has to compete with existing IGBT products anddeliver higher performance at comparable or lower cost. This paper reviews key achievementsmade by the GaN semiconductor industry, requirements of the automotive electric drivesystem and remaining challenges for GaN power devices to fit in the inverter application ofhybrid vehicles.
(Some figures may appear in colour only in the online journal)
1. Introduction
Semiconductor devices play a key role in power electronicsystems. Most of these applications today are enabled bysilicon. However, wide bandgap (WBG) semiconductors, suchas silicon carbide (SiC) and gallium nitride (GaN), possessmaterial properties that are superior to silicon for powerswitching operation. They have been under extensive academicresearch for more than 20 years and promise to replacesilicon with better energy efficiency. In hybrid electric vehicles(HEV), existing silicon IGBT and diodes contribute about 20–25% of traction inverter system cost. The expected increase ofautomotive fuel efficiency over the next decade will continueto drive the expansion of this market, and in turn create ahuge opportunity for power semiconductor devices, valued atover a billion dollars. While silicon IGBT and PiN diodesremain a primary choice for automotive power modules, largeautomakers are already interested in emerging technologiesthat enable higher power conversion efficiency or reduced
system cost. GaN semiconductor is one of the most anticipatedcandidates for the next-generation HEV power conversionapplication.
Meanwhile, high-voltage GaN power device has receivedsignificant research interest due to the unique materialproperties suited for power electronics application. Recenttechnology advancements make its application prospectincreasingly realistic, such as the availability of native andGaN-on-silicon substrates, development of normally-off gatestructures, suppression of the current collapse phenomenon aswell as the demonstration of high-voltage blocking capability.However, the stringent cost and reliability requirements ofautomotive components, together with a highly competitivesilicon IGBT industry create barriers for the transition fromsilicon power electronics to a GaN-based one. In this review,we discuss the encouraging progresses as well as remainingchallenges facing GaN power electronic devices, and evaluatethe prospects of GaN for the high-voltage, high-currentautomotive application.
Figure 1. Schematic diagram of a three-phase inverter used inhybrid electric vehicles.
2. Traction inverter of the HEV drive system
In general, full HEVs today are equipped with at least onethree-phase traction inverter circuit as shown in figure 1 toconvert dc power from the high-voltage battery pack into acform for motor drive. [1] The input dc voltage is typically300–600 V after a voltage booster stage. Six switch pairsconsisting of silicon IGBTs and PiN diodes connected inanti-parallel fashion are configured in three phase legs. Thevoltage and current ratings for the silicon switches are inthe range 600–1200 V and 200–600 A, depending on themotor power rating. While the hybrid vehicle power electronicssystems may also include on-board chargers and auxiliarydc–dc converters besides the traction inverter, it is the three-phase inverter that requires high current devices and thereforeconsumes the largest share of power silicon. As a result, GaNpower switches stand to gain the most valuable automotivepower market if they can supersede silicon IGBTs in thesecircuits.
Automotive power modules in many ways share therequirements for consumer power electronics components aswell as industrial motor drive devices, including low powerloss, normally-off operation and low-cost manufacturing.Because of this, GaN power semiconductors developed forgeneric power electronic applications make an important steptoward their future adoption in automotive systems. However,beyond these common metrics, automotive components oftenrequire greater robustness against environmental factors, suchas a wide range of operation temperature, typically −40 ◦C toas high as 150 ◦C, as well as tolerance of aggressive thermalshock and power cycling, vibration and harsh ambient in termsof chemical presence and humidity. As a result, the durabilityand reliability demand is very challenging. Nevertheless,electrified vehicles need to compete with their traditionalgasoline-powered counterparts to gain customer acceptance,which means the added equipment cost has to be small enoughto be paid back in fuel savings. At the present time, siliconIGBT power modules satisfy these performance requirementsof the traction inverter and offer a cost competitive solutionfor the HEV system. It is expected that WBG semiconductorsdeveloped for the inverter system will yield additional fuelsavings and gain other measurable benefits to offset theincreased cost as compared to the silicon devices, in orderto ensure the new solution is economically feasible for theautomaker and end consumers.
3. Substrates for GaN power devices
Modern GaN devices are usually epitaxial grown on a foreignmaterial, such as silicon carbide or sapphire. This is howevernot an ideal way of producing high-quality semiconductorcrystal since the atomic spacing and thermal properties ofGaN are different from those of the substrates, leading to highdislocation density and film stress issues. Furthermore, highpower switching transistors naturally prefer vertical structures,which require native GaN substrates [2]. The synthesis of GaNsingle crystal substrates has been an extraordinarily difficulttask. The highly successful Czochralski method for siliconcrystal growth would have process requirements of 2225 ◦Cand 64 000 atm to grow GaN, comparable to conditions verydeep within the Earth’s mantle and making it impractical tobuild such a system [3].
Alternative approaches have been investigated forgrowing GaN bulk substrates, such as hydride vapor phaseepitaxy (HVPE) and ammonothermal growth. The state-of-the-art GaN substrates offered by Polish company Ammonoare 2 inch in diameter, and it has been shown that the epitaxialfilm on these native substrates offer much lower defect densitycompared to the heteroepitaxial counterparts. In the meantime,it is still a great challenge to scale up bulk GaN growth to largerwafer sizes. At the current wafer diameter and prices, the nativesubstrate option is not economically feasible for automotiveapplication in the near future [3].
Given the obstacles in GaN native substrate manufactur-ing, there has been substantial effort devoted to the epitaxyon foreign substrate materials. Common choices for GaN het-eroepitaxy include sapphire, silicon carbide and silicon [4]. Inthe past decade, SiC and sapphire substrates have been widelyused in nitride LEDs and RF transistors.
Table 1 compares the properties of the three most widelyused substrates for GaN film growth. An ideal substratematerial should have minimal mismatch with GaN in the latticeconstant and thermal expansion coefficient, as well as a highthermal conductivity. SiC is a superior choice, but is also themost expensive among these options, currently priced above$20 cm−2. Sapphire costs less but it has much lower thermalconductivity than SiC, which can be a significant concern forhigh power devices. Furthermore, sapphire wafers producedtoday are limited in size, predominantly 2 inch, due to lowyield on larger wafers.
For cost-sensitive applications such as the automotiveconverter, silicon substrate is perhaps the most attractiveamong all options, as GaN grown on SiC or sapphire maynever reach device prices comparable to the silicon products.Although silicon substrate has relatively large thermal andlattice mismatch to GaN, its availability in large diameter, plusthe possible integration of GaN device processing in siliconfabrication facilities, promises to bring down the cost of aGaN epitaxial wafer to less than $3 cm−2, according to TimMcDonald of International Rectifier [5].
3.1. Recent progress in the growth of GaN power devicestructures on large silicon substrates
Growing GaN epitaxial layers on silicon is difficult due to theintrinsic mismatch between the lattice constant and thermal
Table 1. Comparison of GaN epitaxy on common substrate materials [4].
SiC Sapphire Si
Lattice constant mismatch 3.1% 15% 17%Linear thermal expansion coefficient ( × 10−6 K−1) (GaN value = 5.6) 4.16 (c-axis) 7.5 2.6Thermal conductivity (W cm–1 K–1) 3.8–4.9 0.25 1.56Typical epitaxial GaN dislocation density >108 cm−2 >108 cm−2 >109 cm−2
Cost Expensive Less expensive Cheap
expansion coefficient. Without optimal strain managementbuffers, excessive wafer bow and film cracks will result. Thechallenge becomes even greater for thick epilayers on large-diameter wafers. Nevertheless, encouraging results have beenreported by researchers at IMEC [6]. AlGaN/GaN/AlGaNepilayer stacks of 3 μm thickness were successfully depositedon 1.15 mm silicon (1 1 1) substrates of 200 mm diameter, thelargest GaN-on-Si wafer for high-voltage devices known tothe authors so far. Breakdown voltages above 600 V weremeasured on devices passivated by a bilayer dielectric ofSi3N4/Al2O3. The team also made an earlier report in 2012,where GaN MISHEMT devices were processed on 150 mmsilicon wafers and shown to exhibit high breakdown voltages[7].
In order to achieve cost competitiveness, GaN-on-Sidevices have to be processed on 150 mm or larger wafers, sincesilicon IGBTs today are already manufactured on 200 mm linesby leading suppliers, with 300 mm lines in pilot production.The success at IMEC marks a significant milestone for thelarge wafer processing of GaN, although it is yet to be shownthat this can be duplicated for higher voltage transistors up to1200 V class.
4. Transistor structures for power switchingapplication
Silicon power transistors today are mainly in the form ofMOSFET, IGBT or thyristor structures, depending on thevoltage and power rating. For WBG materials, bipolar deviceswith odd number of p–n junctions in the main current path arenot suitable for low and medium power applications due toexcessive conduction loss. Hence, unipolar ones are of mostinterest. Furthermore, group III–V material systems, such asGaN, offer heterojunction structures with very high electronconcentration and mobility. These devices are attractive forpower switching application due to low on-state resistance.In the past ten years, GaN power switching devices have beeninvestigated mainly in three forms, the MOSFET (or MISFET),the HEMT (or HFET) and the hybrid MOS-HEMT (or MIS-HFET) [8].
4.1. GaN MOSFET
Although the conventional MOSFET structure does not takeadvantage of the high mobility of 2DEG (two-dimensionalelectron gas) electrons, it has attracted interest because of thenormally-off operation, low gate leakage current and simplestructure.
A GaN MOSFET consists of an n-type source region,an n-type drain region, their ohmic contacts, a gate insulator
Figure 2. Cross-sectional views of GaN MOSFET.
layer (typically SiO2) and a gate electrode [9, 10]. To supporta high-voltage lateral structure, a RESURF (reduced surfacefield) region can be designed. An example of GaN MOSFETstructure is shown in figure 2. It can also be called a MISFET(metal–insulator–semiconductor FET) if the gate insulator isnot an oxide.
Normally-off operation characteristic is required inautomotive motor drive circuits to satisfy fail-safe criteriaand simple gate drive configuration. The GaN MOSFET, ascompared to the conventional HEMT, is preferred in thisaspect. Huang et al of RPI demonstrated enhancement-modedevices on p-type GaN with 3.3 V gate threshold voltage [11].However, the MOSFET inversion channel electron mobility(∼100 cm2 V–1 s–1) is drastically lower than that of the 2DEG(1500–2000 cm2 V–1 s–1for GaN HEMT) [10]. In turn, lowermobility limits the on-resistance performance and creates achallenge for GaN MOSFETs to achieve low conduction loss.One of the best high-voltage GaN MOSFET reported has a730 V blocking voltage and a specific on-resistance value of34 m� cm2 [10]. It is clear that significant effort needs to bemade to reduce the channel resistance.
4.2. Recessed-gate HEMT and hybrid MOS-HEMT
To date most research on GaN power transistor has been basedon the HEMT structure, where spontaneous and piezoelectricpolarization induces a high-density layer of electrons with veryhigh mobility at the heterojunction formed by AlGaN and GaN.This so-called 2DEG enables low conduction resistance and isvery beneficial for efficient power switching operation.
Figure 3. Cross-sectional view of an AlGaN/GaN power HEMT
Illustrated in figure 3, the power HEMT consists of a bufferlayer on the substrate, a GaN channel layer, a top AlGaN caplayer of 20– 30 nm, as well as source and drain metal contactsand a gate electrode [12]. A gate insulator is not needed for RFdevices, but is required for power devices to suppress the gateleakage current. The 2DEG channel offers very low conductionresistance, but also comes with a penalty for its normally-on behavior. As electrons are induced in large quantities bya strong polarization field, the conduction channel naturallyexists even without a positive gate bias.
Normally-off operation in GaN HEMT can be achievedby several methods, although they face limitations and trade-offs. The most widely used method is by gate recess etching.This can be performed by ICP plasma to remove the AlGaNlayer on top of the GaN channel layer. The reduction ofAlGaN thickness results in a lower polarization-induced 2DEGdensity. Depending on the depth of etching, the post recessstructure can be categorized as (1) a recessed gate HEMT or(2) a hybrid MOS-HEMT.
As shown in figure 4, a recessed-gate HEMT has theAlGaN layer partially removed, leaving the heterojunction ofAlGaN/GaN still in place. In such structures, the quantum wellcreated at the AlGaN/GaN interface is intact, and normally-off operation is achieved by electron depletion under the gate.On the other hand, the hybrid MOS-HEMT is subject to fullremoval of the AlGaN layer, interrupting the 2DEG channel inthe gate region. Essentially, to turn on the transistor, the gateregion functions like a MOSFET by inducing electrons to thegate oxide to form a conduction channel.
Clearly, the recessed-gate HEMT structure is preferredfor on-state resistance, since the gated channel maintains highmobility of the 2DEG. State-of-the-art results based on suchstructures carry single digit specific on-resistance for a 1 kVclass device in m� cm2 unit [13, 14]. However, this structurerequires precise control of the gate etching depth yet it doesnot offer a good etch stopper. The manufacturability canbe, therefore, a major hurdle, in terms of threshold voltageuniformity and yield. Furthermore, most reported recessed-gate HEMTs show very low threshold voltages around 1 V,insufficient to leave a margin against noises.
The dependence and sensitivity of gate threshold voltageon the etching depth have been studied by Kanamura et al of
(a)
(b)
Figure 4. Cross-sectional structures of (a) a recessed-gate HEMTand (b) a hybrid MOS-HEMT.
Fujitsu [15]. In that report, a 20 nm AlGaN layer is grownon top of i-GaN, followed by a total of 6 nm triple caplayers of GaN, AlN and GaN. A linear dependence of thegate threshold voltage (Vth) is observed as a function of gaterecess etching depth. 3 V of Vth is reached at etch depth of24 nm, leaving a distance of only 2 nm above the AlGaN/GaNheterojunction. Controlling such an etch depth across a largewafer can be a great challenge. Slight over-etching will cutthrough the AlGaN layer and eliminate the 2DEG channel,while a shallower etch will lead to low threshold voltages. Forexample, a 0 V gate threshold level is seen at 14 nm of etching,compared to 3 V at 24 nm.
Different from the recessed-gate HEMT, hybrid MOS-HEMT undergoes full removal of the AlGaN layer at thegate regions, thereby eliminating the 2DEG channel at thegate altogether. The threshold voltage of the hybrid deviceis less sensitive to the exact etching depth. In 2008, a highthreshold voltage GaN MOS-HEMT (Vth > 5 V) was reportedby Oka and Nozawa [16]. Their device combines AlGaNetching with a negative polarization induced by a thick AlGaNbuffer below the GaN channel to enhance Vth. In other reports,gate threshold voltages are usually in the 2–3 V range, whichare among the highest of GaN power transistors. The main
Figure 5. AlGaN/GaN HEMT with F-negative chargesincorporated under the gate
weakness of the hybrid structure, compared to the recessed-gate HEMT, is the higher on-resistance due to the lowerelectron mobility of the MOS channel. Even though onlya short length is required at the gate region, it could oftendominate the on-resistance of the entire device. For example,Huang et al reported that a hybrid MOS-HFET with a 4 μmgate channel and 20 μm drift length has 84% of its totalon-resistance contributed by the gate channel due to its lowfield-effect mobility [17]. More recently, however, progress inreducing the on-resistance was reported by Ikeda et al [18].The authors demonstrated a hybrid MOS-HEMT device with7.1 m� cm2 specific on-resistance at 1.21 kV breakdownvoltage, making it one of the best performing normally-offGaN transistors above 1000 V.
4.3. Other normally-off HEMT structures
Besides gate recess etching, two other approaches have beendeveloped to shift the gate threshold voltage of a HEMTstructure from negative to positive. They are (1) fluorineplasma treatment under the gate area, introducing negativecharges in this region that dispel 2DEG electrons and (2)adding a p-type AlGaN or GaN layer in the gate region atthe top of device.
Figure 5 shows the principles of the first approach, wherefluoride-based plasma (CF4) treatment is applied under thegate region, shifting the threshold voltage toward the positivedomain [19, 20]. The amount of the threshold shift is dependenton the plasma power and treatment time. While the long-termstability of these implanted fluorine ions remains to be verified,this approach has been effective to achieve a positive thresholdvoltage around 1 V.
The second approach is developed by Panasonic for itsgate injection transistor (GIT). The structure is shown infigure 6 [21]. Some of the best performing normally-off GaNdevices in the 600–800 V range are based on this structure.
The GIT structure is normally off because a p-AlGaNlayer raises the potential at the AlGaN/GaN interface channelabove the Fermi level. This could also be understood as anatural depletion of mobile electrons on the n-side due tothe built-in p–n junction. By applying a positive gate bias,the channel begins to accumulate 2DEG as the quantum well
Figure 6. Gate injection transistor (GIT) featuring a p-AlGaN gatelayer.
reaches the Fermi level, thereby turning the device on. Furtherincreasing the gate bias beyond the turn-on voltage of the p–njunction will lead to hole injection from the p-AlGaN layer tothe channel and result in conductivity modulation, triggeringbipolar current transport mode in the GIT. Under this approach,positive gate threshold around 1 V has been demonstrated.However, it will be difficult to increase the Vth much furtherdue to the limitation of the p–n junction barrier height, whichalso places a physical limit on the maximum gate voltage swingof the GIT structure.
Another popular solution proposed to address thenormally-off requirement is a cascode configuration consistingof a high-voltage, normally-on GaN device and a low-voltagesilicon MOSFET. By avoiding direct focus on threshold-voltage engineering of GaN transistor structures, this approachprovides one of the simplest and fastest way to deliver anormally-off GaN product. A few leading GaN researchteams, such as International Rectifier and Transphorm, aremaking efforts in this direction. However, using hybrid circuitconfigurations in place of a single functional device bringsin additional issues such as increased cost and complexity.The system stability due to leakage inductance also poses achallenge as the current rating scales up.
5. State-of-the-art performances and remainingchallenges
In the past five years, industrial and academic research teamsfrequently reported high-voltage GaN power switches withspecific on-resistance less than 10 m� cm2. Such performancefar exceeds the unipolar limit of silicon devices, and isoften superior to modern IGBTs, although the GaN unipolarlimit is still not within reach. Structures reported includeconventional GaN HEMT with gate dielectrics, hybrid MOS-HEMT, GIT and new concept devices, such as a super HFET byPowdec [22]. Figure 7 compares the key performance meritsof breakdown voltage and specific on-resistance for some ofthese devices.
Several factors play a role in keeping GaN powerswitches from reaching their theoretical performance limits.Resistive components outside of the drift region can contributesignificantly to Ron_sp, such as the MOS channel and ohmic
Figure 7. Specific on-resistance versus breakdown voltageperformance chart for GaN power devices.
contacts. On the other hand, the theoretical breakdown fieldof over 3 MV cm–1 has not been practical for GaN devicesgrown on silicon substrates. High density of dislocationas well as non-uniform field distribution places a limit onthe breakdown voltage for a given drift region distance,typically at 1 MV cm–1 or less. Furthermore, conventionalavalanche breakdown is not observed in existing GaN powerdevice products. Instead, these devices often suffer from fieldcrowding near the drain-side edges of the gate electrode, andtheir breakdown mechanism is heavily associated with thepassivation layers, not the GaN material.
Based on figure 7, GaN transistors engineered forhigher threshold voltage are associated with higher on-resistance due to the gate region treatments applied. But recentperformance data show some exceptions. In 2011, Ikeda et al[18] optimized a hybrid MOS-HFET structure and achieved7.1 m� cm2 resistance with 1200 V breakdown voltage.Very good performance on normally-off devices is alsodemonstrated by HRL using an F-plasma treatment technique[19], and by Panasonic [21]. In the ISPSD conference of 2012,Hwang and colleagues from Samsung Electronics presentedrecord-breaking HEMT devices up to 1640 V. A 3 V thresholdwas achieved with a p-GaN gate cap and a tungsten metal stackforming the Schottky contact to the p-GaN layer. Similar to theGIT structure, the p-GaN HEMT enjoys a continuous 2DEGchannel of high electron mobility, and produced on-resistancevalues as low as 1.45 m� cm2 for a 1 kV device [23].
Despite rapid development in GaN power technologyand the record-breaking performance figures reported to date,substantial barriers still exist that prevent automakers fromadopting these transistors in our electrified vehicles.
5.1. GaN-on-silicon materials for high-voltage devices
As discussed earlier, to make cost-effective GaN powersemiconductors, large wafer device processing has to beproven. IMEC has pioneered the development of 150 and200 mm GaN-on-Si process for power devices up to the
600 V class, which is by far one of the most interestingadvances toward mass manufacturing of high-voltage GaNdevices. Meanwhile, in the traction inverter system of fullhybrid vehicles, dc bus voltages up to 650 V are used, andthe availability of power switches across the 600–1200 Vvoltage range is required. At higher voltage rating, therequired epitaxial layer is thicker. Specifically, a dependenceof maximum breakdown voltage on epilayer thickness wasfound to be around 300 V μm–1 in one study [8]. As thestrain management becomes increasingly difficult for thickerepilayers, the challenge for making a 1200 V device is muchgreater than a 600 V one. In 2011, Ikeda et al reported thegrowth of a thick epitaxial stack of 7.3 μm on 4 inch siliconsubstrates, resulting in GaN MOS-HFETs capable of 1700 Vvoltage blocking [18]. It is yet to be demonstrated that suchdevices can be manufactured on wafers of 150 or 200 mm indiameter.
5.2. Normally-off operation
Normally-off operation is required for fail-safe reasons in theHEV motor drive system. While this application requirementis well known in the device research community, there areoften perceptions that a positive gate threshold voltage (Vth) isall that is needed. Many scientific papers claim successfuldemonstration of normally-off devices that have thresholdlevels below 1 V. In real-world applications, automotivequalified power switching devices (IGBTs) have much higherVth at 5 V or more. The necessity of a high nominal gatethreshold is attributed to margins against a wide range ofoperation temperature, substantial underhood noise signallevels as well as manufacturing distribution. Among thenormally-off structures reviewed, some are inherently limitedin their maximum threshold voltage, such as the GIT, whileothers face difficult challenges due to the trade-off betweengate threshold voltage and on-state channel resistance, ormanufacturing obstacles such as precise etching.
5.3. Surface passivation and gate dielectrics
In GaN HEMT, it is often observed that the forward resistancesignificantly degrades when turned on again after a period ofhigh-voltage drain bias. This is believed to result from thetrapping of electrons from the 2DEG channel, primarily atthe surface of the top AlGaN layer. This behavior is of greatconcern for power switching applications due to the increaseddynamic on-resistance and hence larger power loss. Properpassivation of the AlGaN surface is critical in suppressingthe trap states and the current collapse phenomenon. For thispurpose, silicon nitride seems to be more effective than otherdielectrics, such as SiO2. Furthermore, field plates can beused to reduce field crowding at the drain-side gate edge.Using optimal passivation and field plates, the dynamic on-resistance can be shown to minimize [19]. However, in recentliterature reports, the drain voltage level demonstrated forcurrent-collapse-free operation is often much lower than thebreakdown voltage of the device. Apparently, further researchis still needed to ensure suppression of this phenomenon up tothe rated voltage, as well as over long operation lifetime.
The passivation layer is also known to affect thebreakdown voltage of the GaN HEMT devices. Researchon this topic shows mixed results. A few reports indicatethat the standard silicon nitride passivation layer may reducethe breakdown voltage, especially if not deposited in highquality or in conjunction of optimal field-plate structures[24–26]. Contrary to this, other researchers believe thatproper passivation not only reduces the dynamic on-resistanceproblem, but also increases the breakdown voltage [27–29].It is very likely that the mechanisms of breakdown in thesereports are different, making their conclusions inconsistentwith each other. In principle, proper passivation of the HEMTdevice is critical to mitigate the current collapse problem,but optimal process needs to be verified to ensure breakdownperformance over time.
Traditional AlGaN/GaN HEMTs for RF applications arebased on Schottky gate contacts. However, for power switchingdevices that handle much greater voltages, the gate leakagecurrent must be minimized, preferably by a high-qualitygate insulator. In silicon and SiC devices, the insulator ismade by thermally grown silicon dioxide (SiO2). In contrast,GaN lacks practical native oxides and requires a depositionprocess, which leads to greater quality concerns comparedto natively grown oxides. Commonly used gate insulationmaterials include silicon nitride, aluminum oxide (Al2O3),SiO2 and HfO2 [30]. Thinner insulators are often insufficientfor suppressing leakage, while a thicker one leads to reducedgate capacitance and transconductance (gm) [31]. Leakagecurrents, interface charge states and long-term stability are stillcommon issues for gate insulators. These not only affect deviceperformance parameters, such as Vth or breakdown voltage, butalso cast doubts for the device reliability.
5.4. Short-circuit capability and current rating scalingbarrier
In addition to the more frequently discussed issues, automotiveapplications will encounter additional technical challenges.One of these is the short-circuit tolerance capability. In tractioninverter circuit, a short-circuit event occurs when the motorwindings are accidentally shorted, subjecting the on-statepower switch to high drain bias and pushing it into currentsaturation mode. Extreme power dissipation follows the eventand the gate drive circuit must turn off the device promptlyto protect it from catastrophic failure. The manufacturer’sspecification on short-circuit withstand capability is typically10 μs, which is dependent on the reaction speed of thegate drive circuit. The expected capability of GaN lateralswitches will be more limited than vertical silicon IGBT forthe following reasons. The power dissipation density duringshort-circuit event is proportional to the electric field strength(E) and the current density (J). GaN, as a WBG material, willbe designed according to its breakdown field strength for thedrift region width. Ideally, this leads to up to ten times higherelectric field in the drift region when high drain bias is applied,compared to the silicon counterpart. In addition, high surgecurrent is concentrated at the AlGaN/GaN interface, insteadof spread out in the bulk material in silicon vertical IGBT,
triggering extremely localized heat dissipation. The saturationcurrent for GaN devices may also be determined by 2DEGelectron saturation velocity, a mechanism that may result inmuch higher short-circuit current amplitudes than the IGBT. Inlight of these factors, plus a relatively low thermal conductivityof GaN, it is suspected that the hot-spot temperature surge willbe extremely fast in a short-circuit event, and GaN lateralpower transistors will face serious challenges in the short-circuit requirement aspect. No published research on GaNshort-circuit performance has been identified to date. But thesolution will likely require development of advanced gatedrive and protection circuits that reduce the required devicewithstand time.
Additionally, lateral GaN power devices will suffer fromscaling limitations because of their drain and source electrodesbeing interdigitated at the transistor cell level. Due to thethin metal runner fingers and current crowding phenomenon,on-resistance deteriorates considerably with increasing diesize. Automotive traction inverters typically employ powerswitching devices rated at 200–600 A and 600–1200 V. Suchhigh power ratings have to be supported by a substantial chipsize capable of handling 100 A or more per die, even thoughmultiple chips can be connected in parallel. Devices of thenecessary size have not been reported in the literature to date.It is expected that scaling the GaN transistors to this currentrange will be difficult, both from the device yield and the lateralscaling limit points of view.
5.5. Reliability qualification
As reviewed in this paper, at the present time, focus of theGaN power switch development is still on the functionalityand manufacturability aspects. Since devices of the requiredvoltage and current ratings for HEV inverters are notyet available, reliability studies based on the automotivesystem demand are rare. However, when future devices aremade to meet the performance targets, standard qualificationprocedures for automotive discrete semiconductors will apply,and become the next-stage challenges for GaN [32].
Common reliability tests for automotive semiconductorcomponents are listed in table 2. During and after thesetests, critical parameters are measured for their shift fromoriginal values, including forward voltage drop (VCE_SAT, VF),breakdown voltage (BVCES), collector/drain leakage current(ICES), gate leakage current (IGES) and gate threshold voltage(Vth). Failure criteria are customer defined, for example,at 20% shift or less. Compared with silicon, GaN mayface unique challenges in these tests due to its structuralconstraints. HTGB/HTRB tests on a deposited gate insulatormay produce leakage or Vth shift problems. In other cases,thermal and mechanical stresses can possibly lead to cracks orother crystalline degradation in the heteroepitaxial film. Morestudies are needed to reveal details of GaN devices under thesetest conditions, especially for high-voltage and high-currentrated transistors.
Table 2. Automotive reliability tests for power semiconductors.
Reliability test Typical test conditions
Thermal cycling −40 to 125 ◦C (or higher Tj_max), 1000 cyclesPower cycling �T = 100 ◦C, 20 000 cyclesThermal shock −40 to 125 ◦C (or higher Tj_max), 1000 cyclesHigh temperature gate bias (HTGB) 125 ◦C (or higher Tj_max), VGE = VGE_max, 1000 hHigh temperature reverse bias (HTRB) 125 ◦C (or higher Tj_max), VCE = 90% VCE_max, 1000 hTemperature and humidity bias (H3TRB) 85 ◦C, 85% relative humidity, 80% VCE_max or 100 V, 1000 hVibration Depending on the customer profile and mounting scheme of inverter controller
6. Conclusions
This review presents a vision of Ford Motor Companyon the potential application of GaN power devices in ournext-generation electrified vehicles. The superior materialproperties of GaN for power switching transistors, togetherwith a low-cost manufacturing scheme (GaN-on-silicon),make this technology highly attractive. These properties notonly promise lower conduction and switching losses thanexisting silicon devices, but could also enable reduction inchip sizes due to the lower cooling demand. In the automaker’sperspective, this means valuable cost saving opportunities.Recent literature results on high-voltage GaN power transistorsevidence the rapid progress in the technology, such as growthof 600 V+ structures on large silicon wafers, normally-offoperation of GaN HEMT with various gate designs and thefrequent update on record low specific on-state resistances fora given breakdown voltage.
With that said, several critical technical and commercialchallenges still limit the near-term implementation ofGaN power electronics in automotive converters. GaNsemiconductor materials and fabrication are currently muchmore expensive than silicon, regardless of its manufacturingprocess (native epitaxy or heteroepitaxy). In order for thistechnology to be adopted in the hybrid drivetrain, a cost-effective manufacturing solution must be developed. Basedon present data and prediction, GaN native substrates, SiCand sapphire substrates all seem to be too expensive formass production of GaN high-power transistors. While GaNepitaxy on silicon is a viable solution, prices today are stillfar higher than target. Technically, growth of thick GaNepilayers up to the 1200 V voltage class is still difficulton large wafers. In addition, high gate threshold voltage,optimal surface passivation and control of current collapse,gate dielectrics, short-circuit withstanding capability and diesize scaling remain challenges for making qualified GaNproducts for automotive inverters. These issues are under activeresearch, but conclusive results may not be reached within afew years. It is our expectation that GaN power semiconductorbe first deployed in low-voltage applications in the near future,while certain challenges specific for high-voltage, high-powersystems are addressed. Once widely proven in low-voltagepower electronics, high-voltage GaN transistors may becomea reality for automotive qualification. Looking into the future,competitive GaN solutions for the HEV traction inverter couldbe developed to challenge the silicon IGBT incumbent after5–10 years, if the technical issues discussed are successfullyresolved.
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