Lateral GaN Transistors – A Replacement for IGBT devices in Automotive Applications

Technology News | September 25, 2014

By eeNews Europe

The improvements offered by the GaN devices are yet to be realized in deployed subsystems. Several groups of researchers are experimenting and reporting upon GaN transistors that are aimed at replacing Si IGBTs. The results achieved by GaN Systems are presented and these are compared to other producers of GaN devices.

Introduction

The chart shown in Figure 1 describes the relationship between required motor power and power source voltage for some of the HV/EVs 2010/12 [1]. The dashed lines show the average current flow, while the power source voltage indicates the voltage limits we have to minimally reach. Blocking Voltage ratings of 650V and 1200V for high current large area GaN devices have been difficult to achieve. For that reason ARPA-E projects involving IR/Delphi and DOE projects of GaN Systems/APEI have been directly aimed to meet the needs of the automotive industry [2], [3]. Similarly at HRL, development work was partially supported by ARPA-E and the General Motors Company [4].

Device Technology and Performance

The vehicles shown use Si-IGBT devices in the inverters. However, the material limits of Silicon prevent further substantial improvements of the performance of these devices. As shown in Figure 2, both GaN and SiC devices potentially offer alternative solutions. The chart compares the on-resistance achieved per unit of device area at a given blocking (rated) operating voltage. GaN and SiC devices can achieve substantially lower losses per unit area than Si-IGBT devices. In addition, conventional IGBT devices have an offset voltage of 2-2.5 volts that is present over the entire rated operating current range. This leads to inefficient operation at low currents.

However, the voltage offset is almost temperature invariant. The on-resistance of GaN devices doubles if they are allowed a temperature rise of 150°C. When higher speed operation is required the GaN and SiC devices provide substantially lower losses because they have lower capacitances and near zero charge storage. A complex tradeoff between chip size, switching speed, thermal resistance, converter efficiency and cooling capabilities must be made.

Physical size and weight relate directly to cost. The chart shown in Figure 3 compares the power densities achieved using the various power switches [5]. As shown, boost converters built by HRL using a normally-off GaN transistor, and GaN Systems also using a normally-on GaN transistor. These first generation GaN designs have achieved better than 10 watts per cm3, a 10x improvement upon typical Si based converters. It is clearly possible to provide exceptional performance in advance of the projected time line.

Electrical and Packaging issues

There are two distinctly different approaches used to construct useful normally-off GaN transistors. HRL has described a normally-off device that is a monolithic GaN structure while IR has maintained an adherence to the cascode technique where a series MOSFET provides the normally-off function. The reliability and longevity requirements of the automobile industry would dictate that the multiple bond wires that normally apply to the cascode would preclude the use of the cascode structure. Bond wires also present an inductance challenge.

Figure 4 shows the package types used by IR/Delphi [6] and GaN Systems. As shown the GaN Systems designs use a source sense electrode. This ensures that the devices can be driven cleanly, on-and-off, and free of source power electrode generated noise. IR has taken a different approach. Here there are is direct lateral ‘interposer’ has low inductance. GaN Systems has addressed the issues by stacking the devices where the GaN transistor acts as a carrier of the small low voltage MOS device – the combination allows connections to be made with low inductance connections.

As shown in Figures 5 and 6, IR has provided its cascode design with a very high threshold voltage customized series connected MOSFET [7]. This unique low voltage MOSFET has an even higher threshold voltage than the Si-IGBT. This combined with the inherent low inductance of the package ensures that the device does not suffer noise induced transient switching. The sharp contrast between the two cascode designs is therefore due to IR’s duplication of the IGBT input voltage characteristic while also providing a low input capacitance by using a rather smaller than ideal MOSFET. The IR cascode therefore has a lower than expected mutual conductance but this makes the cascode more controllable and may provide for a safe internal common node voltage. This technique limits the voltage stress on the gate of the GaN device. Indeed, the custom MOSFET may be further tailored to avalanche at a low voltage to protect the gate of the GaN device. Also, the internal lateral passive ‘interposer’ could be designed to provide additional capacitance to tame the voltage excursions of the common node of the cascode. Both the relatively small cascodes from IR and GaN Systems can provide about 80 amps under saturated conditions at room temperature.

As noted by IR [7] GaN devices providing this performance are achieving maximum current density of 1000 A/cm2. IGBTs which have a rated current density of 150 A/cm2 have a maximum current capability of 300 A/cm2. At 150°C the on-resistance of the GaN device doubles and the current density capability is reduced to 600 A/cm2. Under realistic conditions where the case temperature can only be held down to 105-120°C the current density must be limited to 200 A/cm2 but this is still a substantial improvement over IGBTs.

The complexity of the cascode has led HRL and GaN Systems to develop normally-off GaN devices [4]. These devices have very low threshold voltages as shown in Figure 5 and 6. From some viewpoints, these devices must use signal isolators, isolated positive and negative power supplies and the source sense electrode to ensure safe operation. These requirements could add significant costs and complexities. However, the normally-off devices are potentially able to operate up to 200°C, a substantial advantage. Therefore a simpler drive system is needed to reduce the cost and complexity of these circuits. In addition it is desirable to reduce the need to use numerous parallel devices.

GaN devices that have on-chip drivers, that are easy to drive, that can sink and that source 200-400A are needed solutions are presented. Lateral 650V GaN devices achieve between 200 and 500mA per mm of gate width (Wg). If the device is carefully laid out and the electromigration issues are properly considered a 1 cm2 device can achieve a peak current limit of up to 1000A. Large area devices having total device areas of 0.3 to 1.3 cm2 will be available for developmental designs in 2014.

Very large area devices

Very large area normally-off (E-mode) GaN transistors can produce current transitions of 300A within 4 ns. As a result, inductive interconnects can cause formidable problems. Ultra low inductance packaging is a vital requirement. As will be described, on-chip drivers are the only feasible solution to control the driver loop inductance requirement. The circuit shown in Figure 7 has two small on-chip drive transistors, D1 and D2 closely connected to the gate of the major power switch, D3. The critical loop inductance of the drive circuitry is therefore very low. In addition very small, 0.5A pre-drivers can be used to drive D1 and D2. If D3 was driven directly, a very large, high current driver would be needed.

The drive transistors D1 and D2 are integrated with the large area transistor D3, so a very low inductance internal source sense connection has been provided. As a result clean drive waveforms are provided to the gate of D3. Switching transition losses due to output voltage and output current overlap are minimal as shown in Figure 8.

The other sources of switching transition loss include QRR charge storage losses CV2f capacitance charge/discharge losses and IGBT tail current overlap. GaN normally-off devices have no intrinsic body diode structure and they are free of QRR or tail current loss issues. The power losses associate with driving the CISS, COSS and CRSS capacitances of the power switching transistors can be significant in high speed inverters and boost converters.

The intrinsic C-V curves for GaN, IGBT and SJ MOSFETs are shown in Figure 9. The GaN device has significantly lower capacitances than the equivalently rated silicon devices. The COSS capacitance plays the leading role in determining the losses when high speed, high voltage operation is required. The CRSS capacitance primarily determines the drive difficulties that arise from Miller effect. The CISS capacitance also determines some of the drive challenges. The SJ MOSFET has, for example, a CISS capacitance that reaches 200nF in the on-state!

The data shown in Figure 10 was compiled from data sheet information and measurements made using large area GaN devices. The results were normalized to the rating to the GaN transistor. The GaN device has the smallest chip size and a conduction loss which is typically 50% better than the silicon devices. At current loadings below 200A the GaN device has an even better comparative performance than the IGBT because of the IGBTs offset voltage. The table below compares the maximum current conduction losses of currently available 600/650 V IGBTs and SJ MOSFETs with the developmental normally-off GaN-on-Si device described earlier.

The automotive market opportunity

The major automotive market drivetrain opportunity includes the boost converter and inverter system of hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and also battery-electric vehicles (BEVs) that are powered using electricity from the grid. According to Navigant Research’s 2013-2020 Electric Vehicle Market Forecast the total market for these vehicles, as shown in Figure 11, will reach 6 million units by 2020. This market may therefore offer a $1B opportunity for power semiconductor manufacturers.

The simplicity of the BEV approach offers the possibility of substantial cost saving over the long term due to the absence of the gas engine and its supporting equipment. BEVs are expected to achieve a 32% CAGR over the next six years. These projections rely in part upon the belief that the battery cost will drop by 10 to 25% by 2020. Similar intense pressures will be exercised upon the semiconductor costs, making GaN-on-SiC transistors and pure SiC transistors uncompetitive due to the high price of SiC wafers. The battle for the automobile market will therefore be contested by IGBTs, SJ MOSFETs, and GaN-on-Silicon devices. The lowest cost devices will have a blocking voltage of 650V because of the ubiquity of devices that will be made to provide this blocking voltage.

Conclusions

The fabrication of GaN-on-Silicon very large area normally-off transistors will be a challenge that will be met this year. Further success will depend in part upon the achievement low defect densities. However even the best projected defect density will not provide cost effective very large area production worthy devices. High yield devices that have total chip areas of 0.3 to 1.3 cm2 require sophisticated methods that completely isolate the defects. The success of these schemes will determine whether the very large area GaN devices described will yield production worthy, and cost effective production worthy products.

The performance improvement, chip size reduction and clear potential system level cost advantages of very large GaN devices is very evident. If the devices can be yielded and proven to be reliable they will be the obvious choice to replace IGBTs in automobile applications.

References

[1] T. Kachi, D. Kikuta, T. Vesugi, “GaN Power Device and Reliability for Automotive Applications” Reliability Physics Symposium (iRPS) 2012.

[2] Press Release – International Rectifier, Jan 2010

[3] Press Release – GaN Systems, Sept 2013

[4] R. Chu et al, “Normally-off GaN-on-Si Metal-Insulator – Semiconductor Field-Effect Transistor with 600-V Blocking Capability at 200°C” 24th Int. Symp. Power Semiconductor Devices and ICs June 2012.

[5] C. Blake “GaN Technology: What Power Designers Need to Know” PSMA 2013 pp. 330.

[6] H. Lee, “GaN-on-Silicon-Basked Power Switch in Sintered Dual-Side Cooled Package” Power electronics Feb 2013.

[7] M. Briere, T. McDonald, H. Lee, L. Murlino, “Current Handling Capability of 600V GaN High Electron Mobility Transistors” Power Electronic Europe Issue 7 2012 pp. 26-28.