Gallium Nitride Applications in Power Electronics

Transcription

1 Gallium Nitride Applications in Power Electronics Mohammad Taufik 1, Taufik 2 1 Electrical Engineering Department, Universitas Padjadjaran, Bandung, Indonesia 2 Electrical Engineering Department, Cal Poly State University, San Luis Obispo, California, USA 1 m.taufik@unpad.ac.id 2 taufik@calpoly.edu Abstract For just over 50 years doped Silicon (Si) has been the dominant semiconductor material for electronic circuit design and will continue its dominance for the foreseeable future for good reason. For now, competing or complementary process technologies lack the economies of scale to drive costs down in parallel with Si. In addition to development and manufacturing costs, Si process behavior and circuit performance are well understood and consistently predictable well into the deep submicron level. Current production Si channel lengths are about 22nm and 18µm for digital and analog integrated circuit manufacturers, respectively. Other than design specific characteristics, doped Si performance suffices for the vast majority of applications today. This paper will lightly cover Gallium Nitride (GaN) properties and the applications in power electronics. Additionally, GaN will also be compared with Silicon Carbide (SiC) which has very good characteristics for power electronics but is not a III-Nitride compound. Keyword Heterostructures, power transistor, heterojunction field effect transistor (HFET), high electron mobility transistor (HEMT) I. INTRODUCTION As Si based power systems approach theoretical limits of device performance, alternative semiconductor materials were considered, in particular semiconductors having wide energy bandgaps. This group III-Nitride (trivalent-nitride) compound semiconductors offer compelling material properties superior to Si and consequently extend device circuit performance. Such group III-Nitride heterostructures include GaAs, AlN, InP, and GaN, all having important common material properties of a wide bandgap, high electron saturation velocity, high breakdown voltage, high thermal conductivity and other important characteristics when compared with Si. Each of these materials possesses unique characteristics that fulfill niche applications and potentially can expand beyond their intended uses. With regard to GaN, the wide bandgap, high electron saturation velocity, and low frequency noise characteristics eventually led to the development of optoelectronics (ultra-bright blue LED and laser diodes for reading Blue-ray discs) and high efficiency power amplifier designs (Class D, Class E, Class F and Class J). In addition to the wide bandgap and high electron mobility and saturation velocity, high breakdown voltage, and high thermal conductivity (substrate dependent) characteristics inevitably led to applications in power electronics.[1],[11],[12],[15]. The exponential growth of publications over the last three decades show continued growing interest in research and development of applications for III-Nitride compound semiconductors. Fig. 1 Progress in the number of publications per year in group III-Nitride material and device research [1]. The increase can be attributed to a breakthrough in Metal Organic Chemical Vapor Deposition (MOCVD) epitaxial growth in the late 1980s which led to the first electronic and optoelectronic devices that were successfully demonstrated, creating strong interest in the field and leading to a variety of other circuit applications As alluded to earlier, the initial impetus for the development of Group III-Nitride compounds was for optoelectronic and microwave circuit applications. The direct-bandgap materials release energy that can be directly converted to light during electron-hole recombination, which is extremely important in optoelectronic applications. As for microwave circuit applications, the wide bandgap, high electron saturation velocity and in particular the low frequency noise characteristics of III-Nitride materials enable RF power amplifier designs, like those employed in mobile phones and other consumer electronic products [16]. Research and application of GaN to power electronics has been ongoing for about a decade. As 125

2 mentioned earlier, the material characteristics of a high critical electric field, wide bandgap, high electron mobility and saturation velocity, high breakdown voltage, and high thermal conductivity eventually led to applications in power electronics. TABLE I CHARACTERISTICS OF SI WITH WIDE BANDGAP SEMICONDUCTOR. Semiconductor Si GaN SiC-4H SiC-6H Breakdown Field (kv/cm) Bandgap (ev) Electron Mobility (cm 2 /V-s) Thermal Conductivity (W/cm-k) Saturation Velocity 1 x x x 2 x 10 7 (cm/s) 10 7 Table I displays the most important semiconductor parameters with respect to power electronics applications. The electron saturation velocity, which is proportional to the breakdown field voltage (critical electric field), enables high frequency switching by the GaN transistor and diode. The 3.45eV bandgap, a significant three times more than Si s bandgap, requires higher activation energy and therefore less susceptible to high temperature performance degradation, a key feature in power supply design, automotive and other high temperature applications. The thermal conductivity of GaN is not quite as good as Si, but GaN is grown on either one of three substrates: Si, SiC and sapphire, with the latter two the most common. GaN on SiC has the best thermal conductivity of the three and is preferred for high power and high temperature applications. As a reference, GaN on SiC has 10 times better thermal conductivity than GaN on sapphire [1]. The potential advantages of these wide bandgap devices include higher achievable junction temperatures, and thinner drift regions that are due to the associated higher critical electric field values, which also can result in much lower on- resistance than is possible in Si. There are however, several disadvantages associated with the use of heterojunction devices fabricated from wide bandgap materials. Among these is that the ratio of the electron to hole mobility is much higher than in Si, so the use of wide bandgap semiconductors for bipolar devices is not desirable. HFETs are similar to MOSFETs in that they are unipolar devices [l7]-[19]. II. PROPERTIES AND STRUCTURE OF GALLIUM NITRIDE The group of III-Nitride semiconductors includes three main materials, Gallium Nitride (GaN), Aluminum Nitride (AlN) and Indium Nitride (InN). All three materials crystallize mostly in socalled wurtzite structures that have a hexagonal unit cell. An example of GaN crystal structure is shown in Figure 2. An important property of this crystal cell is the lack of inversion symmetry, which leads to very strong polarization effects in group III-Nitride materials. As emphasized earlier, the fundamental property that makes group III-Nitride materials extremely attractive for high- power, high temperature applications is the very large bandgap. Sufficient energy (E g in ev) is required to ionize atoms, enabling electrons to jump the bandgap (the valence band to the conduction band and become free electrons). The bandgap energy values for the most important semiconductor materials are shown in comparison to each other in Figure 3. From Table 1 and as one can see from Figure 3, the bandgap energy of GaN is 3.45 ev and is more than three times higher than that of Si s 1.1 ev. Since the probability of ionization and many other processes depend exponentially on the bandgap energy, the large bandgap is the key factor for high temperature operation, chemical inertness, and high breakdown voltage of GaN devices. The bandgap energy of AlN is even higher, about 6.2eV. AlN applications include optoelectronics, dielectric layers in optical storage media, and as a crucible to grow crystals of GaN. Fig. 2 Schematic of the wurtzite GaN crystal structure. Fig. 3 The bandgaps of the semiconductors materials. 126

3 GaN devices are made as High Electron Mobility Transistors (HEMT), also known as heterojunction FETs (HFETs) which are unipolar (electrons are majority carrier) field effect transistors using a junction between two materials with different bandgaps (a heterojunction) as the channel instead of a doped region, as is generally the case for MOSFETs. An AlGaN/GaN combination can provide the 2DEG 1 channel as illustrated in Figure 4. Fig. 4. General cross-sectional view of a GaN HFET [14] Figure 4 defines the general cross-sectional structure of agan device. Choosing the substrate, dimensions of each layer, the length of gate, drain, source and the distance between them, selecting materials for contacts and finally optimizing passivation layer are fundamental steps in designing GaN devices. Moreover, to achieve higher breakdown voltages and low on-resistance, field-plate technology has to be executed [14]. One of the challenges of working with GaN is choosing the proper substrate. Presently, Si, sapphire, and SiC can be used as substrate with the second two as common choices. Unfortunately, the lattice mismatch of GaN on sapphire and SiC which are 13% and 3.1%, respectively, could introduce defects and dislocations and degrade the device performance. Between sapphire and SiC, due to higher thermal conductivity and also lower lattice mismatch, SiC is preferred for high power and high temperature application nevertheless its higher price. The absence of an inclusive substrate is one of the drawbacks of GaN technology; the substrate must be selected for the target application. After selecting the substrate, an AlN buffer layer is grown on it to improve the interface for the growth of GaN channel layer, and finally the AlGaN layer is grown to complete the heterojunction. Each of these layers has its own thickness based on the particular application [1][23]. Although GaN-based materials have been initially applied to optical devices and commercialized, transistors using GaN materials for power switching applications were developed. Specifically, two types of structures were engineered. One candidate is a lateral structure and the other candidate a vertical structure in the same manner as Si power devices. Furthermore, these device characteristics were improved due to the improvement of both the device structure and the growing technique of GaN-based materials. In the case of lateral devices, the most distinctive feature of GaN-based materials is a two-dimensional electron gas (2DEG) generated at the interface of an AlGaN/GaN heterostructure (Figure 4), resulting in a high electron mobility and a high carrier density due to its spontaneous polarization effect. HFET devices using an AlGaN/GaN heterostructure can obtain a high speed, large current, and high breakdown voltage characteristics. Toshiba developed a high-power AlGaN/GaN HFET on SiC substrate or sapphire substrate, which included documented applications of DC-DC converters utilizing the AlGaN/GaN HFET structure. Panasonic reported a high power AlGaN/GaN HFET on a sapphire substrate using the vertical structure, resulting in a breakdown voltage over 10kV. Japanese manufacturer Sanken showcased another high power GaN transistor, which demonstrated low specific on-resistance, high voltage characteristics and the application of power factor correction circuits [24]. The above examples of high power and high temperature applications employed AlGaN/GaN HFETs fabricated on sapphire substrates and used in DC-DC converters. Despite the high quality epitaxial layers of GaN materials, sapphire is not considered to be an optimum substrate since the thermal conductivity is less than that of SiC or Si. A better selection, Si substrate is a promising candidate for growing the GaN epitaxial layers due to the low cost and easily obtaining a large diameter. However, the epitaxial growth of GaN/AlN layers on the Si substrate is difficult due to the large lattice mismatch and the large thermal expansion-coefficient difference between GaN and Si materials, as discussed earlier. Therefore, the buffer layer structure is important for realizing both the smooth surface and the large thickness of GaN epitaxial layers in the case of using a large diameter Si substrate. The epitaxial growth technique has been improved, resulting in growing a smooth surface and thick epitaxial layers. To improve the breakdown voltage on an Si substrate, the suppression of the buffer leakage current in both the vertical and the lateral direction is very important. Additionally, in order to obtain a high resistive buffer layer, a carbon doping technique was applied to the GaN buffer layers. Alternatively, a vertical structure is very effective for achieving low on-resistance and high breakdown voltage characteristics in similar to conventional Si-based power devices such as power MOSFETs or insulated-gate bipolar transistor 127

4 (IGBTs). As for GaN-based power devices, there have been several attempts for realizing the vertical power devices. UCSB reported a unique structure, CAVET, using 2DEG carriers introduced to a vertical direction. Toyota reported vertical structures combined with regrown p-typed GaN in order to confine the vertical current [24][25]. Also, normally-off operation is strongly required in the case of power switching applications, especially inverters or DC- DC converters due to a fail-safe design. Several approaches have been demonstrated in order to realize normally-off mode operation. First, a lateral MOS structure was developed Furukawa Electric composed of using SiO 2 layer as a gate insulator film on GaN surface [26][27]. The device operated normally off mode like an Si MOSFET structure. When involved with AlGaN/GaN HFETs, a phenomenon called current collapse can occur by the trapping of majority carriers in the defects, resulting in the increase of on resistance. Therefore, the suppression of the current collapse phenomenon should be required in order to realize the lower loss power switching. Research into suppressing or minimizing current collapse occurrence using the gate or source field-plate structure, not only improved the breakdown voltage of the device, but it can decrease the concentration of the electric field when high voltage was applied to the device. The field-plate structure is a very effective method to decrease the electric field of the gate to drain and therefore minimize the chance of current collapse. Additionally, an electrical conductivity of a substrate also affects the current collapse because of the field-plate effect on the back side. Fig. 5 Specific on-resistances (direct bandgap), R DS(ON), as a function of breakdown voltages for Si, GaN, and SiC [20]. where V B - breakdown voltage, E G - bandgap energy, µ n - electron mobility, ε r - relative permittivity GaN HFETs have some of the same capacitance behaviors as other FET devices, but the higher power density relative to the other semiconductor technologies result in smaller devices and thus smaller absolute capacitance values. Figure 6 shows measured C GD and C DS vs. V DS for the Nitronex HFET NPT These are compared to simulated results using a TCAD model. III. PARASITICS Sources of parasitic losses from in Si-based power MOSFETs and diodes are considerably reduced using GaN material. The HFET R DS(ON) is reduced by taking advantage of high-electron mobility in a twodimensional electron gas (2DEG) layer, large critical electric field, and high breakdown voltage material characteristics. Figure 5 plots the on- resistance of Si, SiC, and GaN versus breakdown voltage and highlights the advantage of Si and GaN over Si. Additionally, since GaN is a wide bandgap material, on-resistance will not be affected with temperature changes, unlike the on-resistance of Si. Equation 1 below was derived quantitatively from research done by J. L. Hudgins, et al [20] and it is a more accurate than the initial on-resistance equation by S. Sze, et al. Fig. 6 Nitronex HFET NPT25100 Measured and simulated gate capacitances C GD, C DS vs. V DS IV. APPLICATIONS The performance of HFETs in switch-mode power supply applications have exceeded that of Si considerably. Following are few examples. GaN heterojunction field effect transistors (HFET) were used as switches in an inverter application with the following operating parameters: 20A, on-state R DS(on) = 8mΩ/cm 2 at 370V and switching times of 10ns/11ns on-time/off-time, respectively. The inverter frequency was 370MHz for DC/AC conversion from 30V to 100V AC. The two- 128

5 stage converter topology included an input converter with four switches operating in parallel and a DC/AC stage with in an H-bridge converter configuration [5]. A AlGaN HFET on sapphire substrate was designed to operate at a 470V breakdown voltage in a 200kHz DCDC converter application [6]. A fabricated GaN HFET yielded an on-resistance of 1.9mΩ/cm 2 and a 700V breakdown voltage [7]. A set of GaN HFETs tested in a half-bridge configuration for a DC/AC converter application exceeded a switching power density of 20W/mm 2. The 90ns rise and fall time were limited only by the driver circuit [8]. Another arena for GaN applications would be for theautomotive industry. High-performance solid-state devices are required for future vehicular power electronics, particularly in electric and plug-in hybrid electric vehicles which need high power inverters to drive their propulsion systems. Essentially, power applications for GaN HFETs and diodes can be found in high temperature environments, where radiation hard devices are required, and where fast switching is required. V. CONCLUSION With Si-based power electronics approaching its property limits, GaN heterojunction devices have picked up the performance baton where Si performance has started to tail off. Superior GaN material characteristics, with the appropriate substrate, are crucial in pushing power electronics performance beyond that offered by Si. In particular, properties like higher power density, higher junction temperature, a large critical electrical field, high electron saturation velocity, high breakdown voltage and high thermal conductivity not only contribute to the design of higher efficiency power supplies, but also expand voltage, current and temperature ranges beyond limits offered by Si. Sources of parasitic losses from in Si-based MOSFETs and diodes are dramatically reduced with GaN technology. With GaN HFETs, R DS(ON) is a fraction of MOSFET R DS(ON). This is done so by taking advantage of the following properties: high electron mobility in a two-dimensional electron gas (2DEG) layer, a large critical electric field, and a high breakdown voltage. Transistor gate capacitances C ISS and C OSS, C GS + C GD and C GD + C DS, respectively, are minimized because of the higher power density relative to Si, resulting in smaller devices and thus smaller capacitances. GaN Schottky diodes performance outstrip their Si equivalent counterparts in power electronic applications. Taking advantage of the high breakdown voltage and low leakage current properties of GaN, Japanese electronics manufacturer Powerdec introduced a 600V GaN Schottky diode with an on-resistance over 100 times smaller than existing power diodes. Powerdec also released a 620V/10A rated GaN power diode that measured only 20µA of leakage current. Transphorm, a GaN start-up out of Santa Barbara, California, demonstrated at APEC a DC-DC boost converter. Using their 400V diode and 600V HFET, an efficiency of 99% was achieved (switching speed not published). Improving the efficiency of power supplies require reducing parasitic losses in components. Through careful component selection and balancing tradeoffs, losses can be reduced but significant power savings are at the mercy of the physics of each component: MOSFET, freewheeling diode, and inductor. GaN power devices possess the same parasitic losses that chip away at efficiency, but at a considerably lower level than with Si power devices. A new industry has sprouted based on the advantageous of GaN. Several GaN start-ups such as Transphorm, Efficient Power Conversions, and Nitronex have been releasing GaN- based transistors and diodes into the market. 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