GAN POWER TRANSISTORS - Panasonic

1 GAN POWER TRANSISTORS NEW CONTENDER FOR THE POWER TRANSISTOR THRONE: HOW GAN IS THREATENING THE MOSFET S CROWNWHITE PAPER VERSION GAN POWER TRANSISTORS VERSION CHARACTERISTICS OF GAN ..3 INTRODUCTION ..3 OVERVIEW AND ADVANTAGES ..4 GATE DRIVING PRINCIPLE ..5 Panasonic S HD-GIT STRUCTURE ..6 RELIABILITY ..7 END OF LIFE TEST ..8 SOLVING THE CURRENT COLLAPSE PROBLEM ..8 MECHANICAL STRESS ..9 TESTING TRAPPING AND CURRENT COLLAPSE ..9 ROBUSTNESS IN APPLICATION ..10 APPLICATIONS OF GIT TRANSISTORSSTRESS ..11 Panasonic GATE DRIVER IC ..12 ADVANTAGES IN APPLICATIONS ..13 TABLE OF CONTENTS3 Panasonic GAN POWER TRANSISTORS VERSION For more than 35 years, POWER MOSFETs have dominated the field of POWER converter design in the low to medium POWER range.

2 This has been supported by continuous innova-tion in the components structure and related semiconductor technology. Fast switching characteristics and low losses, as well as ease of use in various circuit topologies also con-tributed to their success. At the dawn of a new millennium, however, silicon POWER MOSFETs are reaching their theoret-ical performance limits, which means that further progress in POWER supplies and POWER management systems will no longer be as easy to achieve with these switching elements. Current trends in POWER supply unit design are focusing on higher efficiencies and POWER densities, that go beyond the capabilities of the silicon MOSFET technology. Development engineers need new switching devices that are able to meet these requirements.

3 And so begins the conception of gallium nitride TRANSISTORS (GaN) . THE CHARACTERISTICS OF GANThe first GaN POWER TRANSISTORS were introduced in the early 2000s, after being used over a decade as a standard fixture in high-frequency technology. The favourable combination of chemical-physical characteristics offered by GaN such as ten times the dielectric strength of silicon, high electron mobility and carrier density, very fast carrier recombinations, and last but not least a high maximum junction tempera-ture of over 400 C open up additional prospects for this material. Applied to POWER TRANSISTORS , these characteristics enable the manufacturing of high-switching frequency capable tran-sistors.

4 These in turn improve the POWER density of POWER conversion systems, thanks to lower conduction losses, and thanks to a reduction of the typical system size and weight for example by reducing the size of passive components under fast switching conditions. Gallium Nitride TRANSISTORS are therefore becoming a realistic and attractive alternative to silicon TRANSISTORS and start conquering the field of POWER GAN POWER TRANSISTORS VERSION AND ADVANTAGESP anasonic Hybrid Drain-Gate Injection TRANSISTORS (HD-GITs) are normally-off GaN-on-silicon TRANSISTORS . They are based on the HEMT principle, using the highly mobile 2D electron gas forming at an algan -GaN heterojunction as conduction layer.

5 The active part of the transistor is completed on the top side with (ohmic) drain & source contacts, a recessed p-GaN gate (ohmic contact) and a p-GaN gate like structure con-nected to the drain. For cost reasons, the transistor is grown on top of 6 inches silicon wafers by MOCVD process. In order to reduce the tensile stress caused by the mismatched crys-tal lattices of Si and GaN, to limit the vertical drain-substrate leakage currents and to prevent deep breakdown paths in the conductive Si substrate, a lattice buffer layer (Figure 1) is inserted between the silicon bulk and the active top side of the buffer plays a central role in the determination of key reliability characteristics of the TRANSISTORS (Figure 2), as we ll develop further below.

6 The transistor is turned on resp. off like a field effect transistor, by applying a gate-source volt-age above resp. below a threshold voltage. In off state, the p-GaN gate depletes the electron gas underneath by lifting the potential well of the algan -GaN junction above the Fermi level. In on-state, the gate behaves essentially like a diode. Unlike in MOS TRANSISTORS however, a small (around 10mA) current is injected from the gate into the conducting layers by electrons tunneling through the algan barrier. Due to the low velocity of holes in the GaN material, the current conduction at the algan -GaN interface is only due to the electron gas, and so the TRANSISTORS are to be understood essentially as unipolar devices on this the HD-GiT gate can be accessed directly, the gate circuit can be designed to control and adjust the transistor s du/dt and di/dt a major advantage as compared with the cascade.

7 Figure 1 Figure 2 GaN epitaxialStrainRelaxationSi (111) substrateAlNAlGaNGaN / AlNSuper-lattice Buffer5 Panasonic GAN POWER TRANSISTORS VERSION lateral structure of the GiT is also advantageous for fast switching, since its parasitic capacitances are typically lower than those of vertical structures, such as for example sili-con-based super-junction MOSFETs (see Figure 3). The Figure-of-Merit (RDS(on) QG) of a 600V/70nOhm GiT is therefore ~350m nC, in other words around one tenth the value of modern silicon DRIVING PRINCIPLE The GiT transistor is controlled as mix of a field effect transis-tor and (bi)polar transistor. As with FETs, a positive threshold voltage needs to be applied between the source and gate to open a conductive channel.

8 At the same time, a small on-time current needs to flow into the gate to increase the conduc-tivity of the conductive layer and to keep the switch-on re-sistance as low as possible in the useful operating area. The transistor is switched off by removing the voltage from the gate; the gate current stops, the channel closes, the transis-tor is blocking again. Unlike with IGBTs however, the charge recombinations at switch off do not result in measurable de-lays or POWER losses due to tail currents (see Figure 4).Figure 3 Figure 46 Panasonic GAN POWER TRANSISTORS VERSION GiT TRANSISTORS allow current to flow in the reverse direction once the source, gate, and drain potential are set in a way that current is fed in at the gate.

9 Unlike with MOSFETs the reverse current does not flow through a parasitic body here, instead is it conducted through the channel. Even though they are reminiscent of a diode, the thresholds in the third quadrant of the static IV curve are not dictated by junc-tion s behaviour, but are simply the threshold voltage of the transistor plus any negative bias voltage that is applied to the gate. In the same way as a MOSFET, the GiT can be switched on in the reverse direction in order to further reduce the loss-es by operating under 0V-offset condition. The GiT recovers extremely quickly from reverse conduction. The recovery en-ergy practically just corresponds to the energy required to charge the output capacitance.

10 The conduction and recovery performances of the GiT in the reverse mode are the same as those of an SiC Schottky diode (see Figure 5). Panasonic S HD-GIT STRUCTURE Developers of gallium nitrite components have to deal with the phenomenon of current collapse. When the transistor is under high voltage stress, conduction electrons can become trapped in defects in the crystal, at interfaces between lay-ers etc., which can leads to a rapid increase of the RDS(on) (on-resistance), leading to a rapid increase in losses and the destruction of the components. This effect can be especially critical for hard switched topologies. So far Panasonic has been the only provider of GaN compo-nents to publicly announce the complete elimination of the problem of current collapse.