Reverse Conduction Mode of GIT GaN Eliminates the Need for Free-wheeling Diode
Consistently recognised as one of the ‘industry best’ GaN power switches, the novel GIT (Gate Injection Transistor) GaN power device developed by Panasonic achieves a true normally off operation that differs from the complicated cascode configuration. The GIT GaN is also free from current collapse for voltage above 600V where by no behaviour from dynamic RDSON is visible. Figure 1 shows a schematic cross section of the normally-off GIT GaN transistor. P-type AlGaN gate which is placed over an AlGaN/GaN heterojunction depletes the channel under the gate at zero bias of gate-source voltage. When the gate-source voltage is greater than the threshold voltage, holes are injected from the gate resulting in increased drain current and lower on resistance by the conductivity modulation. With the drain current controlled by gate voltage similar to conventional power devices such as IGBTs or Si Power MOSFETs, the driving methods and understanding of Si devices can be applied easily when implementing GIT GaN for wide range of application.
Consistently recognised as one of the ‘industry best’ GaN power switches, the novel GIT (Gate Injection Transistor) GaN power device developed by Panasonic achieves a true normally off operation that differs from the complicated cascode configuration. The GIT GaN is also free from current collapse for voltage above 600V where by no behaviour from dynamic RDSON is visible. Figure 1 shows a schematic cross section of the normally-off GIT GaN transistor . P-type AlGaN gate which is placed over an AlGaN/GaN heterojunction depletes the channel under the gate at zero bias of gate-source voltage. When the gate-source voltage is greater than the threshold voltage, holes are injected from the gate resulting in increased drain current and lower on resistance by the conductivity modulation. With the drain current controlled by gate voltage similar to conventional power devices such as IGBTs or Si Power MOSFETs, the driving methods and understanding of Si devices can be applied easily when implementing GIT GaN for wide range of application.
Figure 1: Cross section of GIT GaN structure and its basic operating principle
The GIT GaN power switches demonstrate many characteristics that are superior over conventional Si power devices as described below:
- Simultaneity of low on resistance and high breakdown voltage
GaN switches have ten time higher breakdown field strength and more than twice the saturation velocity of silicon devices. These characteristics are greatly desired for a power device that needs high breakdown voltage and low on-resistance. With only electron migration, the on resistance has a positive temperature coefficient similar to Si Power MOSFET. Thus, the current concentration leading to secondary breakdown suffered in IGBT does not occur. - Zero Offset Voltage at Zero Current
Figure 2 shows the IDS vs. VDS of GIT GaN which does not have an offset voltage at zero current that usually exist in IGBT. This achieves very low conduction losses at low currents. - High Speed Switching
RonQg which is the figure of merit for high speed switching, the GIT GaN exhibits one-thirteenth lower than the best in class Si Power MOSFET. The switching characteristic of GIT GaN is much faster for its low gate charge. Moreover, since there is no accumulation of minority carrier on gate during turn-off and the recombination current does not flow to gate, tail current that flows in conventional IGBTs does not happen as shown in Figure 3.
Figure 2: Comparison of IGBT and GIT GaN current versus voltage characteristics
Figure 3: Switching characteristic of GIT GaN versus IGBT. The long tail current of IGBT adds to the switching losses
Besides the benefits mentioned above, another characteristic of the GIT GaN that makes it very attractive especially for driving inductive load application is the ability of bi-directional conduction complimented by the operation of reverse conduction mode that eliminates the necessity of the free-wheeling diode and utilization of synchronous rectification. GIT GaN does not have a pn-junction between the Drain and Source. Therefore, the usual body diode does not exist. However, GIT GaN is still able to achieve the same rectification behaviour of the external anti-parallel diode or body diode through the reverse conduction mode operation.
Figure 4: Reverse conduction mode operation of GIT GaN
Reverse conduction mode is activated when the gate and source are electrically short-circuited. Drain current does not flow even if positive voltage is applied between drain and source as described in the operation of GIT GaN earlier. However, when the drain voltage becomes lower with respect to the source voltage and also the gate-drain voltage exceeds the gate threshold voltage, a reverse transistor becomes conducting and reverse drain current is able to flow from the source to drain as show on Figure 4. Therefore, reverse conduction mode provides rectification function without the need of additional free-wheeling diode externally. However, do take note that even if rectification behaviour by reverse conduction mode is possible when negative voltage is applied between gate and source of GIT GaN, the forward voltage threshold (Vth) will increase in proportion with the negative bias. The body diode of Si Power MOSFET has a reverse recovery phenomenon which requires time for turn-off by recombination of minority carrier electric charge. Since GIT GaN has no recombination in the reverse conduction mode, the device can turn off at high speed. Figure 5 shows the measured reverse recovery characteristic of GIT GaN being operated as an anti-parallel diode where the recovery loss is almost negligible.
Figure 5: Negligible recovery charge of the diode mode of GIT GaN compare to a Si-FRD
Utilizing the full benefits of reverse conduction mode with synchronous rectification, the operating loss of an inverter can be significantly reduce for a GIT GaN based type compare to the conventional IGBT type. Tail current, forward and reverse offset voltage associated with IGBT implementation and the need of external diode can all together be obsoleted.
Figure 6: Operation of half bridge inverter using GIT GaN utilizing the device bi-directional feature
The basic operation of the half bridge inverter shown in Figure 6 is used to explain the basic of diode free operations to flow the free-wheeling current by bi-directional GIT GaN. The reverse conduction mode needs to be used to suppress the short-circuit current between the two switches, which is enabled by short-circuiting the source and gate. After a certain period of the dead-time, the GIT GaN is switched from the reverse conduction mode to the on-state FET mode to minimize the power loss caused by the forward offset voltage of the reverse conduction mode. Figure 7 shows the conversion efficiency of the GIT GaN based inverter for various output power and the world highest 99.3% is achieved at 1500W .
Figure 7: Comparing the conversion efficiency of GIT GaN inverter for various output power with that of IGBT
Eliminating the needs for the external free-wheeling diode reduces by half the number of devices for a GaN based system from that of an IGBT based type . This simplifies the thermal management of the system as in usual cases the free-wheeling diode is placed close to the IGBT switches. To further extract the potential of GIT GaN, the simplicity of device integration (monolithic) will yield the benefit of system cost reduction and a more compact inverter design. In addition, a normally-off GIT GaN by a single chip also allowed for a flip chip configuration that can achieve very high turn-on switching speed with slew rate of 170V/ns which is 1.7 times quicker than that of the start-of-the-art SJ-MOS . This is all almost impossible to be achieved by a cascode GaN.
REFERENCE
Y. Uemoto, M. Hikita, H. Ueno, H. Matsuo, H. Ishida, M. Yanagihara, T. Ueda, T. Tanaka, D. Ueda, “Gate Injection Transistor (GIT)—A Normally-Off AlGaN/GaN Power Transistor Using Conductivity Modulation,“IEEE Trans. Electron Device, vol.54, no.12, pp3393-3399, 2007.
Y. Uemoto, T. Morita, A. Ikoshi, H. Umeda, H. Matsuo, J. Shimizu, M. Hikita, M. Yanagihara, T. Ueda, T. Tanaka, and D. Ueda, “GaN Monolithic Inverter IC Using Normally-off Gate Injection Transistors with Planar Isolation on Si substrate.”, IEDM Technical Digests, pp. 165-168, December 2009.
T. Morita, S. Tamura, Y. Anda, M. Ishida, Y. Uemoto, T. Ueda, T. Tanaka, and D. Ueda, “99.3% Efficiency of Three-Phase Inverter Using GaN-based Gate Injection Transistors,” Proc. 26th IEEE Applied Power Electronics Conf. and Expo.(APEC2011), Fort Worth, USA, pp.481-483, March 2011.
T. Ueda, “The Challenge and Progress of High-Voltage Normally-Off GaN Gate Injection Transistors,” 1st IEEE Workshop on Wide Bandgap Power Devices and Applications, Columbus, USA, October 2013.
T. Morita, H. Handa, S. Ujita, M. Ishida, T. Ueda, “99.3% Effciency of Boost-up Converter for Totem-pole Bridgeless PFC Using GaN Gate Injection Transistors, PCIM Europe 2014, Conference Digest, pp.325-329, Nuremberg, Germany, May 20-22, 2014.
