“GaN (gallium nitride)”, a compound of Gallium (Ga) and Nitrogen (N), possesses high breakdown voltage and low conduction resistance characteristics that enable miniaturization and high-speed switching. Unlike conventional transistors made from silicon that require a bigger chip area to keep the on-resistance low, GaN devices having small sizes (and thus low parasitic capacitance) allow high speed switching and miniaturization with ease.
Embodying Panasonic's original and reliable Hybrid Drain-Gate Injection Transistor (HD-GiT) technology, the X-GaN™ familly of 600V, enhancement mode GaN-on-Silicon transistors delivers simple to implement GaN switching performance, while contributing to keeping total system costs under control.
For quick evaluation in an existing design: embedding a driver circuit for the GiT transistor, our "daugther board" are meant to be easily nserted in an existing application board to quickly evaluate the performance of GaN.
For characterization of the switching performance / as reference driver circuit: based on optimized, reference driver circuit designs, our "chopper boards" are setup for double pulse measurement (available) or half-bridge operation (coming soon).
As application demonstrators: a range of application demonstrators - PFC, LLC, compact power supply design...
“GaN (gallium nitride)” is a compound of Gallium (Ga) and Nitrogen (N) classified as wide bandgap semiconductor - with a bandgap about 3 times higher than the one of silicon. It exhibits physical characteristics making it especially interesting in the context of power electronics:
a breakdown field about 10 times higher than silicon,
high electron mobility and carrier density,
very fast carrier recombinations,
a high theoretical maximum operating junction temperature above 400°C.
GaN makes it thus possible to develop small power transistors chips with low parasitic capacitances, supporting high currents and delivering revolutionary performance in terms of switching speed, and on-resistance. Besides, GaN transistors prove very robust against radiations, and bear potential for high temperature operation.
Panasonic X-GaN™ transistors
Overview and benefits of the Hybrid Drain Gate Injection Transistor (HD-GiT) structure
Panasonic X-GaN™ transistors are an evolution of the High Electron Mobility Transistor (HEMT) structure, where a 2D electron gas is formed at the interface between two materials of different bandgap. In order to make the transistors useful in concrete applications, the following techniques have been employed:
the transistor are grown on silicon wafers to keep the costs manageable,
a p-doped recessed gate structure makes the transistor normally-off,
a second p-doped structure injects holes under the drain when under reverse voltage stress (blocking state), solving the current collapse issue inherent to conventional GaN based transistors.
The HD-GiT also inherits from the HEMT the capability to conduct current in the reverse direction through its channel - that is to say with the same excellent conduction capability than in forward mode - practically eliminating the need to use antiparallel diodes to handle flyback currents.
Operating regions of the HD-GiT
The HD-GiTs can operate in the first and third quadrants.
From the static Id-Vds curve point of view, the transistors behave essentially like FETs in forward operation. The next paragraph explains the difference.
In reverse mode, the characteristic looks similar to a diode. The mechanism involved is however totally different and is explained below as well.
Forward conduction mode and gate injection
The X-GaN™ transistors are turned on with two steps:
a gate-source voltage higher than the threshold voltage of the transistor is applied, electrons can now flow at high speed between gate and drain
a constant current - order of magnitude of a mA - is injected through the gate. The hole injection induces a strong amplification effect in the channel.
Practically only the electrons contribute to the current flow when the transistor is conducting.
The transistors are turned off like FETs by simply setting the gate-source voltage below the threshold. The small amount of charges involved and the very fast recombinations in the GaN material ensure that no detrimental side effects (like e.g. tail current) can be observed practically.
Reverse conduction & reverse recovery
Per construction the X-GaN™ GiT transistors can conduct current in the reverse direction as soon as the source, gate and drain potential are set in such a way that current is injected in the gate. The conduction and recovery performances of the GiT in this operating mode are comparable with what a discrete antiparallel SiC Schottky diode delivers - whithout the need to actually implement it.
Although reminiscent of a diode, the threshold voltages in the third quadrant of the static I-V curve are not the built-in voltage of a junction but simply the threshold voltage of the transistor, plus any negative bias applied to the gate potential vs. the source. In the same way as a MOSFET, the GiT can then be turned-on in the reverse direction to further reduce the losses by operating at 0-offset condition.
Thanks to the low reverse recovery charge stored in the transistor, and thanks to the fast recombinations in GaN, the GiT recovers extremely fast from a reverse conduction operation, making it suitable to use as fast switch in topologies like totem pole PFCs.
The solution to GaN transistors' current collapse issue
Conventional GaN-based transistors generally suffer from current collapse effect: during operation, electrons subjected to a high electric field can get trapped in deep levels traps close to the channel. The time constant of the natural de-trapping mechanisms being many orders of magnitude larger that the typical switching period of the transistors, the amount of trapped charges quickly increase, increasing the resistivity of the channel and leading to the destruction of the devices in a very short time.
The original gate structure of Panasonic X-GaN™ transistors solves this issue, and current collapse free operation was demonstrated up to 850V*
Under voltage stress - typically in blocking mode - the high Vds voltage induces the injection of holes into the channel by the p-doped structure connected to the drain. These holes recombine with the trapped charges and maintain the high conductivity of the channel.
* “Suppression of current collapse by hole injection from drain in a normally-off GaN-based hybrid-drain-embedded gate injection transistor”, Tanaka & al in Appl. Phys. Lett. 107, 163502 (2015)
Benefits in applications
The performance of power converters using GaN transistors can thus be improved along the two axes of increased efficiency and system size reduction. Concrete benefits for the application can include:
a reduction of the system size, weight, and bill of material,
by downsizing or eliminating the cooling system (low losses),
by downsizing the passive components (high switching frequency),
an increase of the maximum output power from a size-constrained converter,
helping the end customer to save on the electricity bill by increasing the efficiency to levels not previously achievable.
The right trade-off in term of power density of the final design of course ultimately depends on the requirement of each application.
Discover the full story of the development of Panasonic X-GaN power transistors.
X-GaN User Report - JAXA - | Panasonic Semiconductor
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