What are called vertical transistor designs employing GaN have shown a lot of promise for making possible switches able to operate at around 1,200 V. But new research described at the recent IEEE International Electron Devices Meeting indicates it may be possible to fabricate GaN transistors able to handle voltages in the 3,300 to 5,000-V range, thus ushering in the possibility of directly switching utility grid currents with GaN transistors.
Commercial GaN transistors typically operate at up to 600 V. One factor limiting their breakdown voltage is the planar layout of the transistor itself, with GaN feature geometries built atop foreign substrates such as sapphire, silicon carbide (SiC) or silicon. Current flows across a narrow cross section on the surface of the transistor, so the planar dimensions of the transistor are a main determinant of breakdown voltage.
In contrast, current in vertical transistors flows through the wafer rather than across the surface. That is because the substrates are GaN rather than a foreign material. This approach theoretically makes better use of GaN material properties such as ultralow conduction loss under high voltage and high temperature. Free-standing GaN also has much lower dislocation densities, useful for efficient vertical high current conduction and blocking. Moreover, there is more space to attach device leads, enabling higher current loads. One drawback of the approach is that a GaN substrate is more expensive than the alternatives, but hopes are that the increased use of the material will result in lower costs.
The IEDM paper — by researchers from MIT, semiconductor company IQE, Columbia University, IBM, and the Singapore-MIT Alliance for Research and Technology — describes what researchers call a GaN vertical fin MOSFET. This design consists of only n-type layers of GaN, thereby eliminating the need for material regrowth or p-GaN layers. With this device geometry, current is controlled through narrow, fin-shaped vertical n-type GaN channels that are surrounded by gate metal electrodes.
Other vertical GaN designs generally need complicated epitaxial regrowth or p-type GaN layers. Both are undesirable because the re-growth step significantly boosts device the cost and complexity, and adding p-type material produces transport properties that lead to high device on-resistance.
On both sides of each GaN fin are electrical contacts that act as a gate. Current enters the transistor through another contact, on top of the fin, and exits through the bottom of the device. The narrowness of the fin ensures that the gate electrode will be able to switch the transistor on and off.
When the vertical fin device is held at zero gate bias, electrons in the fin channels are depleted because of a work function difference between the gate metal and GaN. Researchers control the threshold voltage of the transistors by changing the fin width. The GaN vertical fin MOSFETs have exhibited a blocking voltage of 1,200 V with an on-resistance of 0.2 mΩ·cm2, with forward current in excess of 104 A/cm2; and a leakage current of only 10-5 A/cm-2 at a reverse bias of 600-800 V.
The researchers say their design, which incorporates recent innovations on GaN trenches and fins, can pave the way to a new generation of vertical GaN power transistors that feature a less expensive way of growing epitaxial layers, excellent blocking capability, and normally-off operation.
George Alongo says
My TV (Sony Wega Trinitron-KV-2704E) is showing the Red button being ON but no display on the screen.
Please assist on what to do.
Regards,
George