transistor architectures  technology


Another feature of our transistor is its barrier thickness of only 9 nm, which leads to a significant improvement in the modulation efficiency of the electrons in the GaN channel by the gate electrode. Our device also contains a 3.3 nm InGaN back-barrier structure underneath the GaN channel that helps to mitigate short-channel effects top barrier scaling. And last but by no means least, we have introduced an oxygen plasma treatment step prior to gate metal deposition that increases the frequency of the transistor by at least 30 percent, due to elimination of transconductance dispersion at high frequencies.


Another device that is showing great promise for sub- mm wave applications is the GaN/AlGaN transistor that has a nitrogen-face GaN structure. This device has several advantages over its conventional equivalent with a gallium-face. One of the biggest benefits results from the formation of the two-dimensional electron gas on top of the AlGaN layer, which occurs because the polarization in this transistor is inverted compared to the standard Ga-face AlGaN/GaN/AlGaN one.


Thanks to this switch in the direction of polarization, a top nitride barrier is not needed – an omission that paves the way to obtaining a very low contact resistance. What’s more, the bottom AlGaN layer that induces the two-dimensional electron gas also leads to excellent channel confinement, a characteristic that enables a high output resistance even in deep- submicron devices. Unlocking the benefits of these nitrogen-face devices has traditionally been very tough, because it is tricky to grow high-quality GaN with a nitrogen-face. But continued efforts in this direction have recently led to substantial improvements in material quality, and researchers at the University of California, Santa Barbara, have reported some excellent results.


That team has produced N-face devices with a 0.7 µm gate with a power-added efficiency of 74 percent at 4 GHz and fT


/ fMAX gate length have shown a maximum fT


while the optimization of the gate structure allows an fMAX


More recently,we have set a new benchmark for InAlN/GaN transistors,working in close


collaboration with the University of Notre Dame and the companies TriQuint and IQE. Our 30 nm gate length device produces an fT


made from this material system are delivering promising results. Transistors made by a team at the National Institute of Information and Communication Technology in Japan with gate lengths of 250 nm and 60 nm have delivered fT


and fMAX combinations of 52 GHz and 60 GHz,


and 107 GHz and 133 GHz, respectively. However, even more impressive results are possible by applying a combination of a re-growth contact and back-barrier structure. Engineers at HRL have pioneered this approach, and produced a 45-nm gate length device with an fT


of 260 GHz and a fMAX of 394 GHz. The latter value is a record for GaN transistors.


The results outlined above showcase the tremendous improvements in all three classes of novel nitride transistor over the last few years. And the race is certainly on to break the 500 GHz barrier, a target that would have seemed far-fetched in the not-to-distant- past. Once that record has been claimed, many will rejoice at the fabrication of transistors that combine speed with great efficiencies and output power levels at mm- and sub-mm wave frequencies that are orders of magnitude higher than what exists today. Hopefully the wait will be a short one.


of 15 / 42 GHz. Devices with a 100 nm of 163 GHz,


as high as 310 GHz in transistors with a gate length of 70 nm. These transistors also feature an unalloyed ohmic contact with a contact resistance of just 0.027 ohm-mm, which was formed via InGaN re-growth. This technology has also been used to produce self- aligned devices.


The third class of novel device – a heterostructure transistor formed from the pairing of AlN and GaN – has also been piquing the interest of the wide bandgap microelectronics community, due to its potential for forming highly scaled transistors. Thanks to a very large polarization discontinuity between the two nitrides, such structures can yield a charge density well in excess of 2x 1013


cm-2 , which in turn leads to a room-temperature


sheet resistance of 150-180 ohm/. In addition to this strength, the ultra-thin AlN barriers that are typically used in these devices can significantly mitigate degradation caused by short-channel effects. Devices


Figure 2: Small signal, high frequency


performance of the InAlN/GaN HEMT


produced by MIT. The gate length of this transistor is just 30 nm


August / September 2011 www.compoundsemiconductor.net 35


© 2011 Angel Business Communications. Permission required.


of 300 GHz, the highest cut-off frequency ever reported for GaN transistors


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