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AlGaN barrier layer so that its surface potential no longer contributes to 2DEG depletion. In addition, we have found that the SiN layer aids device stability at elevated temperatures. Specifically, it significantly enhances device reliability in high-temperature, accelerated-lifetime tests.


These in-situ deposited SiN films can also lead to lower channel resistance. This equips engineers with another route to adjusting the top part of the FET so that it can hit particular device specifications. GaN FETs are lateral devices, and optimising their performance demands a trimming of conduction losses. This means that for switching applications, aluminium-rich barriers are preferred in a typical AlGaN/GaN structure, because this leads to a higher piezoelectric field, a higher current density and a lower specific on-resistance.


We have shown that one of the major benefits of the SiN cap layer is that it enables increases in aluminium concentration, which are possible without any significant material degradation. This is not the case in transistor structures with an uncapped or GaN-capped AlGaN/GaN 2DEG, where relaxation of the strained top AlGaN layer typically prevents high aluminium content in the top layer.


It seems that in-situ SiN capping layers postpone relaxation of the strained AlGaN layer grown on GaN. This delay of relaxation has enabled us to demonstrate a SiN/Al0.35 (Rsh


Ga0.65


limit for the breakdown voltage of our devices – recent work has yielded FETs with a breakdown above 2 kV.


N/GaN/AlGaN FET with sheet resistance


) well below 300 Ω/on 100 mm and 150 mm silicon (111) substrates. As one would expect, our SiN passivation process improves the performance of nitride FETs grown on other types of substrate too, such as SiC.


For our SiN/AlN/AlGaN design that is detailed in Figure 3, sheet resistance falls to 235 Ω/with our passivation technology. In this structure, Hall measurements indicate that the electron sheet concentration is 2.15 x 1013 1250 cm2


cm-2


In every industry, conversion of hero results in the lab to production processes takes time. However, it is clear from our efforts that GaN can already be used today to make products in the 30 V-200 V and 600 V range, and it will not be long before variants operating at 1200 V can be added to the list.


This will pave the way for the replacement of two silicon MOSFETs with a single GaN HEMT, a move that will trim the cost and weight of a power converter. To make this happen, we are focussing our development on 1200 V epiwafers on 150 mm silicon.


and electron mobility is


/Vs. These are very promising values and they enable the fabrication of devices with a high transconductance, even when gate length is relatively large. What’s more, they highlight the potential of this device for high-frequency operation.


The neutralization of surface charges provided by the SiN layer also unlocks the door to an innovative approach for making enhancement-mode devices. This form of transistor, which is required for power converters, can be made by combining a thin AlGaN barrier layer with local removal of SiN under the gate.


From 600 V to 1.2 kV Today we can reproducibly manufacture GaN-on-silicon wafers with a breakdown voltage above 600 V and a very low leakage current. This is by no means the upper


When we release this product, it will be added to our existing range of 4-inch and 150 mm epiwafers for high voltage and/or high-frequency applications. The production capacity for these products is being ramped up at our new site, while in parallel we are developing manufacturing processes for 200 mm GaN epiwafers operating at 600 V and 1200 V.


Although today the demand for these larger epiwafers is weaker than that for those with diameters of 150 mm or less, larger sizes will spur a cost reduction and enable GaN to deliver further success in a field where, up until now, no compound semiconductor has seriously challenged silicon.


© 2012 Angel Business Communications. Permission required.


Further reading


J.Derluyn et al.IEDM Tech. Dig.1 (2009)


F


.Medjdoub et al.Electron. Lett.47 1345 (2011)


J.Derluyn et al. J.Appl. Phys.98 54501 (2005)


March 2012 www.compoundsemiconductor.net 17


Figure 3.


SiN/AlN/GaN heterostructure


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