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surface states - which can be modified during processing and device operation - can severely degrade the dynamic properties of the device. To prevent this EpiGan deposits a unique in-situ SiN capping layer, which is grown by MOCVD as part of the epitaxy process on top of HEMT epi-wafers. The interface between this capping layer and the top nitride surface is incredibly smooth, and it enables perfect passivation of surface states (Figure 1).
The capping layer can properly control the filling of the surface states during device operation. It is believed that SiN can provide enough charge to neutralize the surface charge of the AIGaN barrier layer so that its surface potential no longer contributes to 2DEG depletion. In addition, the SiN layer aids device stability at elevated temperatures.
The in-situ deposited SiN films can also lead to drastic reduction of the channel resistance, This enables adjusting the top part of the FET so that it can meet particular device specifications. GaN FETs are lateral devices, and optimizing their performance demands a trimming of conduction losses. This means that, for switching applications, aluminium-rich barriers are preferred in a typical AIGaN/GaN structure, because it yields a higher piezoelectric field, higher current density and lower specific on-resistance.
One of the major benefits of the SiN cap layer is that it enables higher aluminium concentration 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 AIGaN layer typically prevents the obtention of a low channel resistivity.
For the SiN/AIN/AIGaN design detailed in Figure 2, sheet resistance falls to 235Ω/. with EpiGaN passivation technology. In this structure, Hall measurements indicate that the electron sheet concentration is 2.15 x 1011 mobility is 1,250 cm2
cm-2 and electron /Vs. These are very promising
values and they enable the fabrication of devices with high transconductance, even when the gate length is relatively large. 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 AIGaN barrier layer with local removal of SiN under the gate. By offering a very smooth, clean and uniform protecting surface for active layers, the use
Fig 3: The uniformity of the in-situ SiN layer
of in-situ SiN also enhances the controllability of the device manufacture , further to reduce the cross- contamination potential issues when using a III-V material in a Si CMOS fab. The excellent uniformity of the in-situ SiN layer is shown on Figure 3.
From 600 V to 1.2 kV Today EpiGaN is able to manufacture GaN-on- silicon wafers with a breakdown voltage above 600V and a very low leakage current. But this is by no means the upper limit for the breakdown voltage of these devices. Recent work has yielded FETs with a breakdown above 2kV.
GaN can already be used to make power products in the 30 to 200V and 600V range, and it will not be long before variants operating at 1,200V 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 power converters. To make this happen, EpiGaN is focusing on the development of 1,200V epi-wafers on 150mm silicon.
Future products based on this process will complement the existing range of 4-inch and 150- mm epi-wafers for high-voltage and/or high- frequency applications. The production capacity for these products is currently being ramped up at the new Hasselt facility. In parallel, manufacturing processes for 200mm GaN epi-wafers are being developed.
Although today the demand for these larger epi- wafers is weaker than that for those with diameters of 150mm or less, larger sizes will spur a cost reduction and enable GaN to deliver success in a field where, until now, no compound semiconductor has seriously challenged silicon.
© 2012 Angel Business Communications. Permission required.
Today EpiGaN is able to
manufacture GaN-on-silicon wafers with a breakdown voltage
above 600V and a very
low leakage current
Issue 2 2012
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