This page contains a Flash digital edition of a book.
INDUSTRY GaN DEVICES


with transistor processing, so allow a high level of integration. The D-mode transistors that are normally on are well established, with 600 V-rated devices exhibiting an on-resistance of just 0.8 mΩ cm2


. However, most research efforts are now focused on the realisation of highly performing E-mode devices, because they are easier to use in circuits – they do not have to be paired with a silicon transistor to form the preferred, normally-off mode of operation. It is the E-mode form of the device, which is normally off, that will lead to a ramp in sales of GaN transistors.


Two-pronged approach


One of the common approaches to making an E-mode device is to recess the AlGaN barrier and then deposit a gate dielectric. An alternative, popular process involves growing a p-type layer on top of the AlGaN, and then etching the p-type layer in the active region only. Both approaches, which form MISHEMTs and J-HEMTs respectively, have their pros and cons. This is why our partners – whose names cannot disclosed for reasons of confidentiality – are pursuing both options to de-risk their future investment on processing development.


MISHEMTs that we have produced exhibit a uniform threshold voltage of around +1.2 V, and a low leakage current that is in the pico-amp range at a gate voltage of 0 V (see Figure 4 a). Under forward gate bias with a 10 V gate voltage, current is below 1 nA/mm, and the breakdown voltage exceeds 15 V (see Figure 4 b).


At a gate voltage of 0 V, the breakdown voltage exceeds 600 V, demonstrating the true E-mode nature of these devices. They deliver state-of- the-art performance for an E-mode transistor, with an on-resistance of 1.5 mΩ cm2


Figure 3. Gold cannot be employed as a contact metal in silicon lines, so imec’s engineers have developed a gold-free alternative that has a low contact resistance and a high level of reproducibility (a). There is a small degree of variation in contact resistance between within six wafers within one lot (b).


, which is far


superior to that of a standard silicon equivalent. We are now trying to take these 600 V devices to a higher level of maturity, and to increase the threshold voltage beyond 2 V, while trimming the on-resistance to below 1 mΩ cm2


.


We have also used our 200 mm GaN-on-silicon platform for the development of power rectifiers. The biggest challenge with this class of device is to combine a low turn-on voltage with a low leakage current. We have succeeded in this regard by pioneering an innovative, proprietary device architecture named the gate-edge terminated (GET) diode. This class of diode has a turn-voltage that is below 1 V, and a leakage current less than 1 μA/mm at high reverse voltage (see Figure 5).


An additional strength of these devices is their incredibly short reverse recovery time, which


Figure 4. E-mode MISHEMTs produced at imec combine a uniform threshold voltage of typically 1.2 V (a) with a low leakage current (b) and a breakdown voltage in excess of 600 V (c).


Copyright Compound Semiconductor October 2014 www.compoundsemiconductor.net 51


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76