technology  GaN transistors


Constructing these chemical sensors involves the mounting of FET chips in the centre of a PCB board with Cu/Au-based conductive pathways. Gwent Electronic Materials gave support to that task. These transistors are adhered to 48 pads on a PCB board by ultrasonic wedge bonding, a process that enables fast connection of the nano-crystalline diamond electrode to transistors with different geometries. Covering this sensor with synthesis glass allows observation of the device with an optical microscopic while it is being used (see Figure 6 (a)).


Measurements reveal that the sensor has a reproducible pH sensitivity of 55 mV/pH, a sensitivity in the range of 20 mA/mm per pH (see Figure 6(b)) and a resolution as high as about 0.05 pH. One of the great strengths of the sensor is its fast response, which is limited by the seconds required to realise complete intermixing of the electrolyte.


Dissipating the heat


The potential benefits of coating GaN-based HEMTs with diamond have been recognized for many years: This yields a corrosion-resistant protection layer in harsh environments and forms an ideal heat-spreading layer for dealing with forced cooling from above. Converting the promise into a reality requires deposition at 700 °C or more of nano- or poly-crystalline diamond films in an atmosphere rich in hydrogen radicals at temperatures in excess of 700 °C. This requires an extremely stable semiconductor heterostructure, device contacts and passivation, which may also act as nucleation layer.


The Technical University of Ulm fabricated the first submicron HEMTs overgrown with a 1 µm-thick nanocrystalline diamond heat spreader by bias


Figure 7. Current gain (h21


gain


(MSG/MAG) as a function of frequency for a 2x0.5x75µm3 HEMT with 1µm thick nanocrystalline diamond heat spreader.The bias conditions were fixed at Vds Vgs


=8V and =-1V


enhanced nucleation. The small signal performances of the components are satisfactory, with an ft and an fmax


of 16.8 GHz


6.4 GHz (see Figure 7). This result constituted a world first for the MORGaN project.


Ultra-powerful HEMTs


Constructing 1 kW transistor technology was another goal of the MORGaN programme. To try and hit this output power, researchers at III-V Lab fabricated HEMTs with a 36 mm periphery and a 0.7 µm gate length on semi-insulating SiC substrates (see Figure 8). These transistors featured a 2 µm-thick GaN layer grown on SiC and a 10 nm-thick InAlN layer. Optimising device topology preserved microwave power gain and addressed thermal constraints. Due to the operating frequency, these devices tend to dissipate more power than they emit. When driven by 10 µs pulses, the basic cell 2 mm device delivers 13.2 Wmm-1


with a Power Added Efficiency (PAE) of 70 percent. Switch operation


) and power


Figure 8.(a) Scanning electron microscopy image of a 36 mm periphery,0.7 µm gate-length HEMT power bar for L to S-Band operation.(b) Pout


13.4 dB measured on wafer at 3.5GHz and Vds


= 41.2 dBm (13.2W – 6.6W/mm) with PAE of 70 percent and Gp = 35 V (2mm device & short pulse setup)


of April / May 2012 www.compoundsemiconductor.net 49


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  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175