wide bandgap electronics industry
electric field at the drain-edge of the gate, contributing to a reduction of the electron temperature and trapping. If a plate is not in place, hot electrons diffuse into the bulk, where they became trapped. Insert a field plate and trapping falls, due to a lowering of electron temperature that limits electron spillover from the channel into the bulk. Our gate-lag transient simulations revealed key insights into the so-called current collapse phenomena, with the addition of field plates aiding the recovery of the drain current following a gate off-on switching pulse.
Thanks to improvements in epitaxial material and processing, devices now exhibit nearly ideal characteristics. Although extensive reliability studies of GaN devices are still underway, the nitride community has entered a phase in which simulation of their devices takes on a more conventional role: It provides a tool for designing and optimising device structures for specific applications.
For power switching, gate-drive circuitry is greatly simplified if the switching FET operates in enhancement mode, because the device is then normally off. Interest in this class of device has recently taken off, because it has tremendous commercial potential for power switching.
One interesting and promising variant of the normally off nitride transistor is the p-type GaN gate device that has been pioneered by Oliver Hilt and co-workers from the Ferdinand-Braun-Institute in Leibniz, Germany. As the paper presented by this group at last year’s International Symposium on Power Semiconductor Devices and ICs did not report some of the key dimensions of their transistors, we have had to adopt reasonable assumptions to create a structure consistent with the device performance results (see Figure 2 for details).
In this p-type gate device, highly doped regions are created under the source–drain electrodes that stretch down to the GaN channel to emulate metal spikes and to control contact resistance. Magnesium-doped GaN is used as the p+ gate to deplete the channel at Vg
=0,
yielding a normally off transistor. An AlGaN buffer is used to increase the threshold voltage, and increasing the aluminium content in this layer reduces the on- resistance.
We assume that the Al0.05 Ga0.95 N buffer is completely
relaxed, and the subsequent channel and barrier layers are strained to match the lattice constant of Al0.05
Ga0.95 N,
but with 20 percent of relaxation. The large polarization divergence at the AlGaN barrier surface (barrier/nitride interface) produces a large sheet of negative polarization charge. One might expect holes to accumulate at that interface and completely deplete the
channel of electrons. However, in reality it is still not clear whether the polarization charge is compensated by fixed charges or interface trap states. According to our simulations, accumulated holes at the surface of the AlGaN barrier are completely compensated by deep, single-level trap states.
To assess the device performance for power-switching applications, we perform voltage sweeps of Id
-Vg -Vg , Id -Vd
and off-state breakdown voltage. To match the sub- threshold slope of the Id
curve reported by Hilt and
his co-workers, we add traps to the AlGaN barrier/GaN channel interface. As expected, our simulations reveal that the off-state leakage current and breakdown voltage are strongly influenced by the passivation nitride thickness and field-plate length. What’s more, traps in the buffer affect both the sub-threshold slope and off-
Figure 4.
Id-Vgand Ig-Vgcurves for a structure
with a 1.8 µm field plate and
6 µm gate- drain spacing,
biased at Vd=15V
Figure 5.
Id-Vdcurves for structure with
a 1.8 µm field plate and
6 µm gate- drain spacing. The negative output
conductance shown in the top trace is due to self- heating
October 2011
www.compoundsemiconductor.net 23
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 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200