technology III-V MOSFETs
III-Vs and the silicon roadmap
Silicon foundries could switch production from silicon MOSFETs to those based on III-Vs and germanium by the end of this decade. Making this transition is far from trivial, but progress is being made in gate dielectrics, contact resistance, peak current flow and material quality. Richard Stevenson reports.
T
ime and time again, critics have claimed that there will soon come an end to the shrinking of silicon transistors to smaller dimensions. Some have argued that photo-lithography cannot extend beyond optical wavelengths – but tools have been built that can do just that; others have warned that electrons cannot zip about fast enough when transistors reach the nanoscale – but adding a little strain into the material has put that issue to bed; and other critics have pointed out that high leakage currents will put an end to device scaling – but this issue has not been a show-stopper, thanks to a switch from silicon dioxide to high-k dielectrics, such as hafnium dioxide.
Today, claims that the days of the silicon transistor are numbered are still being made – and there’s a good chance that this time this view could well be right. That’s because this belief is not just held by those outside the silicon industry, but also some within it:
Alternatives to silicon are now on the International Technology Roadmap for Semiconductors (ITRS), with III-Vs and germanium predicted to make an impact at the 11 nm node that could be rolled out in 2015.
Iain Thayne from the University of Glasgow, UK, explained the reason for the invasion of these new materials into silicon lines at the recent CS Europe conference in Frankfurt, Germany. Thayne, whose efforts at developing III-V transistors initially focused on RF and millimetre-wave front-end applications, argued that compound semiconductors must be introduced to maintain performance as dimensions are reduced.
“Increasing the density of transistors in silicon leads to heating, which will soon approach an air-conditioning limit,” said Thayne. He explained that preventing over-heating in the circuits that will be built with tomorrow’s transistors requires a reduction in the voltage of the power supply, but no compromise in performance. The only way to satisfy these conditions is to replace silicon transistors with those based on III-Vs and germanium.
He went on to explain that scaling efforts are focused on increasing the density of transistors. Although every new node has a shorter gate length, it also has a reduction in gate pitch, which is scaled even more aggressively (see Figure 1).
Figure
1.According to the ITRS roadmap,between 2011 and 2024 reductions in gate pitch will be more rapid than those in gate length.(Image reproduced courtesy of IEEE Spectrum)
Sceptics within the silicon industry have argued that III-Vs will never be suitable for logic circuits, because the drive currents produced by this class of transistor are not high enough, due to the low densities of states associated with compound semiconductors. But Thayne’s colleague Asen Asenov has spotted fundamental flaws in this argument: Although the low density of states in III-Vs leads to a lower effective capacitance, these materials combine a high mobility with a low mass, resulting in the injection of carriers with high velocities and increased ‘ballisticity’. What’s more, the lower density of states means that
26
www.compoundsemiconductor.net June 2012
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