Resistive-metal-electrode Schottky diodes for temperature sensing

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Schottky diodes, which comprise a metal electrode deposited on a clean semiconductor surface, are perhaps the simplest of all semiconductor devices. There is an inbuilt potential barrier which means that it is much easier to pass current in one direction through the device than in the opposite direction. In other words the device has a rectifying action and from that it derives its great utility. This assumes that the current is driven across the barrier i.e. one of the electrical contacts to the outside world is attached to the metal electrode and the other to the semiconductor substrate. Here we consider what happens if the resistance of the metal electrode itself (let's call it Rm) is measured while the temperature of the device is changed. What we have noticed is that as the temperature increases the apparent value of Rm decreases quite substantially over a limited temperature interval of ~50K. In preliminary work on PtSi/p-Si diodes (platinum silicide or PtSi is a type of metal and p-Si is 'doped' silicon in which the majority current carriers are holes, rather than electrons) a factor of up to 5 drop in Rm has been observed in the temperature range 100-150K. The reason is that at low temperature (< 100K) the current carriers in the metal are confined to the metal electrode by the built-in potential barrier. However, as the temperature increases the current carriers (electrons in the metal and holes in the semiconductor) can progressively start to cross the barrier between the two - what this means is that the semiconductor opens up as a parallel current carrying channel and so Rm decreases. In fact Rm is now really the resistance of the semiconductor wafer in parallel with the metal electrode. There is a proviso, which is that the metal electrode should have a higher resistance than the semiconducting substrate. This is kind of the wrong way round but can be arranged if the metal electrode is very thin (<10 nm); in fact Rm can be very much greater than the resistance of the semiconductor substrate if it is also long and narrow (width 1 micron or less). Since this is not normally the case and electrical measurements are normally taken across the barrier, there appears to have been something of a blind-spot regarding this effect in semiconductor device research. This preliminary project aims to exploit and enhance the effect so that the observed change in resistance, Rm, is a factor of 100 or 1000 or perhaps even more.In effect the device will be an extremely sensitive temperature sensor and that is the immediate application that is envisaged. Temperature sensors form a crucial component in various control circuits and are used widely in the PC, automotive and communications sectors. The temperature range in which the resistance change occurs depends on the height of the potential barrier which in turn depends on the specific metal-semiconductor combination. The good news is that the range of such combinations and of corresponding potential barriers is such that the temperature sensitive window can probably be designed to fall anywhere in the range from 50K to 500K (or let's say approximately -225oC to +225oC). Longer term, we think that such devices could be also adapted to highly sensitive gas sensing.