Indium phosphide tunnels into low power memory

Indium phosphide tunnels into low power memoryFaster than GaAs, speedy in linear circuits, indium phosphide is a pretty useful material. It is in high speed low power memory though, that it is exciting researchers at Raytheon TI Systems. Steve Bush Indium phosphide is showing a lot of promise in a novel form of high speed low power memory. One company that is making a significant commitment to InP is Raytheon TI Systems, formerly a research arm of Texas Instruments. “It has a lot of advantages over GaAs,” said Raytheon’s Alan Seabaugh, “You can make a 1.5ps switch in InP, and we have never heard of anything faster.” The material is also no slouch in linear circuits. “Indium phosphide HEMTs [high electron mobility transistors] are useful in the 20 to 100GHz range, maximum ft is around 340GHz,” said Seabaugh. So InPis fast in both digital and analogue circuits, but it is as a memory, using a strange device called a resonant tunnelling diode (RTD), that most excites the Raytheon researchers. Tunnel diodes have an ‘N’ shaped V-Icurve. That is, between certain voltages, the device has a negative resistance and current falls with increasing voltage. Resonant tunnelling diodes (RTDs) are a type of tunnel diode made using heterostructures rather than P-N junctions. Fast RAM… Two resonant tunnelling diodes and a heterostructure FET – the fastest known three terminal device – give this indium phosphide memory sub-nanosecond access times while it consumes 200 times less power than the equivalent GaAs SRAM. The central node of two tunnel diodes connected in series can only assume one of two stable states. If two RTDs are connected in series across a fixed voltage supply of around half a volt, the central node will assume one of two stable voltages (see diagram). The state of this node can be set to one value or the other by injecting or extracting current briefly through a transistor. The transistor and diodes forms the basis of a memory cell which can be read like a DRAM, but is inherently bi-stable and does not need periodically refreshing. The disadvantage it that the cell needs a continuous current flow which is where its power consumption comes from. Raytheon has managed to make RTDs with very low quiescent currents, reducing power consumption to 50nW per bit while maintaining sub-nanosecond access times. Silicon DRAM bits use around 1pW, but are far slower. GaAs SRAMcells are as fast as the the RTD cell, but use 200 times more power. Raytheon reckons that a mature InP memory technology could have a density of 2Mbit/cm 2 and dissipate only 5mW/Mbit.
Indium phosphide shows a lot of promise for memory. However, there is a long way to go. Raytheon is still at the prototype stage, with a 16-bit memory. 100mm wafers are the latest thing in InP, with 75mm still the norm and the material is expensive and brittle.

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