Successor of the Transistor The Crossbar Latch:

Presenters: Joseph Adorno Brian McSkimming Edward Sniezak

Current Size of Transistors Advanced chip manufactures, which currently make processors with feature sizes of 90 nanometers, say they could probably reduce feature sizes to 32 nanometers.

Current Transistor Technology To turn the MOSFET on in saturation mode Vds must be greater then or equal to Vgs – Vt.

Problems with Current Transistors As transistor sizes decrease, very small gate lengths can cause transistors to leak electricity, devices to consume large amounts of power, data to corrupt, and device performance to vary.

Heat and Power problems Power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature. If the heat produced by the transistor is more than the heat sink can dissipate, the thermal runaway occurs and the transistor will be destroyed.

Technology Roadmap

Hp’s Solution

The Crossbar-Latch

Crossbar-Latch Crossbar latches are not transistors, so they do not experience the, size, power, leakage or heat problems that transistors do. Therefore, chip makers can use large numbers of small latches, rather than transistors, to make chips more powerful.

Crossbar-Latch Crossbar latches are devices created between the two junctions where one tiny wire – currently 30 nanometers across, but could be decreased to as small as 2 to 3 nanometers across – crosses two other parallel wires at right angles. Each junction has a switch. The switch, a tunnel junction, consists of a single molecular layer sandwiched between two metal wires. The layer is normally an insulator. But if it is thin enough, charged particles can tunnel through the junction. If the junction’s resistance to such tunneling current is high, the switch is open; if it is low, the switch is closed. Applying a high or low voltage across the switch toggles the barrier between low and high resistance.

Crossbar-Latch The schematic illustrates the basic function of a latch in a crossbar latch The schematic of a actual crossbar latch with L being the signal line, and Ca and Cb are the two control lines

Crossbar-Latch According to “The crossbar latch: Logic value storage, restoration, and inversion in crossbar circuits”, HP lab researchers describe and demonstrate a microscopic device consisting of a single wire acting as a single line, crossed by two control lines with an electrically switchable molecular-scale junction where they intersect.

Crossbar-Latch By applying a sequence of voltage impulses to the control lines and using switches of opposite polarities, HP Lab researchers demonstrated that the crossbar latch can perform the NOT function which is essential for general computing operations.

Crossbar-Latch These are experimental demonstrations of a working crossbar latch. (a) The pulse sequence on the two control lines Ca and Cb to achieve the latch function. P1, P4, and P7 are the test pulses, P2 and P3 are open, P5 and P6 are closed.

Crossbar-Latch These are six trial runs varying only the input signal voltage. Input voltages of 0.3 and 0.4 volts latched correctly and inverted correctly with signal restoration while 0.2 volts failed to latch

Crossbar-Latch The advantage of using the crossbar latch, the two latch control lines may be driven by conventional circuitry outside the nanoscale circuit, which can be connected to a large number of logic signal lines. The disadvantage of using the crossbar latch is the limited lifetime (of 100 s of cycles) and switching speeds (.1kHz), both require improvement before reliable applications.

“Transistors will continue to be used for years to come with conventional silicon circuits, but crossbar latches could very well replace transistors in computers someday, just as transistors replaced vacuum tubes and vacuum tubes replaced electromagnetic relays before them.” - quote by Phil Kuekes, the holder of the patent for the crossbar latch.

- Nanotechnology and Science Bibliography - Nanotechnology and Science by: Mitin, Kochelap, and Stroscio - The crossbar latch: Logic value storage, restoration, and inversion in crossbar circuits. Journal of Applied Physics, Issue 97, Volume 3 - Researchers work on Transistor Successor IEEE article, May 2005