The cryotron is a four-terminal superconductive computer component which uses the property of superconductor critical fields to switch between superconductive and normal states. Two terminals comprise the control and the other two terminals comprise the gate. Passing current through the control will change the gate from superconducting to normal. The cryotron exhibits current gain which allows cryotrons to directly control other cryotron circuits. Cryotrons may be used to construct flip-flops, analog-to-digital converters, counters, shift registers, line drivers, and recognition units for content addressed memory.
Cryotrons may be made of thin-film materials or drawn wires. The gate conductor is a single straight conductor and the control winding is wound over the gate conductor.
Tc the critical temperature
Superconductors are materials which become superconductive below a specific temperature; this is called the critical temperature, Tc.
|| 8 oK |
|| 7.2 oK|
|| 5.1 oK|
|| 4.4 oK|
|| 3.7 oK|
|| 1.2 oK|
Liquid helium, under atmospheric pressure is 4.2 oK
Hc the Critical Field
The critical field is the intensity of magnetic field which will cause a superconductor to change from its superconductive state to its normal state. It is represented by Hc.
All conductors carrying electrical current generate a magnetic field; this is also true of superconductors. When the magnetic field created by the current in a superconductor becomes greater than the critical field, the superconductor will return to its normal state. If the current is sufficient, the conductor temperature will rise above Tc .
Choice of superconductors
The function of the control is to carry current, which generates a magnetic field greater than Hc to turn the gate off without itself changing into the normal state. To meet this goal, the control conductor is chosen for an Hc that is higher than the Hc of the gate conductor. Before the end of 1954 Dudley Buck was making cryotron with 0.0508 mm (0.002 inch) niobium for the control wire. Gate conductors were constructed of 0.229 mm (0.009 inch) tantalum or 0.015 mm (0.0006 inch) lead-plated copper.
The cryotron was developed by Dudley Allen Buck at the Massachusetts Institute of Technology. Though announced to the world in a press release dated 6 February 1957, the superconductive switch was established in his notebook entry of 15 December 1953; in February 1954 Buck began using the term cryotron in his notebook entries.