Handbook of High Temperature Superconductor Electronics Part 10 pptx

A number oftests of the RSFQ circuits using HTS Josephson junctions have been reported, butmost of the circuits that have been reported are small-scale circuits because thefabrication te

High-Temperature Superconducting Digital Circuits

com-There have been many efforts to develop circuits for exploring the tages of ultrahigh-speed processing system by superconducting digital circuits us-ing metallic superconductor materials, such as Pb and Nb Two examples of theseefforts are the IBM project (1969–1983) (1) and the Japanese MITI project

advan-(1981–1991) (2) Successful demonstrations for the low-T csuperconductor (LTS)circuits have been made, such as a 4-kbit RAM that has 42,000 junctions and op-erates at 620 MHz (3) and a computer-communication-network logic circuit that

has 4300 junctions and operates at 2 GHz (4) It has, nevertheless, become clearthat the first-generation superconducting digital circuits, so-called “latchinglogic” circuits using the zero- and a finite-voltage states for logical “0” and “1”states, cannot compete with high-speed semiconductor circuits after paying theircooling penalty The main drawback of the “latching logic” is that it is clocked bylarge radio-frequency (RF) current from outside of the chip The operation fre-quency is restricted to a few gigahertz, because a large amount of current (e.g.,several amperes) cannot be supplied at a higher-frequency.

Much attention has thus been directed to the single-flux-quantum (SFQ)logic, which codes the binary information not by using the dc voltage, but by us-ing single quanta of magnetic flux ( 0 h/2e  2.07  10
15Wb) Supercon-ducting digital circuits using the SFQ logic were originally proposed by Nakajimaand Onodera in 1976 (5), and since 1985 have been dramatically improved by theMoscow State University group, represented by Likharev and Semenov (6) TheirSFQ circuits, called rapid single-flux-quantum (RSFQ) circuits, have become themost popular SFQ circuits and are expected to operate at a frequency greater than

100 GHz Several high-speed RSFQ circuits based on tunnel-type LTS Josephsonjunctions have been reported, and the highly important of these is an analog-to-digital converter circuit, which was made by Semenov et al and has thousands ofjunctions and operates at frequency up to 11 GHz (7)

High-T csuperconducting (HTS) digital circuits are more suitable for use inSFQ circuits than LTS ones, because HTS Josephson junctions are naturally over-

damped, which means that their I–V curves do not show hysteresis, and the

junc-tions in SFQ circuits must be overdamped juncjunc-tions The tunnel-type LTS son junctions, on the other hand, are underdamped ones and require some shuntresistance between the two electrodes of each junction This makes the character-

Joseph-istic voltage (I c R nproduct) values lower, which results in lower operating speeds,

and also complicates the layout and the fabrication process The I c R nproduct ofHTS junctions can also be expected to be larger than that of LTS junctions because

it intrinsically depends on the gap voltage of the superconductor A number oftests of the RSFQ circuits using HTS Josephson junctions have been reported, butmost of the circuits that have been reported are small-scale circuits because thefabrication technology for HTS junctions is still in a primitive stage

9.2 OPERATING PRINCIPLE OF SFQ DIGITAL CIRCUITS

Magnetic flux is quantized in a superconducting closed loop and the minimumunit is a SFQ Figure 9.1 shows the simplest loop for the SFQ circuit, which is asuperconducting closed loop including a Josephson junction As magnetic-fluxcrossing of superconducting lead is forbidden by the Meissner effect, the Joseph-son junction plays the role of a “gate” for going in and out of the loop When theJosephson junction switches to a voltage state, magnetic flux goes in and out

through the junction If the product of the junction critical current I cand loop

in-ductance L is 0 LI c 2 0, only a SFQ can exist in this loop after resetting the

junction to superconducting state and the SFQ makes a circulating current Icirinthe loop Figure 9.2 shows another explanation of SFQ storage and release in the

superconducting loop When a dc bias current I bis supplied to a superconductingloop including a Josephson junction, almost all of the current goes through the

junction because of inductance L in another branch (Fig 9.2a) Here, I bis smaller

than the critical current I c of the junction If a signal current I sis then supplied to

the junction and the sum of I b and I s is larger than I c, the junction switches to a

voltage state and I b and I sflow through the inductance branch (Fig 9.2b) After

resetting the junction to the superconducting state and I b and I sturning off, the rent flowing through the inductance branch is preserved in the loop (Fig 9.2c)

cur-The preserved current Iciris 0/L when the L and I cvalues satisfy 0 LI c

2 0 The Icirpreservation in the loop corresponds to SFQ going in the loop The

Iciris released by supplying I s in the opposite direction The currents I s and Icirare

F IGURE 9.1 An explanation of a SFQ storage in a superconducting loop cluding a Josephson junction.

in-F IGURE 9.2 The basic operations of a SFQ gate.

added because they flow in the same direction at the junction and their sum

ex-ceeds I c(Fig 9.2d). Then, the junction turns on and Icirdissipates at the junction

(Fig 9.2e) This corresponds to a SFQ going out of the loop

Figure 9.3 shows a series of SFQ circuits: a DC/SFQ converter, a Josephsontransmission line (JTL) and a SFQ/DC converter The DC/SFQ converter, whichconsists of junctions J1 and J2 and inductance L1, makes a SFQ pulse by a dc cur-

rent Iin input If Iinincreases beyond a threshold value, a SFQ pulse is generated

by J2 turning on and is transferred to the right direction in Figure 9.3 The

DC/SFQ converter resets to its initial state when Iinfalls below a certain value.The reset of the circuit is accompanied by the generation of a SFQ pulse across J2,which does not propagate to the right The JTL consists of three superconducting

loops including junctions J3–J5 and inductances L2–L4 Because the product of I c and L for each superconducting loop is less than 0 in the JTL, the SFQ pulsepropagates through the JTL without being stored in these loops The SFQ/DC con-

verter contains a SFQ storage loop with J5, L5, and L6, in which the LI cproduct

is larger than 0, and a readout SQUID consisting of junctions J7 and J8 and avoltage output terminal between them The SFQ from the JTL is stored in thisloop, and the stored SFQ is converted to dc voltage by the readout SQUID

Because this circuit is biased by dc current I bshown in Figure 9.3, the rf biascurrent indispensable to latching circuits for their reset operations is unnecessary

in SFQ circuits This is the main reason that SFQ circuits are so much faster thanlatching circuits Any logic functions and memory operations can be implemented

using SFQ circuits by combining LI c 0loops and LI c 0loops A detailedexplanation of the RSFQ circuits can be found in Ref 6

In the SFQ circuits, binary information is propagated as very short voltagepulses instead of dc voltage in the superconducting latching circuits as well as in

all semiconductor circuits The voltage pulse V(t) has a quantized area given by

F IGURE 9.3 A series of SFQ circuits: DC/SFQ, JTL, and SFQ/DC.

The switching speed % of the simple SFQ loops like those in Figures 9.1–9.3 is stricted by the characteristic frequency of the ac Josephson effect Using a critical

re-current of Josephson junction I c and its normal resistance R n, we can represent %

The I c R n product is one of the most important parameters for evaluating the

Josephson junctions used in SFQ circuits If the I c R nproduct is 1 mV, which is areasonable value for HTS Josephson junctions, % can be as little as 2 ps

The power consumption for one switching of a Josephson junction in a SFQ

gate is about I c R n %  I c ... nproduct ofHTS junctions can also be expected to be larger than that of LTS junctions because

it intrinsically depends on the gap voltage of the superconductor A number oftests of the RSFQ... n

products of HTS Josephson junctions can be expected to be larger than that of LTSjunctions because of the larger energy gaps of HTS materials The development of

superior-quality... main parts of a kind of analog-to-digital converter, are indispensable to the resistance Resistance used for dividingbias current to each SFQ loop in parallel has a value of a few tens of ohms