RADIATION HARD MEMORY CELL

Abstract: Pairs of cross-coupled transistors (T₁ + T₂, T₃ + T₄) are configured as a bistable regenerative circuit. Isolation means, such as diodes (I₁ to I₄), are provided in the cross-coupling paths to ensure that if the logic state of one transistor is tem­ porarily changed by radiation striking the circuit, the logic state of the other transistor it is paired with will not change and the logic state of the unchanged transistor will be utilized to maintain the logic state of the other pair of transisfors. DTL and SDFL circuits are disclosed as the preferred embodi­ ments.

Description: This invention relates to bistable regenerative solid state memory cells, and particularly to such cells where redundant elements are employed to protect the logic state of the cell against upset due to radiation. Figure 1 shows a basic known solid state bistable regenerative memory cell. Random access memories (RAMs) use a matrix of such cells, each adapted to be uniquely written into or read. FR-A 15 32 056 corresponding to GB-A 11 99 931 describes and shows in Figure 7b a bistable electrical circuit comprising redundant circuit components as mentioned in the pre-characterizing portion of claim 1. The redundancy of components increases the reliability of such binary logic circuit in case one of the components, in particular a transistor, should become open-circuited by a fault or should become short-circuited. By the provision of double or triple redundancy of critical components, the failure of one component would not render the circuit inoperative, but the redundant component takes over the function of the failing component. Radiation, such as alpha particles or cosmic rays, cause transients in memory cells as they penetrate into the cell. These transients can trigger the bistable circuit by changing the current or voltage at a transistor node thereby causing the cell to switch logic states. In bipolar memory cells, the causes of such radiation-induced upset are primarily (1) collector nodes of OFF transistors can short to ground, and (2) base nodes of OFF transistors can short to collector nodes turning OFF transistors ON. The first mechanism is the dominant cause of single event upset (SEU) in cosmic ray environments. This mechanism, with small variations in charge collection volume and circuit upset threshold, is also found in other semiconductor technologies such as NMOS/Si and CMOS/Si. The second upset mechanism is found only in bipolar devices. Here a parasitic current path is formed within the device itself rather than between the device and the substrate. The second upset mechanism involves a smaller amount of upset charge which can still cause upset at high LET values (LET denotes linear energy transfer, which is the amount of energy deposited in the semiconductor by the radiation). It is estimated that if the first upset mechanism is eliminated completely, then the second upset mechanism will still limit the error rate to about 1E-7 (i.e., 1x10⁻⁷) errors per bit-day. Various techniques have been proposed for hardening bipolar circuits to SEU. However, all of these techniques address only the first type of upset mechanism listed above. They propose to either remove the collector from electrical contact with the substrate or reduce the charge injected into the collector node. The most effective of these other techniques - dielectric isolation - completely isolates the collector from the substrate. Eliminating SEU-induced collector-to-base shorts is much more difficult. Pulse rise time can be used to discriminate SEU currents from normal device currents. This forms the basis for CMOS hardening by means of cross-coupled resistors or extra capacitance. But this technique degrades the speed of bipolar circuits so much that they lose their competitive advantage against CMOS. Photocurrent compensation techniques are also ineffective against the second form of SEU because the upset current varies with the direction of ion incidence. Geometric effects practically rule out the location of a second charge collection region which can produce an equal but oppositely directed photocurrent from the same ion path. Finally, circumvention techniques are almost totally ineffective because of the difficulty of sensing a time-random ion event at every transistor node and recovering fast enough (e.g., 0.2ns) to prevent an upset. A bit error rate of 1E-10 error/bit-day is being sought for memory elements in a cosmic ray environment. This corresponds to approximately three years without an error for a one megaword memory. Only one technology (CMOS/SOS) can meet this goal at present, but CMOS/bulk Si is constantly being improved and may be able to meet this goal in the near future. No bipolar technology, however, can approach this goal. Ever more stringent error/bit-day rates are highly desirable with the enormous increase in memory capacity being projected for general purpose computers. In a general purpose computer an error in the register bit or memory bit can be retained and used in a number of calculations. If the bit is a critical value, the consequences of even one error can be disastrous. It is the object of the present invention to provide a memory cell which is extremely resistant to both types of SEU. It is further highly desirable to have such a cell which is adaptable to either unipolar or bipolar transistor circuits. These objects are achieved by the invention as characterized in claim 1. If as a result of radiation striking the cell transient electrical signals are generated in said second regions, the cell will nevertheless not change its current logic state. The present invention is a bistable solid state regenererative memory cell wherein redundant switching means, such as bipolar or unipolar transistors, are connected in a cross-coupled arrangement . Each transistor has a load connected between the transistor and a voltage referenced current source. Means to control the current or voltage applied to the base or control gate of each transistor is provided between such base or control gate and the collector (or drain) of the other transistor to which it is cross-coupled. Diodes are the preferred control means. Read and write circuitry is readily provided to allow the cell to be incorporated in a RAM or a logic array. The redundant transistors and diodes serve to retain the logic state of the cell despite an SEU occurring at the collector (or drain) of one of the transistors. Further improvements and details are subject of the subclaims. In the drawings Figure 1 is a prior art solid state regenerative bistable cell. Figure 2 is a generalized schematic of the present invention. Figure 3 is a DTL version of the present invention. Figure 4 is the cell of Figure 3 with read and write circuitry added. Figure 5 is an SDFL version of the present invention. Figure 2 shows the present invention, i.e., a solid state memory cell 10. Two cross coupled pairs of switching means, such as transistors, are shown as T1/T2 and T3/T4. Each transistor includes a control region (i.e., 12, 14, 16 and 18) which is either the gate of a field effect transistor or the base of a bipolar transistor. The control region in each transistor controls the voltage or flow of current between a first region (i.e., 20, 22, 24 and 26) and a second region (i.e., 28, 30, 32 and 34) of each transistor. Load means L₁, L₂, L₃ and L₄ are connected between a first voltage reference point V₁ and second regions 28, 30, 32 and 34, respectively. Isolation means I₁, I₂, I₃ and I₄ for controlling the voltage or current applied to the control regions are connected, respectively, between second region 28 and control region 16, second region 30 and control region 18, second region 34 and control region 12 and second region 32 and control region 14. Load means L₅ may be provided between common base node 36 and a second voltage reference point V₂. Similarly, load means L₆ may be provided between common base node 38 and second voltage reference point V₂. Load means L₅ and L₆ are important to the WRITE process when the transistors are bipolar. Writing into the cell will be discussed further below. Load means L₇, L₈, L₉ and L₁₀ are each connected between the control regions of one of the transistors and their respective cross coupling isolation means. These further harden the circuit against radiation and will be discussed further below. Note that nodes 36 and 38 are convenient points for connecting a WRITE means. Operation of cell 10 (Figure 2) is as follows. Assume that T₁ and T₂ are ON, and T₃ and T₄ are OFF. The voltages at the second regions 28 and 30 are low (e.g., a logic 0) since T₁ and T₂ are in their low impedance state. The current through I₁ and I₂ is insufficient, therefore, to raise the potentials of control regions 16 and 18 enough to switch ON either T₃ or T₄. Thus T₃ and T₄ remain OFF. In turn, the second regions 32 and 34 are high (e.g., logic 1). I₃ and I₄ are devices which will provide sufficient voltage or current to control regions 12 and 14, respectively, to keep T₁ and T₂ ON when second regions 32 and 34 respectively are in a high state. Since I₃ and I₄ are connected in common at node 36, if either I₃ or I₄ are providing sufficient voltage, or current, then both T₁ and T₂ will be ON. Similarly, due to their common connection at node 38, either I₁ or I₂ will provide sufficient voltage or current to keep T₃ and T₄ ON if either node 28 or node 30, respectively, is in a high state. If the transistors in Figure 2 are bipolar, the electrical signals controlling them will be currents transmitted through the isolation means I₁ through I₄ to their respective base regions. If transistors in Figure 2 are FETs, the electrical signals transmitted by the isolation means I₁ through I₄ will be voltages applied to their respective gate regions. Consider the invention if the memory cell is hit by a single event pulse. For example, in DTL circuit 40 of Figure 3, assume that initially T₃ and T₄ are OFF, and that T₃ is hit by a heavy ion which funnels charge into its collector node 32. Then collector node 32 is pulled to ground. Diode I₃ gets reverse-biased, isolating node 32 from the rest of the circuit. Diode I₄, however, stays forward-biased maintaining the base current of T₁ and T₂. Thus T₁ and T₂ stay ON, the collector nodes 28 and 30 of T₁ and T₂, respectively, stay low and diodes I₁ and I₂ remain reverse-biased. Transistor T₄ stays OFF, ensuring that the state of the cell is not changed. As soon as the transient charge on collector node 32 of T₃ is dissipated (e.g., 0.5 ns) the potential at collector 32 rises to the same level as collector 34 of T₄. Cell 40 has thus survived an SEU event with its information intact with only the appearance of a transient state which may or may not be observed on later readout. Readout from the cell can be accomplished by numerous convenient means. For example, for cell 40 one could use (a) Schottky diodes coupled to collector nodes, (b) multiple emitters on transistors or (c) Schottky diodes coupled to base nodes. Figure 4 shows scheme (a) with transistors T₅ and T₆ coupled to regions 28 and 34, respectively. It is not necessary to sample both current paths on each side of the flip-flop for readout. If a single event pulse affects one current path while readout is being accomplished on the other path, the event will go unnoticed and the correct logic state will result. If a single event pulse affects the same current path that is used for readout, the effect is the same as if a single event pulse hit one of the transistors in the logic chain between the cell and a following register or output pin. Therefore, there is little reason to harden one part of the chain unless the other (larger) part of the chain is hardened also. Voting on three successive readouts of the cell will eliminate all errors incurred during cell readout, both inside and outside the cell. Writing the cell requires forcing a current into the base node of the OFF transistors which is larger than the base current provided inside the cell to the ON transistors. This is accomplished, conveniently, with transistors T₇ and T₈ coupled, respectively, to nodes 42 and 44 as shown in Figure 4. The time for the cell to switch from one state to another is related to the time required to pull charge out of the base regions of the initially ON transistors (e.g. T₃ and T₄). When the initially OFF transistors are switched ON by the externally supplied base current pulse, the diodes on the collectors of these transistors becomes reversed-biased. This means that there is no way for the base charge of the initially ON transistors to be withdrawn unless one supplies a leakage path ground. This is accomplished (in Figure 4) by base pull-down load means L₅, L₆, L₇ and L₈. The cell must be accessed from both sides in order to WRITE both states successfully. This means that somewhere in the WRITE circuit there is a possibility of single-point failure. This failure can result in an incorrect state being written into the cell. In Figure 4, the single point of failure occurs on the means whereby current is diverted from the base of transistors T₇ and T₈. Consideration of errors which occur in the WRITE mode leads one to the conclusion that it is not effective to harden just the few transistors adjacent to the storage cell unless all transistors in the logic chain leading into the cell are hardened to near the same level. This follows because all the transistors in the logic chain have the same duration of susceptibility (the path delay time) and all are likely to have the same critical charge and critical time. Therefore, the probability of upset is approximately the same. This means that hardening the last N of M susceptible transistors in the logic chain will result in a reduction of the critical volume, and enhance the SEU error rate, by a factor N/M. In order to harden the cell against SEU induced collector-base shorts, load means L₉, L₁₀, L₁₁ and L₁₂ are preferably employed. Even though the transistors T₁ through T₄ are placed in separate isolation regions, the transistors on one side of the cell share a common base node. Therefore, collector-base current from one transistor can affect the base current of the other transistor on the same side of the cell, turning both OFF transistors ON at the same time. This can be alleviated by adding base resistors such as load means L₉ through L₁₂ as shown in Figures 2 and 4. Ion-produced single event upset currents from one transistor must go through both resistors (e.g. L₉ and L₁₀) before affecting the second transistor on the same side of the cell. This is a modified version of cross coupled resistors, which in the past has not been very effective for bipolar SEU hardening. But in the present cell the collector-substrate component of upset current cannot produce upset, and the collector-base component is much smaller than the normal collector-substrate component. Therefore, the resistor values can be relatively small and the circuit is not slowed very much. A final improvement which is highly desirable to harden the cell against SEU is to provide isolation means and load means which are inherently not susceptible to SEU. If they are susceptible, SEU could inject current into the base of both OFF transistors on one side of the cell, turning them both ON. Load means L₅ through L₁₂ and isolation means I₁ through I₄ are the most important means to be hardened against SEU. Hardening of these elements of the cell can be accomplished by providing thin film polysilicon resistors as the load means and using an implanted dielectric under Schottky diodes as the isolation means. By itself, the gated feedback cell is of course only part of an integrated circuit. For a complete circuit, this cell can be embedded in either a RAM, an edge-triggered flip-flop, or a register file (which is an array of edge-triggered flip-flops.) The area penalty for RAM design is approximately 100%. The area penalty for an edge-triggered flip-flop can be as low as 25% if only the output latch is protected. This may be possible in the edge-triggered flip-flop scheme because the remainder of the edge-triggered flip-flop is used only during the input operation. This makes the cell of the present invention an attractive solution for register files and stand alone flip-flops. Implanted resistors with implanted dielectric isolation can also be used instead of polysilicon resistors to harden the load means. In addition to implanting a dielectric under a Schottky or junction diode to harden diodes, some other techniques available for hardening diodes are (a) implanted polysilicon junction diodes, (b) Schottky diodes in recrystallized silicon over insulating substrate material, and (c) Schottky diodes in double epitaxial material, where the second epitaxial layer is grown in a V-shaped region surrounded by oxide with the original epitaxial layer as the floor. Presently, implanted dielectric under the Schottky diode is the preferred option. Edge-triggered flip-flops are only one of three types of synchronous clocking schemes commonly used by logic designers. The other two are master-slave latches and multi-phase clocking with logic between the latches. Each of these clocking schemes can be implemented in various circuit families. The gated feedback cell can be used with any of these clocking schemes. The choice of clocking scheme invovles a tradeoff of system speed, clock skew problems, minimum path delays problems and ease of architectural design. The choice will be highly application dependent. Figure 5 shows a version of the present invention applied to GaAs SDFL technology. READ and WRITE means are not shown. However various READ and WRITE means well known to those skilled in the art can be employed. Figure 5 employs metal-semiconductor FETs in the depletion mode; however, enhancement mode FETs can be employed as well as modification doped FET's (MODFET's). When MESFETs are employed, GaAs is the preferred substrate material; however, silicon or other semiconductor materials can be used for the substrate with the present invention.

Claims: A solid-state gated bistable feedback memory cell (10), comprising: a) first (T₁), second (T₂), third (T₃), and fourth (T₄) switching means, each of said switching means having a first region (20, 22, 24, 26), a second region (28, 30, 32, 40) and a control region (12, 14, 16, 18), wherein said first and second regions are doped so as to be partially conductive and wherein said control region controls the flow of current between said first and second regions, and wherein said first regions (20, 22) of said first and second switching means are connected at a first node (V₂) and said first regions (24, 26) of said third and fourth switching means are connected at a second node (V₂); b) first (L₁), second (L₂), third (L₃), and fourth (L₄) load means, wherein said load means and said switching means form a bistable regenerative device with logic values provided at said second regions, characterized in that c) said first load means (L₁) is connected to and between said second region (28) of said first switching means (T₁) and a third node (V₁); d) said second load means (L₂) is connected to and between said second region (30) of said second switching means (T₂) and said third node (V₁); e) said third load means (L₃) is connected to and between said second region (32) of said third switching means (T₃) and a fourth node (V₁); f) said fourth load means (L₄) is connected to and between said second region (34) of said fourth switching means (T₄) and said fourth node (V₁); g) first (I₁), second (I₂) third (I₃) and fourth (I₄) isolation means are provided for controlling electrical signals applied to said control regions; wherein h) said first isolation means (I₁) is connected to and between said first load means (L₁) and said control region (16) of said third switching means (T₃); i) said second isolation means (I₂) is connected to and between said second load means (L₂) and said control region (18) of said fourth switching means (T₄); j) said third isolation means (I₃) is connected to and between said third load means (L₃) and said control region (14) of said second switching means (T₂); and k) said fourth isolation means (I₄) is connected to and between said fourth load means (L₄) and said control region (12) of said first switching means (T₁); l) said control region (12) of said first switching means (T₁) is connected to the control region (14) of said second switching means (T₂), said control region (16) of said third switching means (T₃) is connected to the control region (18) of said fourth switching means (T₄); m) said first and second nodes are connected to a first voltage reference point (V₂) and said third and fourth nodes are connected to a second voltage reference point (V₁), so that different voltages may be applied to said first and second voltage reference points to supply electrical energy to said cell; n) fifth load means (L₅) is connected to and between said control region (12) of said first switching means (T₁) and said first node (V₂), and to and between said control region (14) of said second switching means (T₂) and said first node; and o) sixth load means (L₆) is connected to and between said control region (16) of said third switching means (T₃) and said second node (V₂), and to and between said control region (18) of said fourth switching means (T₄) and said second node; wherein p) if transient electrical signals are introduced at only one of said second regions, the logic values of the remainder of said second regions will be unaffected andsaid bistable circuit will not be triggered to switch logic states due to radiation striking only one of said logic nodes at any given time. The cell of claim 1, characterized by : first and second common nodes (38, 36; 44, 42), wherein said first and second isolation means (I₁, I₂) are connected in common at said first common node (38) with said first isolation means (I₁) connected to and between said second region (28) of said first switching means (T₁) and said first common node and said second isolation means (I₂) connected to and between said second region (30) of said second switching means (T₂) and said first common node (38), and said third and fourth isolation means (I₃, I₄) are connected in common at said second common node (36) with said third isolation means (I₃) connected to and between said second region (32) of said third switching means (T₃) and said second common node, and said fourth isolation means (I₄) connected to and between said second region (34) of said fourth switching means (T₄) and said second common node (36). The cell of claim 2, characterized by seventh (L₉), eighth (L₁₀), ninth (L₁₁) and tenth (L₁₂) load means, wherein said seventh load means (L₉) is connected to and between said second common node (42) and said control region (12) of said first switching means (T₁), said eighth load means (L₁₀) is connected to and between said second common node (42) and said control region (14) of said second switching means (T₂), said ninth load means (L₁₁) is connected to and between said first common node (44) and said control region (16) of said third switching means (T₃) and said tenth load means (L₁₂) is connected to and between said first common node (44) and said control region (18) of said fourth switching means (T₄) (Fig. 4). The cell of one of the preceding claims, characterized in that said switching means (T₁ to T₄) are bipolar transistors, said first regions (20, 22, 24, 26) are emitters of said transistors, said second regions (28, 30, 32, 34) are collectors of said transistors and said control regions (12, 14, 16, 18) are bases of said transistors (Fig. 3). The cell of claim 1, wherein said switching means (T₁ to T₄) are field effect transistors, said first regions (20, 22, 24, 26) are sources of said transistors, said second regions (28, 30, 32, 34) are drains of said transistors and said control regions (12, 14, 16, 18) are gates of said transistors (Fig. 5). The cell of claim 5, characterized in that said field effect transistors are metal-semiconductor field effect transistors (MESFETs). A cell according to one of the preceding claims, characterized in that each of the isolating means (I₂ to I₄) includes at least one diode.

Inventors: BELT, RONALD A., HAVEY, GARY D.
Applicants: HONEYWELL INC.
Application year: 1986
Main classification: G11C11
IPC codes: G11C11, G11C5, G11C29