UPROM cell for low voltage supply

Abstract: The present invention concerns a redundant UPROM cell (1) incorporating at least one memory element (P0) of the EPROM or flash type having a control terminal (CG) and a conduction terminal (X) to be biased, a register (2) with inverters connected to the memory element and MOS transistors (M1,M3) connecting said memory element (P0) with a reference low voltage power supply (Vdd). There is provided a precharge network (5) for the conduction terminal (X) of the flash cell and said network (5) incorporates a complementary pair of transistors (M4,M5). The second transistor (M5) of said pair (M4,M5) is a natural N-channel MOS type. With the UPROM cell (1) is associated a circuit portion (10) for generating at output (U) a live signal (UPCH) to be applied to the control terminal of the second transistor (M5) with the portion (10) comprising a timing section (7) and a generation section (8) for said second live signal (UPCH).

Description: Field of application The present invention relates to UPROM memory cells for low supply voltage. Specifically but not exclusively the present invention concerns a redundant UPROM cell incorporating at least one EPROM or flash memory element having a control terminal and a conduction terminal to be biased, a register with inverters connected to the memory element and MOS transistors connecting the memory element with a reference low-voltage supply. As known, one of the greatest difficulties encountered in providing memory devices integrated on semiconductors on a very large scale is the yield of the associated production process. It often happens that at the end of the production process the defectiveness of the memory cell matrix is such as to make the device unusable. The cell matrix occupies the preponderant part of the circuit area of the device and there is high probability that a serious manufacturing defect will reside in the circuit portion occupied by the matrix. In this context the production of non-volatile memory devices of the so-called EPROM and flash type has proven to have very low yield. Prior art The prior art has sought to remedy the low yield of the production process of flash memories. The solution thus far adopted consists of equipping the cell matrix with additional rows and/or columns - termed redundant - which could be used if necessary to replace rows or columns which prove defective or display malfunctions after testing of the device. Those skilled in the art know well the design methodologies using redundant rows and columns and the associated selection circuitry. The latter allows readdressing the memory in such a manner as to replace the addresses containing defective bits with operating ones present in the redundant rows or columns. A known type of memory device able to operate with multiple power voltage levels is described in the US Patent No. 5,357,458 to Yu et al. Currently, the continuing evolution of technology and the market trend for semiconductors lead to designing memory devices capable of operating with ever lower supply voltages. This involves not a few problems due to the fact that to obtain a memory device efficient and fast in response, in particular in the reading phase, it is necessary that the redundant cells and circuitry also meet certain stringent specifications. Specifically the UPROM memory cells incorporated in the selection circuitry which contains the binary codes of the addresses to be redounded must be able to operate effectively even with a low power supply. The technical problem underlying the present invention is to conceive an UPROM cell having structural and functional characteristics such as to allow extremely fast reading of the memory while being able to operate with low supply voltage. This would allow overcoming the limitations and shortcomings of the present solutions proposed by the prior art for low voltage memory devices. Summary of the invention The solution idea underlying the present invention is to precharge the drain terminal of the flash cell incorporated in the UPROM cell to prevent the problem of the presence of extra charge current for parasite capacitors present in the UPROM cell. On the basis of this solution idea the technical problem is solved by an UPROM memory cell of the type indicated above and defined in the characterizing part of the annexed claims 1 and following. The characteristics and advantages of the cell in accordance with the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings. Brief description of the drawings FIG. 1 shows a basic diagrammatic view of an UPROM memory cell, FIG. 2 shows a diagrammatic view of a preferred embodiment of the cell according to the present invention, FIG. 3 shows a diagram of the behaviour as a function of time of some voltages present in the cell of FIG. 1, and FIG. 4 shows a diagrammatic view of a voltage generator associated with the UPROM memory cell in accordance with the present invention. Detailed description With reference to the above figures reference number 1 indicates as a whole and diagrammatically the structure of an UPROM memory cell. The cell 1 is integrated in a semiconductor memory device and in particular the EPROM or flash type operating on low supply voltage. The memory device is not shown in the drawings but is understood to be the type comprising a cell matrix organized in rows and columns and connected to a control, selection and decoding circuitry. The cell 1 is powered by a supply voltage Vdd having a value between 2.5V and 3.6V and is inserted between the power supply Vdd and a second reference voltage GND, for example a signal ground. This UPROM cell comprises a memory element represented by a floating gate cell P0 of the EPROM or flash type containing a binary code of an address to be redounded. This cell P0 is a floating gate transistor with its body terminal connected to ground and a control gate CG to which is applied a live signal UGV. The behaviour of the signal UGV as a function of time is shown in FIG. 3. On the source terminal of the transistor P0 is present a signal redsrc while the other conduction terminal (drain) is connected to the power supply Vdd through a complementary pair of MOS transistors M1,M3. Respective parasite capacitors C1 and C2 are present between the drain terminals of the transistors P0 and M3 and ground GND. The basic structure of the UPROM cell 1 also comprises a first inverter I1 and a second inverter I2 each having its respective input A and output B terminals connected to the output and input of the other inverter. The first MOS transistor M1 of the complementary pair is the P-channel type and connects the input A of the first inverter I1 with the power supply Vdd. The second MOS transistor M3 is the natural N-channel type and connects the input A of the first inverter I1 with the drain terminal of the cell P0 in a source follower configuration. The control terminal of the cell P0 receives the signal UGV while to the respective control terminals G1 and G3 of the transistors M1 and M3 is applied a signal POR# and a drain voltage signal Vb. The behaviour as a function of time of the signal Vb is shown in FIG. 3 while the signal POR# represents the negated form of a POR starting and initialization signal known as 'power on reset'. This POR signal is generated inside the memory device by a power on reset circuit portion. The POR signal is applied to the control terminal G2 of an enablement transistor M2 of the natural N-channel type inserted between the output B of the first inverter I1 and ground GND. The inverters I1 and I2 make up a register of the latch type and the transistors M1 and M2 allow performance of the initialization phase of this latch. M1 and M3 allow updating of the state of the latch depending on the state of the non-volatile cell P0. The UPROM cell 1 also comprises an output buffer I3 having its input connected at a common node B to the output of the first inverter I1 and to the input of the second inverter I2. On the node B is generated a signal q1 which downstream of the output buffer 13 becomes a negated signal q1#. There is now described operation of the UPROM cell of Figure 1 starting from an initial state in which the memory device is turned on. When turned on, the power on reset circuitry portion produces a pulsed POR signal having high logical value, i.e. corresponding to a logical '1'. The signal POR#, which is the negation of the above, will consequently correspond to a logical '0'. This situation allows an initial unbalance of the latch 2 which will go into a state of rest or stand-by with the output of the first inverter I1 at low logical value and the output of the other inverter I2 at high logical val. At the end of the POR pulse there are supplied to the flash cell P0 the control voltages UGV and Vb and a reading of its informative content is performed. If the flash cell P0 is canceled, i.e. if it has a threshold voltage lower than 2.5V, it will begin to conduct when the two voltage values UGV and Vb have reached their rated value of approximately 3V and 1.5V respectively. At this point the latch 2 can be unbalanced and brought into a state in which the output of the inverter I1 has high logical value while the output of the inverter 12 has low logical value. If the flash cell P0 were written, i.e. if its threshold voltage were higher than 4V or 5V, it would not absorb current and therefore the latch 2 would remain in its stand-by state. It is important to note that the sizing of the inverters I1 and I2 is especially critical and must allow for the following conditions: the latch 2 must be unbalanced with the minimum current absorbed from a canceled flash cell P0, and the latch 2 must not modify its state during the reading of a written flash cell P0 because of the charge current of the parasite capacitor C1. This current begins to flow as the voltage Vb increases. The two above conditions conflict with each other because: the first condition can be satisfied if the pull-up of the second inverter is very resistive since in this manner the presence of current in the flash cell P0 is able to lower the voltage on the node A, and the second condition can be satisfied with a more conductive pull-up so as to supply a charge current of the parasite capacitor C1 without overly disturbing the voltage on the node A. Satisfying these two conflicting requirements is very important in the case of low voltage power supply because decrease in the voltage also decreases the reading current of the flash cell. Advantageously in accordance with the present invention the structure described above was improved in order to supply an extra charge current for the parasite capacitor C1. All this was obtained by keeping unchanged the pull-up of the second inverter I2. Advantageously in accordance with the present invention, a precharge network 5 is associated with the basic structure of the UPROM cell 1. The precharge network 5 comprises a complementary pair of transistors M4 and M5. The transistor M4 is the MOS P-channel type with the gate terminal connected to ground and the source terminal connected to the power supply Vdd. The transistor M5 is the natural MOS N-channel type and receives on its gate terminals a UPCH signal. The drain terminal of the transistor M5 is connected to the source terminal of the transistor M4. The source terminal of the transistor M5 is connected to a node X which coincides with the drain terminal of the transistor P0 and hence of the flash cell. The presence of the transistor M4 is useful only for the purpose of protecting the natural transistor M5 from electrostatic discharges. Advantageously the signal UPCH applied to the gate terminal of the transistor M4 allows performance of a precharge of the drain terminal X of the flash cell. FIG. 3 shows the behaviour of the signal UPCH as a function of time. It is readily understood that first the value of the voltage UGV on the control gate of the flash cell is set and then the precharge pulse UPCH on the transistor M5 is set. The reading pulse given by the signal Vb arrives only subsequently. To avoid undesired stress on the flash cell the value of the precharge pulse has a controlled amplitude equal to that of the signal Vb. FIG. 4 shows diagramatically the structure of a generator circuit 10 for the precharge pulse UPCH. The circuit portion 10 comprises virtually two sections, one 7 for timing and the other 8 for generation of the UPCH voltage. The timing section 7 controls the time of intervention of the remaining generation section as clarified in the description below. The section 7 has an input terminal I which receives a signal START for enablement of generation of the signal UPCH. The signal START is supplied by a circuitry assigned to generation of the signal UGV for the control gate of the flash cell. This circuitry is not described in detail since it is the subject of a correlated patent application of this same applicant. The section 7 comprises a logical gate I4 of the NOR type having a first input directly connected to the input I. The other input of the gate I4 is connected downstream of a series of inverters 3, 4 and 6. The input of the first inverter 3 coincides virtually with the input I. A capacitor C1 is inserted between the input of the second inverter 4 and ground GND. A fourth inverter 9 connects the section 7 with the section 8 downstream of the logical gate I4. The generation section 8 comprises a logical gate I6 of the NOR type equipped with a feedback loop. The output of the logical gate I6 of the section 8 is connected to the control terminal of a natural N-channel transistor M2 having one conduction terminal connected to the power supply through a transistor M1 and the other conduction terminal connected to ground through a transistor M3 in diode configuration connected in turn to another transistor M4. The control terminal of the transistor M1 is connected to a first input of the gate I6 while the connection node between threshold transistors M3 and M4 is connected in feedback to the other input of the gate I6. The control terminal of the transistor M4 is directly connected to the output of the logical gate I4 of the first section 7. Between threshold transistors M2 and M3 there is an output node U on which is taken the output voltage UPCH. This output node U is connected to ground through a transistor M5. A capacitor C2 indicates the presence of a parasite capacitor of the line UPCH. The control terminal of the transistor M5 is connected to the first input of the logical gate I6. There is now discussed operation of the circuit portion 10. When the gate I6 has one of its inputs at high logical value its output will consequently be at low logical value and will keep the transistor M2 off. Under these conditions the transistor M5 is on and holds the output node U at the value of ground. When the input I rises to high value the output of the gate I6 is allowed to switch in turn to the value '1'. In this manner the transistor M2 is turned on allowing passage of current through the transistors M1, M3, M4. The voltage on the output UPCH can reach the rated operation value set by the release threshold of the gate I6 and by the threshold voltage of the transistor M3. If the voltage UPCH had a value lower than that indicated, the output of the gate I6 would allow the transistor M2 to conduct more current and thus increase the output voltage. But if the voltage UPCH were higher than the selected value, the fedback input of the gate I6 would assume a potential value higher than the release threshold of the gate to take the output to a low potential and extinguish the transistor M2. In this manner UPCH would be reduced until it reached the predetermined value. The feedback loop formed by the logical gate I6 and the transistors M2 and M3 has two great advantages, to wit: the output voltage UPCH is kept equal to the release threshold of the logical gate I6 and of a natural N-channel transistor M3. The threshold of the latter component is equal to approximately 0.5V; the value of UPCH is kept steady even in case of power supply Vdd affected by an electrostatic discharge, and the charge transient of the parasite capacitor on the output line UPCH is very fast, less than 10ns, just because of the feedback of the loop. The UPROM cell in accordance with the present invention has the great advantage of being especially sensitive and is thus suited to operation with low supply voltages. These characteristics are made possible with a negligible increase in circuit area occupied. Variations and modifications can be made to the UPROM cell of the present invention within the scope defined by the following claims.

Claims: Redundant UPROM cell (1) incorporating at least one memory element (P0) of the EPROM or flash type having a control terminal (CG) and a conduction terminal (X) to be biased and a register (2) with inverters connected to the memory element and a first MOS transistor (M1) and a second MOS transistor (M3) connected, in series to each other, bewteen a reference low voltage power supply (Vdd) and said memory element (P0) and receiving respective control signals (por#, vb) at their control gates (G1, G3), said second transistor (M3) being connected between said conduction terminal (X) of said memory element (P0) and an input node (A) of said register (2), said first transistor (M1) being connected between said input node (A) and said reference low voltage power supply (Vdd), characterized in that it comprises a precharge network (5) for said conduction terminal (X) incorporating a complementary pair of transistors (M4,M5). Cell in accordance with claim 1 and characterized in that the first transistor (M4) of said complementary pair is for protection from electrostatic discharges. Cell in accordance with claim 1 and characterized in that the second transistor M5 of said pair (M4,M5) is a natural N-channel MOS type. Cell in accordance with claim 3 and characterized in that it further comprises a circuit portion (10) to generate at output U a live signal (UPCH) to be applied to the control terminal of said second transistor (M5) with said portion (10) comprising a timing section (7) and a section (8) for generation of said second live signal (UPCH). Cell in accordance with claim 4 and characterized in that the timing section (7) comprises a series of inverters (3,4,6,9) and at least one logical gate (I4). Cell in accordance with claim 4 and characterized in that said generation section (8) comprises a logical gate (I6) with two inputs and with an output connected to the control terminal of a transistor (M2) and with the conduction terminals of said transistor (M2) being respectively coupled in feedback to said two inputs of the logical gate (I6). Cell in accordance with claim 1 and characterized in that the first transistor (M4) of the complementary pair is a MOS P-channel type transistor with control terminal connected to ground (GND). Cell in accordance with claim 1 and characterized in that said first and second MOS transistors (M1,M3) connecting said memory element (P0) with the reference low voltage power supply (Vdd) are a complementary pair incorporating a natural MOS N-channel transistor. Cell in accordance with claim 3 and characterized in that the source terminal of said natural N-channel MOS transistor (M5) is directly connected to said conduction terminal of the flash cell (P0).

Inventors: BRANCHETTI MAURIZIO, GHEZZI STEFANO, MACCARRONE MARCO, BRANCHETTI, MAURIZIO, GHEZZI, STEFANO, MACCARRONE, MARCO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G11C16
IPC codes: G11C16