Voltage regulator for programming non-volatile and electrically programmable memory cells

Abstract: A voltage regulator for programming non-volatile memory cells, which comprises an amplifier stage (4) being powered between a first (VPP) and a second (GND) voltage reference and having a first input terminal connected to a resistive divider (2) of the first reference voltage (VPP) and an output terminal fed back to said input through a current mirror (3), and a source-follower transistor (MOUT) controlled by the output and connected to the cells through a programming line (VP). Also provided is a MOS transistor (MG2) which connects to ground the programming line (VP) and a corresponding resistive path (7) connected between the current mirror (3) and the second voltage reference (GND).

Description: Field of the Invention This invention relates to a voltage regulator which allows the terminals of a memory cell to be suitably biased. More particularly, the invention relates to non-volatile memory device including a voltage regulator for programming non-volatile memory cells, which regulator comprises a differential amplifier stage being powered between a first and a second voltage reference and having a first input terminal connected to a voltage divider of said first reference voltage, a second input terminal connected to a programming line and an output terminal connected to a source-follower transistor which is coupled to said first input through a current mirror and connected to the cells through the programming line In a well-known manner, a non-volatile memory cell comprises a MOS transistor having a "floating" gate terminal, that is exhibiting a high DC impedance to all the other terminals of the cell and the circuit in which the cell is connected. The cell also has a second electrode, known as the control gate electrode, which is driven by appropriate control voltages. The other electrodes of the MOS transistor are the usual drain, source and body terminals. By changing the value of the voltage applied to the control gate, the amount of the charge present in the floating gate can be changed. Thus, the transistor can be placed in either of two logic states: a first state with a "high" threshold voltage, and a second state with a "low" threshold voltage. By applying a voltage in between these two values to the control gate, the state of the transistor can be "read", since the transistor will either present a low or a high impedance between its drain and source terminals according to the value of the threshold voltage. The transistor functions, therefore, as a logic storage element. Furthermore, since the floating gate presents a high impedance to any other terminals of the cell, the charge stored in the transistor can be held for an indefinite length of time, even if the power supply to the circuit in which it is connected is removed. Thus, the cell has non-volatile memory characteristics. The operation whereby charge is stored in the floating gate is referred to as the cell "programming", whereas the operation whereby said charge is removed from the floating gate is referred to as the cell "erasing". A non-volatile memory circuit integrated to a semiconductor usually includes a very large number of such cells. The cells are laid in rows (word lines) and columns (bit lines). The cells in one row share the line which drives their respective control gates. The cells in the same bit line share the drain electrode. In order to program a given cell, it is necessary that the appropriate positive voltage values be impressed on the word line and the bit line that locate it. A memory cell programming is strongly affected by the voltage applied to the drain terminal, that is the voltage VP present on the bit line to which it belongs. Also known is that with non-volatile memory cells, in particular of the flash type, a reduced value of said drain voltage VP gives rise to insufficient and slow cell programming, whereas an exccessively large value results in the cell being partially erased (the so-called "soft erasing" phenomenon). Thus, the optimum range for VP is fairly narrow, typically between about 5 and 6 V. The foregoing considerations lead to conclude that the memory circuit should be provided with a particularly sophisticated and accurate voltage regulator to supply the bit line with the appropriate voltage during the programming step. Background Art It is well known that one of the methods employed to program memory cells utilizes the phenomenon of the injection of hot electrons through the gate oxide of the cell. For the injection process to be triggered, it is necessary that the cell terminals be suitably polarized; the drain-source voltage should attain a value in the 5 to 6 volts range, in order to accelerate the electrons present in the channel, while the gate-source voltage should be a value of about 12 volts, so as to attract the accelerated electrons. However, as previously mentioned, what is critical is the value of the drain voltage VP, and this should be held as stable as possible during the programming. The fluctuations in said voltage are tied to the following factors: technological changes in the manufacturing processes of the various components; increased threshold voltage of the cell during the programming, and consequent decrease in current draw; and, voltage drops across the selection transistors present in the bit lines. The last-mentioned is a significant effect especially with large capacity memories, wherein in view of the small dimensions involved, the selection transistors have a particularly small W/L (channel width to length) ratio and, therefore, high series resistance. Thus, there evidently exists a need for a voltage regulator of sufficiently refined design to take account of all the factors involved. The prior art solves these problems by providing a circuit wherein the output voltage from the regulator exceeds that sought for the bit line by an appropriate amount which is continually dependent on the actual current flowing through the selection transistors in the bit line. To accomplish this, a polarization technique of the adaptive type is employed which provides for a positive feedback structure. An example of this prior approach is shown schematically in Figure 1 and disclosed in EP 92830340.9 (EP-A-0576774), which falls under Article 54(3) EPC. In essence, a voltage generator generates a constant voltage (VPROG) equal to the voltage that should appear on the selected bit line if optimum programming conditions are to be provided. That voltage is added a varying voltage, designated Vcomp. The value Vcomp is obtained by mirroring the current supplied from the regulator to the bit line on elements which are mirror-images of the bit line selection transistors, thereby producing the same voltage drop across them.More specifically, the regulated voltage is supplied from a source-follower MOS transistor, designated MOUT, which is driven by a suitably fed-back operational amplifier (4). This arrangement includes two feedback loops: a first loop inherent to the voltage regulator, and the second loop (comprising a MOS transistor, designated MPR2, and a generic block H) to compensate for the voltage drop at the bit line. The second loop supplies a current to a drop resistance Rdrop which is proportional to the current from the regulator, and hence, tied to that drawn by the cell being programmed. In a storage device, the programming usually takes place in parallel through all the cells of one word; of course, only those cells whereat a voltage rise is to occur, i.e. which are to contain a logic "1", would be actually programmed. The current drawn from the bit line at the programming stage will depend, therefore, on the pattern of bits to be stored. To avoid altering the compensation, the drop resistance should be varied accordingly. However, this prior approach has some drawbacks, as follows: the bias current drawn by the MOS M9, shown in Figure 1, is carried over to the drop resistance to cause an undesired constant component to appear in the compensation voltage Vcomp. This constant voltage component is particularly objectionable where a single cell is programmed, because in this case, the current drawn by the transistor M9 is then no longer negligible against that required for the programming; and while the cells are being read, the bit line is polarized to a voltage of about 1 V, and connected to the line VP via a p-channel MOS transistor. This transistor has its gate voltage Vg0 = 0 V (as impressed by a switch being also supplied VP) and its source voltage Vs0 = 1 V, and is therefore in conduction. At the reading stage, the programming voltage VP is zero and the p-channel MOS transistor connects the capacitance associated with the junction of the conducting transistor MOUT to the bit line; as a result, the capacitance of the bit line is greatly increased and its reading speed greatly decreased. This is a baffling phenomenon in that it only comes out where the bit line takes a higher voltage than the p-channel threshold voltage; this may happen when the supply voltage exceeds its rated value. Accordingly, this phenomenon should be carefully suppressed. The underlying technical problem of this invention is to provide a voltage regulator which can supply the voltage actually needed for proper programming, but also hold the programming line and bit line decoupled while the latter is being read. Summary of the Invention The solutive idea on which the invention stands is to provide a voltage regulator which comprises an amplifier stage controlled by a compensation voltage which is made independent of the voltage drop due to a resistive path connecting the programming line to the device ground reference. In addition, this regulator should contain a structure which allows the reading speed of the cells in the bit line to be improved, while decoupling it from the programming line to thereby eliminate the above-outlined drawbacks. Based on this solutive idea, the technical problem is solved by a voltage regulator as previously indicated and defined in the characterizing parts of the appended claims. The features and advantages of a circuit according to the invention will be apparent from the following detailed description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 is a diagram illustrating the principle which underlies a voltage regulator of a prior approach. Figure 2 shows diagramatically a regulator according to the invention. Figure 3 shows schematically a circuit generating signals which appear in the regulator of Figure 2. Detailed Description Figure 1 relates to an embodiment of a voltage regulator according to the prior art as previously discussed. With reference to Figure 2, generally and schematically shown at 1 is the circuit architecture of a voltage regulator according to the invention. The regulator 1 comprises an amplifier stage 4 incorporating two MOS transistors of the n-channel type, with source terminals connected together and to the drain terminal DG1 of a MOS transistor MG1 which functions as a current generator. The drain terminals of said transistors are connected to the drain terminals of another two MOS transistors of the p-channel type, in a current mirror configuration. The amplifier 4 is connected between a first or programming voltage reference VPP, through a further MOS transistor MX, and a second voltage reference, in essence a signal ground GND. The gate terminal of the last-mentioned MOS transistor MX is applied a first reference voltage VR1, whereas the voltage reference GND is applied to the source terminal SG1 of the transistor MG1. The input I1 of the stage 4 is connected to the drain terminal D1 of a MOS transistor M1, which has its body terminal B1 connected to the voltage reference GND and source S1 and gate G1 terminals connected across a resistive element R2. The resistive element R2 is connected in a resistive divider, generally denoted by 2. Said divider 2 comprises a series of three resistive elements and is placed between the voltage reference VPP and the output terminal, O3, of a current mirror generally denoted by 3. Said current mirror 3 comprises four MOS transistors, and allows a current IC to be carried over to the input I3 of the divider 2 which is proportional to the current IC' supplied to a transistor MOUT to be described. Advantageously, a resistive path 7 is connected to the current mirror 3 which comprises a MOS transistor M2. This transistor M2 has its drain terminal D2 connected to the current mirror 3, its body B2 and source S2 terminals connected to the voltage reference GND, and its gate terminal G2 controlled by a signal A. The point of interconnection between the resistive divider 2 and the output of the mirror 3, denoted by I3, is also connected to a network of compensation resistive elements, generally denoted by 5. That network 5 consists of a set of MOS transistors which function as resistive elements and are selected on the basis of the bit pattern to be programmed in the memory word; basically, for each bit to be programmed there corresponds a different compensation transistor. Said network 5 is connected between the voltage reference GND and a second polarization signal VR2. Advantageously, connected in parallel with the network 5 is a resistive path 8 comprising a MOS transistor M3 which has its gate terminal G3 connected to the reference VR2, its drain terminal D3 connected to the node I3, its body terminal B3 connected to the voltage reference GND, and its source terminal S3 connected to a point CT of a series of two MOS transistors M4 and M5. The MOS transistors M4 and M5 are connected between the voltage reference GND and the network 5 of resistive elements, being controlled at their gate terminals G4 and G5, respectively, by a signal PP (Parallel Programming) and an enable signal A. These transistors function substantially in lieu of the network 5 during the memory testing procedure. The gate G5 of the transistor M5 is connected to the gate terminals GG1 and GG2 of the transistors MG1 and MG2, and receives the enable signal A. The transistor MG2 is connected between the current mirror 3 and the voltage reference GND, and has its body BG2 and source SG2 terminals connected together. The output O1 of the amplifier stage 4 is connected to ground through the drain terminal D6 of a further MOS transistor M6 which has its body B6 and source S6 terminals connected to the voltage reference GND. The gate terminal G6 of the MOS transistor M6 is applied a negative logic signal AN which allows the output O1 of the amplifier 4 to be connected to ground during the turn-off step. This signal AN is expediently also applied to the gate terminal G7 of a MOS transistor M7, connected in series with another MOS transistor, M8. These MOS transistors, M7 and M8, are included to a decoupling element, generally denoted by 6. In addition, they have their body terminals connected together and to the voltage reference GND; the source terminal S7 of transistor M7 is connected to the programming line VP. The transistor M8, on the other hand, is configured as a diode, with its gate and source terminals connected together and to a further voltage reference VDD. The output O1 of the amplifier stage 4 is connected to ground via a parasitic capacitance C1 which ensures the amplifier stability. Further, the output O1 is connected to the source-follower transistor MOUT. The signals A and AN are generated by a circuit, generally shown at 9 in Figure 3. This circuit 9 comprises a logic gate PL of the NOR type having two inputs A9 and B9 and an output O9. The inputs A9 and B9 are applied the external signals VO and PW, with the former being brought to negative logic by means of an inverter IN1. The output O9 is connected to the source terminal of a MOS transistor M9 whose gate terminal G9 is applied the external signal VCCLOW. This signal reveals dangerous drops in the supply voltage to the circuit. The body B9 and source S9 terminals of that transistor M9 are connected to the voltage reference GND. The output O1 supplies the signal A and signal AN, the latter being obtained from A by means of an inverter IN2. Thus, these signals can enable the on and off states of the regulator 1. The operation of the voltage regulating circuit of this invention will now be discussed. The current mirror 3 allows the compensation to be effected of the voltage drop caused by the series resistance of a bit line for which programming is sought. Said compensation is provided by adding, to the desired reference voltage as obtained through the resistive divider 2 from the voltage reference VPP, a voltage which varies according to the current drawn by the cell or cells being programmed. The provision of the transistor MG2, as necessary to maintain a resistive path from VP to ground, brings about an undesired voltage offset component. The circuit arrangement of this invention overcomes this problem by the inclusion of the transistor M2, which will substract a fraction of the bias current from the mirrored current. Said fraction is determined by the W/L ratii of the transistors provided in the current mirror 3. This allows any offset voltage present on the programming line to be carried over to the input and compensated. Furthermore, to improve the regulation linearity as the number of the programmed cells varies, this arrangement provides for the connection of a permanent resistive path 8 in parallel with the resistive elements selected in the network 5, through the transistor M3. Finally, the decoupling element 6, comprising the series of the transistors M7 and M8, can bias, upon the regulator being turned off, the programming voltage VP to a value equal to the supply voltage VDD minus the value of the threshold voltage VT of one, M8, of the transistors, thereby causing the selection p-channel MOS transistors to be held in a cutoff state while the memory cells of the bit line are being read. In this way, the capacitance of the bit line will increase not even during the reading step on account of the connection to VP, and the reading process will not be slowed down. Through such innovations, the voltage regulating circuit 1 of this invention can overcome the drawbacks with which the prior art is beset, and improve both the stability and reliability of a non-volatile memory programming and the speed of operation of the memory matrix in the read mode.

Claims: A non-volatile memory device including a voltage regulator for programming non-volatile memory cells, said non-volatile memory cells being coupled to a programming line (Vp) through a p-channel selection MOS transistor, the voltage regulator comprising a differential amplifier stage (4) being powered between a first (VPP) and a second (GND) reference voltage and having a first input terminal connected to an output of a voltage divider (2) coupled between said first reference voltage (VPP) and a first node (I3), a second input terminal (I2) connected to said programming line (Vp) and an output terminal connected to the gate of a source-follower transistor (MOUT) which has its drain coupled to said first node (I3) through a current mirror (3) and its source connected to said programming line (VP), the voltage regulator further comprising a decoupling element (6) connected between a supply voltage (VDD) and said programming line (VP) to feed said programming line with a predetermined voltage value upon the voltage regulator being turned off during a read mode. A non-volatile memory device according to Claim 1, characterized in that said decoupling element (6) comprises two MOS transistors (M7,M8) connected serially between the power supply line and the programming line. A non-volatile memory device according to Claim 2, characterized in that the transistors (M7,M8) of the decoupling element (6) have their body terminals (B7,B8) connected together and to the second voltage reference (GND). A non-volatile memory device according to Claim 2, characterized in that the second transistor (M8) of the decoupling element (6) is configured as a diode. A non-volatile memory device according to claim 1, characterized in that it comprises a MOS transistor (MG2) connecting the programming line (VP) to ground and a resistive path (7) connected between the current mirror (3) and the second voltage reference (GND). A non-volatile memory device according to Claim 5, characterized in that said resistive path (7) comprises at least one MOS transistor (M2). A non-volatile memory device according to Claim 6, characterized in that said transistor (M2) has its drain terminal (D2) connected to said current mirror (3) and its source (S2) connected to ground. A non-volatile memory device according to Claim 1, characterized in that it comprises a network (5) of resistive elements, selected on the basis of a bit pattern to be programmed in a memory word, connected between the divider (2) and the second voltage reference (GND), and in that it further comprises a resistive path (8) connected in parallel with the resistive network (5). A non-volatile memory device according to Claim 8, characterized in that said resistive path (8) includes a MOS transistor (M3) connected between the second (GND) and a third (VR2) voltage reference. A non-volatile memory device according to Claim 8, characterized in that said transistor (M3) is connected in parallel with a set of transistors forming said resistive network (5).

Inventors: GOLLA CARLA, MACCARRONE MARCO, OLIVO MARCO, PADOAN SILVIA, GOLLA, CARLA, MACCARRONE, MARCO, OLIVO, MARCO, PADOAN, SILVIA
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: G11C16
IPC codes: G11C16, G11C17, G11C5