Monolithically integrated selector for electrically programmable memory cells devices

Abstract: A selector switch monolithically integrated to a CMOS technology circuit for electrically programmable memory cell devices having at least first and second input terminals for coupling to first and second voltage generators (HV and LV), respectively, and an output terminal (OUT). First (P1) and second (P2) field-effect selection transistors are respectively connected, via first and second terminals, between the first input terminal and the output terminal and between the second input terminal and the output terminal. These transistors are driven through control terminals at non-overlapping phases and have body terminals connected at a body circuit node (BODY) which is coupled to the first and second voltage generators through a bias circuit block (WBC) effective to bias the node to the higher of the instant voltages generated by the first and second generators.

Description: This invention relates to high and low voltage selection circuits for non-volatile memories, and in particular to a high-speed selector switch integrated monolithically to a CMOS technology circuit. There is a growing demand for integrated circuits to be provided with different voltages for internal distribution according to which of their operative phases are to be activated. For example, semiconductor non-volatile storage devices (such as Flash, EPROM, E 2 PROM memories), require different internal voltages for wordline biasing at different phases of the device operation. For a NORed memory, for instance, a reading operation involves biasing the wordline of the addressed cell to a voltage equal to the supply voltage (V dd =3V or 5V), whereas that same wordline is to be applied suitably higher voltages (e.g., V pp =12V) during the programming phase; V dd being a voltage supplied from outside the device, and V pp either a voltage supplied from outside or produced inside from V dd . Furthermore, for multi-level EPROM storage devices, the wordline bias voltage during the read phase is a higher value than the supply voltage V dd (being usually of 3V or 5V). In fact, for a larger number of levels than two to be reliably allocated, the range of possible cell currents must be stretched to values of 100 to 120 µA; such current values can only be obtained, however, by using gate voltages in the 5V to 6V range. In addition, during specific operative phases (such as those activated for testing), the wordline of the selected cell is to be brought to an analog voltage ranging from 0 to V pp for DMA (Direct Memory Access) reading, as the skilled ones in the art will recognize. Accordingly, integrated circuits that are to operate in this way must be provided internally with a selector switch, to each time pick up and distribute an appropriate voltage to the working blocks, under control by suitable control signals. Specifically with a non-volatile memory, there are essentially two voltages (referenced LV and HV hereinafter) involved, and the selector switch is mainly concerned with biasing the wordlines. In this case, the selector switch will switch the power supply to the final decoding stages, as shown in Figure 1 of the drawings, between the low voltage LV and the high voltage HV, according to whether the addressed cell is to be read or programmed, respectively. It can be appreciated that, if positive voltages are to be output to the selector switch with no voltage drops, the use of p-channel pass transistors becomes mandatory. In the instance of n-well CMOS technology, the use of p-channel transistors within the selector switch creates quite a few problems because the source and drain regions of the transistors must be reverse biased, relative to the wells wherein the p-channel transistors are formed, all the time. Thus, the well regions would have to be connected to that terminal of their associated transistor which is at the highest voltage value. A voltage supply switching device is disclosed for example in the European patent application No. 0 322 002. However, as described in further detail hereinafter, it often is impossible to identify the highest voltage terminal of the p-channel transistors which make up the selector switch, because of its dependence on the specific operative phase being activated. The present invention is aimed to solve this problem by an arrangement which is effective to each time find out the highest voltage present within the circuit and bias the well regions thereto. State of Art A first simple circuit diagram of the selector switch is shown in Figure 2 to include just two p-channel transistors (P1 and P2) which are driven by a control signal PH for turning on either transistor to the exclusion of the other. It is readily seen, however, that this simple approach rules out the possibility of using HV<LV,.since there would occur forward biasing between the well region of P1 and the drain region of this transistor. This is an important limitation placed on non-volatile storage devices, because proper operation of the selector switch cannot be assured in such modes as DMA testing and reading. In fact, the operation specifications usually provide for the user's ability to set up at will the value of HV during the read phases, with the single stipulation that it should not exceed the value of the supply voltage V dd . In addition, considerations of constructional simplicity point to the advisability of having the DMA mode implemented through the HV terminal for accessing the node OUT of the selector switch directly. During such operations, the applied voltages may take values in the range of 0V to 12V. Thus, the value of HV can drop below that of LV in both the DMA testing and the reading modes, and this prior approach tends to be used in a less degree. On the other hand, the problem can be solved by using more complicated schemes, with 3 or 4 transistors and a peculiar interconnection of the wells, like that shown schematically in Figure 3 for a three-transistor construction. But these prior approaches, when applied to commercial devices of the EPROM type, are ill-suited for use in applications where the switching rate of the selector switch (and hence of the voltage at the node OUT) is a parameter of major consideration. In fact, the use of such circuit constructions brings about a problem of transient control of the common node N of the transistors P1 and P3. This node evolves, because of the great difference in capacitive load between it and the output terminal OUT (up to approximately 800 pF in a 4096-row memory), toward the voltage HV, upon switching the output node OUT from LV to HV (P1 and P3 being both turned on), at a definitely higher rate than the output node, so that the n-p junction formed of the N region and the well of P3 becomes forward biased. Unfortunately, the circuit setups currently investigated to limit this bias effect invariably result in the LV/HV switching time at the output node being increased. The significance of this problem can be appreciated by considering that the program algorithm for a non-volatile memory cell (also referred to as the program-and-verify algorithm) involves the application, to the selected wordline, of a large number of high-voltage pulses, regularly interspersed with an equal number of low-voltage pulses, during which the programmed state of the cell is verified. On the other hand, during HV-to-LV switchings of the node OUT (P1 and P3 being turned off, P2 being turned on), the problem arises that the two transistors P1, P3 cannot be turned off simultaneously. In fact, the value of HV would then remain "stored" dynamically in the intermediate node N, while the output node goes quickly to LV; here again, the n-p junction between the N region and the well of P3 becomes forward biased. Therefore, during the HV-to-LV switching transient of the node OUT, P1 should be turned off first (in order to isolate the node N from HV), and P3 only turned off after a while (i.e., once the voltage at OUT has attained the value of LV). In this way, N and OUT are "equalized" during the transient through the transistor P3, still on. Accordingly, the phase edges of the control signals PH and PH' must have an interval (on the order of a few µs) of non-overlap, thereby introducing unavoidable delay in the overall switching time. Thus, the prior solutions do not suit applications which require very fast (e.g., on the order of 100-200 ns) switching of the node OUT between the voltages HV and LV. The underlying technical problem of this invention is to provide a monolithically integrated selector switch for devices comprising electrically programmable memory cells, which can combine versatility of application with circuit simplicity and operability. This problem is solved by a monolithically integratable voltage-selecting switch as previously indicated and defined in the characterizing parts of the appended claims to this specification. The features and advantages of a voltage-selecting switch according to the invention can be better understood by reading the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings. In the drawings: Figure 1 shows a circuit architecture for storage devices, including a voltage selecting switch for final decoding stages; Figures 2 and 3 show circuit diagrams of conventional voltage-selecting switches suitable for implementation with CMOS technology; Figure 4 shows a timing diagram of a voltage-selecting switch according to the invention; and Figures 5 and 6 show circuit diagrams, partially in block form, of main elements of a voltage-selecting switch according to the invention suitable for implementation in multi-level storage devices. Proposed Solution A voltage selecting switch according to the invention combines the construction and operation simplicity of the two-transistor selector switch with the versatility afforded by multi-transistor structures. In fact, having solved the problem of proper well biasing, a structure including but two transistors creates no problems of intermediate node control, as no such nodes exist therein, and is highly advantageous in terms of switching speed of the selector switch. As shown in Figure 4, a selector switch according to the invention comprises two transistors P1 and P2 which are used as pass transistors (and form the HV/LV selector switch proper), and a WBC (Well Biasing Circuit) capable of each time biasing the transistor wells (connected together at the node BODY) to the higher of the two instant voltages HV and LV. The transistors P1, P2 are made to conduct at non-overlapping phases by control signals PH and /PH. The inventive solution can likewise be applied to two- or multi-level EPROM storage devices. The tables herein below show possible voltage values in different modes of operation: MULTI-LEVEL DEVICE Mode LV HV Read Mode 5.5V 0 to V dd =3V Program Mode 5.5V 6 to 12V DMA Mode 5.5V 0 to 12V TWO-LEVEL DEVICE Mode LV HV Read Mode V dd =3V 0 to V dd =3V Program Mode V dd =3V 12 DMA Mode V dd =3V 0 to 12 The implementation of the proposed solution for multi-level applications involves, in the program mode, biasing the wells to a systematically varying voltage. To ensure proper turn-on and turn-off of the transistors P1 and P2 at each operation phase, the generator of the PH and negated PH phases requires to be supplied each time with the higher of voltages HV and LV and must, therefore, be powered through the node BODY. Actually, an optimum solution would be that of each time sensing a voltage HV'', which is the program voltage and provides HV. The value of the voltage HV'' varies with the mode of operation. In the program mode, it may be V pp =12V, and in the read mode (which is made available to the user of the device), it may lie somewhere between 0V and V dd · Thus, the transistor P1 can be connected to the line of the voltage HV, while the block WBC is coupled directly to the high-voltage terminal HV'' (whereto P1 is actually also connected indirectly). In two-level applications, HV and HV'' would obviously be the same. In the embodiment of the invention shown in Figure 5, the bias circuit block includes a comparator COMP having a non-inverting (+) input terminal, an inverting (-) input terminal, and an output terminal. Third P3 and fourth P4 field-effect transistors have respective control terminals connected to the comparator output terminal, with the control terminal of the third transistor P3 being so connected through an inverter circuit INV. The bias circuit block further includes an input circuit block IN_COMP, whereby the non-inverting input terminal of the comparator is coupled to the first voltage generator HV''. The third P3 and fourth P4 transistors are series connected together through respective first and second terminals, between said input circuit block IN_COMP, also having a supply terminal of the comparator connected thereto, and the second generator of voltage LV, having the inverting (-) input terminal of the comparator connected thereto. These transistors are connected together at the body circuit node BODY. As shown in Figure 6, the input circuit block IN_COMP comprises fifth P5 and sixth P6 field-effect transistors of the complementary type which are connected, in series with each other through respective first and second terminals, between a power supply line V dd and a ground GND of the circuit whereto the selector switch is integrated. They have a respective control terminal connected to a linking circuit node in series between the fifth transistor P5 and the sixth P6. Seventh P7 and eighth P8 field-effect transistors of the complementary type are connected, in series with each other through respective first and second terminals, between the voltage generator HV'' and the ground GND. They have respective control terminals connected to the series linking circuit node, and are connected in series with each other at a circuit node which forms an output node of the input circuit block. This output node of the input circuit block is connected directly to the non-inverting (+) terminal of the comparator, and via a voltage step-up interface ELEVATOR, to the supply terminal of the comparator. The construction of the voltage step-up circuit is well within the capabilities of skilled ones in the art. A resistive element R10 is connected between the first voltage generator HV'' and the third transistor P3. By means of the circuit block IN_COMP connected to its non-inverting input terminal, the comparator COMP can output a logic zero in the presence of a voltage HV''≤V dd (as in the read mode), or otherwise, a value same as HV'' (as in the program mode). This solution ensures that the comparator will be turned off (with attendant advantages in terms of power consumption), since it forces the supply to the comparator to be canceled at all the phases (read and test modes) where its operation is unnecessary. In fact, the voltage for well biasing is always LV, and the selection of this voltage is ensured by the logic zero value presented at the output by the turned off comparator. The time when HV'' reaches and exceeds the value of LV (at which time WBC should theoretically turn off the transistor P5 and turn on P4 in order to connect the well node BODY to the rising one of the voltages) should be preceded by activation of the comparator to enter the program mode operation. In fact, any delay in sensing this transition would result in the wells being forward biased, relative to the source diffusions connected to HV'', and the more so the higher the rate of change of the voltage HV''. WBC unavoidably introduces a delay in input-to-output propagation, and accordingly, the activation of the comparator is to take place substantially in advance of the crossing time of HV'' with respect to LV. This is provided by the input block to the comparator, IN_COMP, outputting a voltage same as HV'' as soon as HV'' overtakes V dd (that is, while it is still below the value of LV if V dd =3V or 5V). While from a logic standpoint, the supply to the comparator (V alcomp ), the non-inverting input of the comparator (V + ), and the high voltage for well biasing (HV loc ) should be one signal during the transition to the program mode, from a physical standpoint it is advisable to have the three voltages clearly separated, as shown in Figure 6. The block IN_COMP has three outputs, each dedicated to generating a different signal: one output will relate to the comparator input signal (V + ), another to the comparator supply (V alcomp ), and the third to the local HV signal (HV loc ) used to bias the wells. In this way, during the initial phase of the transition from the read mode to the program mode (i.e., when HV'' begins to rise but is yet to reach LV), the three signals will be coincident, but become different upon HV'' crossing LV. In particular, the non-inverting input V + of the comparator will keep evolving toward the steady-state value of 12V (following HV''), without undergoing any delay within the range of LV (with substantial advantages in terms of unbalance of the comparator inputs, and hence of comparator switching speed). On the contrary, the local HV loc will be slowed down considerably within the very range of the trigger voltage LV, with substantial advantages in terms of limitation to possible well/diffusion forward biasings. In addition, HV loc will suffer no voltage drop in its steady state from the current draw of the comparator, since this current is supplied from an independent leg (the leg V alcomp ). In summary, the invention has an important advantage in that the switching rate of the selector switch is high (switching time at the node OUT on the order of 100 ns, as against some µs of conventional designs having 3 or 4 transistors). The simple construction of the selector switch itself, as provided by the use of just two pass transistors, avoids the need for intermediate nodes, whose transient control would bring about an unavoidably slower switching rate for the selector switch. For a two- or multi-level non-volatile storage device, the high switching rate results in an important reduction of the programming times, in view of the programming involving continual HV/LV and LV/HV switching of the addressed wordline (by the program-and-verify technique). Changes and modifications may be made unto the invention within the scope of the following claims. For example, a second, highly conductive path may be added in parallel with a resistor R10 which would be activated with delay from the time when LV is crossed. In this way, the transient RC time constant (as set by R10) would be quite unrelated to the impedance (as set by the second path) presented by HV loc in the steady-state condition.

Claims: A selector switch monolithically integrated in a CMOS technology circuit for electrically programmable memory cell devices having at least first and second input terminals for coupling to first (HV) and second (LV) voltage generators, respectively, and an output terminal (OUT), first (P1) and second (P2) field-effect selection transistors being respectively connected, via first and second terminals, between the first input terminal and the output terminal and between the second input terminal and the output terminal, said transistors being driven through control terminals at non-overlapping phases and having body terminals connected at a body circuit node (BODY) coupled to the first and second voltage generators through a bias circuit block (WBC) effective to bias said node to the higher of the instant voltages generated by the first and second generators, characterized in that the bias circuit block comprises a comparator (COMP) having a non-inverting (+) input terminal, an inverting (-) input terminal, and an output terminal, third (P3) and fourth (P4) field-effect transistors having respective control terminals connected to the output terminal of the comparator, with the output terminal of the third transistor (P3) through an inverting circuit (INV), and comprises an input circuit block (IN_COMP) wherethrough the non-inverting input terminal of the comparator is coupled to the first voltage generator (HV,HV''), between said input circuit block (IN_COMP), whereto a supply terminal of the comparator also is connected, and the second voltage generator (LV), whereto the inverting (-) input terminal of the comparator is connected, there being connected, in series with each other through respective first and second terminals, the third (P3) and fourth (P4) transistors, which are connected together at said body circuit node. A monolithically integrated selector switch according to Claim 1, characterized in that the input circuit block comprises fifth (P5) and sixth (P6) field-effect transistors of the complementary type which are connected, in series with each other through respective first and second terminals, between a power supply line (Vdd) and a ground (GND) of the circuit whereto the selector switch is integrated and have each a control terminal connected to a linking circuit node in series between said fifth (P5) and sixth (P6) transistors, and comprises seventh (P7) and eighth (P8) field-effect transistors of the complementary type which are connected, in series with each other through respective first and second terminals, between the first voltage generator (HV") and ground (GND) and have respective control terminals connected to said circuit linking node in series, said seventh and eighth transistors being connected in series with each other at a circuit node forming an output node of the input circuit block. A monolithically integrated selector switch according to Claim 2, characterized in that said output node of the input circuit block is connected directly to the non-inverting (-) terminal of the comparator and, through a voltage step-up interface circuit (ELEVATOR), to the supply terminal of the comparator, and in that it comprises a resistive element (R10) connected between the first voltage generator (HV") and the third transistor (P3). An electrically programmable non-volatile memory cell device, characterized in that it comprises at least one voltage-selecting switch as claimed in any of the preceding claims.

Inventors: MANSTRETTA ALESSANDRO, PIERIN ANDREA, TORELLI GUIDO, MANSTRETTA, ALESSANDRO, PIERIN, ANDREA, TORELLI, GUIDO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: G11C16
IPC codes: G11C16