Wordline Driver Circuit for EEPROM Memory Cell

This invention relates to circuitry for applying reading, programming and erasing voltages to wordlines of floating-gate-type, electrically-erasable-programmable-memory (EEPROM) arrays and to the control gates of the individual memory cells attached to those wordlines.

This application discloses and claims circuitry that is an improvement over the wordline driver circuitry described in U.S. Pat. No. 4,823,318, issued Apr. 18, 1989 and also assigned to Texas Instruments Incorporated.

EEPROM arrays of the type used with the circuitry of this application are described, for example, in co-pending U.S. Pat. applications No. 07/274,718 (a continuation of U.S. Pat. application No. 07/056,196, now abandoned); No 07219,528; No. 07/219,529 and No. 07/219,530; each of those applications being also assigned to Texas Instruments Incorporated. EEPROM arrays of the type described in the foregoing applications require circuitry that will switch as many as four different values of voltage to each wordline for the purposes of reading, programming and erasing information stored on the floating gates. One of those voltages, the erasing voltage, must be negative with respect to the bitlines or source-drain regions.

During read operation of EEPROM cells of the floating-gate type, a voltage Vsv of perhaps +3 volts above reference potential is applied to the wordline and control gate of the selected cell, with the source connected to reference potential Vss, which may be the circuit ground, and with the drain connected to a read voltage of perhaps +1.5 volts above reference potential. Non-selected wordlines are usually tied to reference potential Vss.

During programming operation of such EEPROM cells, a voltage Vpp of perhaps +12 to +16 volts above reference potential is applied to the wordline and control gate of the selected cell, with the source of the selected cell at reference potential Vss and with the drain of the selected cell either allowed to float or tied to a low voltage or reference potential Vss. The programming voltage Vpp is typically applied for a period of 10 milliseconds and produces a shift in voltage threshold of approximately 4.5 volts or more. Non-selected wordlines may be connected to reference potential Vss or, to prevent disturb on non-selected cells, the non-selected wordlines may be connected to a voltage of perhaps +6 to +8 volts above reference potential Vss.

During erasing operation of such EEPROM cells, a voltage Ver of perhaps -10 to -12 volts below reference potential Vss is applied to the wordline and control gate of the selected cell or, in the cases of a so-called "flash-erase" type of EEPROMs, to all of the wordlines and control gates with the sources of the erased cells being at perhaps +4 to +6 volts above reference potential Vss and with the drains allowed to float or tied to a low voltage or reference potential Vss. The erasing voltages are typically applied for a period of 10 milliseconds and produce a voltage threshold of approximately one volt. The non-selected wordlines are usually tied to reference potential Vss during erasure.

The various EEPROM wordline voltages may be generated from the approximately +5 volt external supply voltage Vdd using charge-pump capacitors located on the memory chip. Circuits for switching from one voltage to a second voltage are well-known. For example, such circuits are used to change the bitline voltages from one value to another value when changing from the programming mode of operation to the erasing mode of operation. Such circuits for switching between a positive read voltage, a positive programming voltage, and a reference voltage are known in the prior art pertaining to electrically-programmable-read-only-memories (EPROMs). However, in the case of EEPROMs, there is a need for improved circuits that will not only switch wordline reading and programming voltages, but which will also switch negative erasing voltages to the selected wordline. Switching of negative voltages presents a unique problem in that such circuits must be designed to prevent P-N junctions between the diffused areas and the substrate of such integrated circuits from becoming forward biased during application of negative erase voltages.

In addition, there is a need for circuitry that will supply a third value of positive voltage to non-selected wordlines during programming operation and to thereby lessen the probability that non-selected memory cells will be disturbed.

The circuit of this invention includes a circuit means for switching positive voltages to a wordline, a first isolating transistor connected between the output of the positive-voltage-switching means and the wordline, and a second isolating transistor connected between an erasing voltage source and the wordline. The isolating transistors may be P-channel transistors with substrates or tanks connected, for example, to the programming voltage source during programming and to the read voltage source during reading and erasing.

In one embodiment, the positive-voltage-switching means provides three values of positive voltage as well as the reference voltage to the wordline. The three values of positive voltage are used for programming, for reading, and for disturb protection and all three values may be different from the supply voltage value. The positive-voltage-switching means includes a two-transistor inverter with a separate feedback transistor and includes a third isolating transistor.

The novel features of this invention are set forth in the appended claims. The invention, its features, and its advantages are described below in conjunction with the following drawing:

FIG. 1 is a circuit diagram of the improved circuit of this invention.

Referring to FIG. 1, positive-voltage-switching circuit 10 is illustrated as having four input voltages. Voltage input Vdd may be a voltage generated externally from the chip and may be in the range of +4 to +6 volts above reference voltage Vss, which may be ground potential. Voltage input Vpp is a programming voltage input that may be, for example, in the range of +12 to +16 volts above reference potential Vss. Sense voltage input Vsv is a read voltage input that may be, for example, approximately +3 volts above reference potential Vss. Optional disturb-protect voltage input Vdp may be, for example, in the range of +6 to +8 volts above reference potential Vss. Voltage inputs Vpp, Vsv, and Vdp may be derived on the chip from external voltage source Vdd through use of capacitive charge-pump circuits, which are known in the prior art.

Input voltages Vpp and Vsv with switch SW1 comprise a programing/sense voltage source. Input voltages Vdd with Vss and switch SW2 comprise a supply/reference voltage source. Input voltage Vdp with Vss and SW3 comprise a disturb-protect/reference voltage source.

Positive-voltage-switching circuit 10 supplies the positive programming and reading voltages Vpp and Vsv as well as reference potential Vss to terminal WL, and may supply a positive disturb-protect voltage Vdp to terminal WL, which is connected to a wordline of a floating-gate memory array.

Terminal Ver/HI is connected to a pulsed negative erase voltage source. The pulses may, for example, be in the range of -12 to -14 volts below reference potential Vss and may be perhaps 10 milliseconds in length. The source Ver/HI is characterized by having a high impedance during times when the pulse voltage is not active. The negative erasing voltage supply Ver may also be generated on the chip from the external power supply Vdd through use of a capacitor-type charge pump.

First isolating transistor T1 is connected with source-drain path between the output terminal A of positive-voltage-switching circuit 10 and the wordline terminal WL. The gate of P-channel first isolating transistor T1 is connected by electronic switch SW4 to a negative signalling voltage source Vcp during reading operations, to read voltage source Vsv during programming operations in which positive voltage pulses are applied to a selected wordline WL, and to supply voltage source Vdd during erasing operations in which negative voltage pulses are applied to all wordlines WL or to selected wordlines WL. Signalling voltage source Vcp is typically -3 volts to -5 volts below reference voltage. The substrate or tank of first isolating transistor T1 is connected, for example, by electronic switch SW1 to sense voltage Vsv during reading or erasing and to programming voltage source Vpp during programming operation. As is well-known, the tank or substrate voltage must be equal to or higher than the voltage on wordline WL to prevent forward biasing of the junctions between source or drain and the tank or substrate. Input voltages Vcp, Vsv and Vdd, together with switch SW4, comprise a supply/sense/signalling voltage source.

Second isolating transistor T2 is connected with source-drain path between erasing voltage source Ver/HI and wordline terminal WL. The gate of P-channel second isolating transistor T2 is connected to the drain of transistor T2, which is connected to supply source Ver/HI. The tank or substrate of second isolating transistor T2 is also connected, for example, by electronic switch SW1 to sense voltage Vsv during reading and erasing operations and to programming voltage source Vpp during programming operation to prevent forward-biasing of junctions.

Positive-voltage-switching circuit 10 may include an inverter with P-channel transistor T4 and N-channel transistor T5 having source-drain paths connected in series between electronic switches SW1 and SW3 and with gates connected to a source-drain terminal of optional third isolating N-channel transistor T3. The commonly connected source-drain terminals of transistors T4 and T5 are connected to output terminal A of positive-voltage-switching circuit 10. The inverter may include a P-channel feedback transistor T6 with source-drain path connected between switch SW1 and the same source-drain terminal of third isolating transistor T3 and with gate connected to the output terminal A of positive-voltage-switching circuit 10. The tanks or substrates of P-channel transistors T4 and T6 are connected to switch SW1. The other source-drain terminal of optional third isolating transistor T3 is connected by electronic switch SW2 to supply voltage Vdd or to reference voltage Vss, depending upon whether the wordline WL is selected or deselected. Optional third isolating transistor T3 protects circuitry attached to switch SW2 from possible harm caused by programming pulses from source Vpp. The T5 terminal of the inverter is connected by optional switch SW3 to disturb-protect voltage supply Vdp during programming operation and to reference voltage Vss during reading and erasing operations.

During programming, or writing, mode of operation of a particular driver circuit that is connected to a selected wordline, the voltage applied by switch SW2 to the inverter is low and, therefore, transistor T5 is non-conductive and transistor T4 is conductive. Transistor T2 is nonconductive because of the characteristic high impedance of erasing voltage source Ver/HI. Because switch SW1 is in the Vpp position and switch SW4 is in the Vsv position, the voltage at the terminal WL will be equal to Vpp. For a particular driver circuit that is connected to a non-selected wordline, the voltage applied by switch SW4 is Vsv (+3 volts for example) and the voltage applied by switch SW2 is Vdd (+5 volts, for example). Transistor T4 will be nonconductive, transistors T1 and T5 will be conductive and the voltage at the terminal WL will be disturb-protect voltage Vdp.

During erasing mode of operation of a particular driver circuit, the voltage Vcp applied by switch SW4 is high (+5 volts, for example) and therefore transistor T1 is nonconductive. Transistor T2 is connected in diode configuration and conducts during the negative erasing pulse applied by source Ver/HI. For non-flash EEPROM applications, the driver circuits connected to non-selected wordlines may be activated such that transistors T1 and T5 connect those wordlines to reference potential.

During reading mode of operation of a driver circuit attached to a selected wordline, the voltage applied by switch SW2 to the inverter is low (0 volts, for example) and, therefore, transistors T4 and T5 are as described for programming operation. Because the voltage applied by switch SW1 is equal to Vsv rather than Vpp, the voltage at the terminal WL will be equal to Vsv. For a driver circuit attached to a non-selected wordline, the voltage applied by switch SW2 to the inverter is high, causing transistor T5 to conduct and, with switch S3 connected to reference potential, connects terminal WL to that reference potential.

While this invention has been described with respect to an illustrative embodiment, this description is not intended to be construed in a limiting sense. Upon reference to this description, various modifications of the illustrative embodiment, as well as other embodiments of the invention, will be apparent to persons skilled in the art. It is contemplated that the appended claims will cover any such modifications or embodiments that fall within the scope of the invention.