Abstract:
Non-differential sense amplifier circuitry for reading out Non-Volatile Memories (NVMs) and its operating methods are disclosed. Such non-differential amplifier circuitry requires exceptionally low power and achieves moderate sensing speed, as compared to a conventional sensing scheme.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an integrated circuit for sensing stored information of a non-volatile memory (NVM). In particular, the present invention relates to circuitry and operating method that are applied to sense information stored on NVM devices (e.g., electrically erasable programmable read-only memory (EEPROM), read only memories (ROMs), phase-change memories (PCMs), and magneto-resistive random memories (MRAMs)). 
     2. Discussion of the Related Art 
     In an integrated memory circuit, a readout circuitry detects and determines the content stored in a selected NVM cell. In many NVM cells, the stored information is represented by one of several possible values of its electrical conductance characteristics. The selected values are kept even after the NVM device&#39;s power is withdrawn or cut off. For instances, an EEPROM cell can represent its stored information by adopting in a metal-oxide-semiconductor field effect transistor (MOSFET) one of several threshold voltages. The selected threshold voltage is achieved by storing a known amount of charge in between the MOSFET&#39;s control gate and its channel. A ROM cell can represent a binary stored value by having a connection or not having a connection between a MOSFET and a bit line. A PCM represents a binary stored value by being in either a high electrical conductance state or a low electrical conductance state, according to whether a silicon layer in the device is in an amorphous phase or a polycrystalline phase. Basically, the stored information in an NVM cell can be determined by measuring an electrical conductance in the NVM cell, and by requiring that the conductance characteristics remain even after power is turned off. 
     Therefore, one way to read out the stored data in a selected NVM cell is to apply a bias voltage to the NVM cell and measure the resulting current. In a conventional readout scheme, e.g., using readout circuit  100  of  FIG. 1 , cell current from NVM cell  102  responsive to the input bias voltage on word line  101  is pre-amplified at current amplifier  103  and compared with a reference current generated by reference current generation circuit  104 . The two currents may be compared using differential amplifier comparator  105 . The output value of comparator  105  represents either a high NVM electrical conductance, corresponding to detecting a large resulting current, and a low NVM electrical conductance, corresponding to detecting a low resulting current. In this readout process, large steady output currents are supplied from the memory cells (e.g., NVM memory cell  102 ) being in a high electrical conductance state, pre-amplifier  103 , differential amplifier comparator  105 , and reference current circuitry  104 . Large steady currents lead to a high power requirement for reading out the stored information from the NVM cell. 
     SUMMARY 
     According to one embodiment of the present invention, readout circuitry and operating methods thereof eliminate the requirement for a large steady current during both sensing and standby modes of operation, thus achieving low power consumption during reading out of NVM data. In one embodiment, readout circuitry of the present invention does not require a reference current or voltage. Thus, circuit complexity inherent in generating a reference current is avoided. 
     According to another embodiment of the present invention, a non-differential type readout circuitry avoids the offset caused by device mismatch. 
     The present invention is better understood upon consideration of the detailed description below, in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention and to show how it may be carried into effect, reference is now made to the following drawings, which show the preferred embodiments of the present invention, in which: 
         FIG. 1  shows a prior art NVM cell readout circuit  100 . 
         FIG. 2  shows readout circuit  200 , in accordance with one embodiment of the present invention. 
         FIG. 3  illustrates the operations of readout circuitry  200  of  FIG. 2 , in accordance with one embodiment of the present invention. 
         FIG. 4  shows a circuit schematic for a NOR-type flash EEPROM array  400 , in accordance with one embodiment of the present invention. 
         FIG. 5  show simulation results that illustrate the operations of readout circuitry  420  included in the NOR-type flash EEPROM array  400  of  FIG. 4 , in accordance with one embodiment of the present invention. 
         FIG. 6  shows circuit schematic for a NAND-type flash EEPROM array  600 , in accordance with one embodiment of the present invention. 
         FIG. 7  shows circuit schematic for a NOR-type ROM array  700 , in accordance with one embodiment of the present invention. 
         FIG. 8  shows circuit schematic for a phase change memory (PCM) array  700 , in accordance with one embodiment of the present invention. 
         FIG. 9  shows readout circuit  900 , in accordance with a second embodiment of the present invention. 
         FIG. 10  illustrates the operations of readout circuitry  900  of  FIG. 9 , in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 2 and 3  show, respectively, readout circuit  200  and its operations, in accordance with one embodiment of the present invention. As shown in  FIG. 2 , the source electrode and the drain electrode of P-type MOSFET MP 1  are connected to read voltage bias V R  and to bit line  201  connected to NVM cell  202 , which stored information is to be read. When readout circuit  200  is in a standby mode (i.e., between times t 1  and t 2  in  FIG. 3 ), signal “Sense Enable” (i.e., waveform  302  of  FIG. 3 ), which is applied to the gate electrode of MP 1 , is at zero volts, so that bit line  201  is pulled (“pre-charged”) to read voltage bias V R . In standby mode, the NVM cells connected to bit line  201  are not selected (i.e., word line  205  is at a “low” state; see, waveform  303  of  FIG. 3 ), and thus are not activated. Consequently, the electrical conductance of NVM cell  202  is low. No active steady current path exists for a current to flow from read voltage bias V R  to the ground reference through an NVM cell attached to bit line  201 . The voltage V R  on bit line  201  turns on NMOS transistor MN 3 , which pulls node  204  (signal  D , waveform  307 ) to the ground reference, thus rendering PMOS transistor MP 4  conducting, pulling node  203  (signal “D,” waveform  306 ) to core “high” voltage V CC . As a result, complementary data output terminals  203  and  204  (signals “D” and “  D ” of read circuit  200 , respectively) are set to “high” and “low” states (“default states”). 
     In reading mode (i.e., between times t 0  and t 1  of  FIG. 3 ), the voltage at the gate terminal of PMOS transistor MP 1  (i.e., signal “Sense Enable,” waveform  302  of  FIG. 3 ) is brought to a “high” level (e.g., core high voltage V CC ), thus turning off PMOS transistor MP 1 . When word line  205  is selected (waveform  203  of  FIG. 3 ), the gate electrodes of a “row” of NVM cells (e.g., NVM cell  202 ) are activated, so that their respective electrical conductance values may be probed. The NVM cells with high conductance begin to discharge their corresponding bit lines (representing the stored values in the high conductance NVM cells) from voltage V R  to the ground reference. Discharging bit line  201  leads to a voltage drop at the gate terminals of PMOS transistor MP 3  and NMOS transistor MN 1 . The voltage at node  206  begins to rise which reduces the current in PMOS transistor MP 2  and increases the rate of discharge at bit line  201  (i.e., a positive feedback action). With the positive feedback action of PMOS transistor MP 3  and NMOS transistor MN 1 , PMOS transistor MP 2  is rapidly turned off, thus isolating bit line  201  from read voltage bias V R . Bit line  201  continues to discharge through NVM cell  202  until its voltage reaches ground reference (and node  206  reaches read voltage bias V R ). Conducting NMOS transistor MN 2  turns on PMOS transistor MP 5 . At the same time, NMOS transistor MN 3 , which is shut off, turns off PMOS transistor MP 4 . Thus, PMOS transistors MP 4  and MP 5 , and NMOS transistors MN 2 , and MN 3  operate to convert the voltage on bit line  201  (which switches from a “high” voltage state to a “low” voltage state) to “low” and “high” voltage states on data output terminals  203  and  204  (i.e., signals “D” and “  D ” of read circuit  200 , respectively), from their default “high” and “low” voltage states. See, waveforms  304  and  305  of  FIG. 3 . No steady current flows after read circuitry  200  reaches the steady state. The only currents generated during the read process are the transient currents from discharging bit lines and MOSFET switching in the circuitry. As mentioned above, in this detailed description, the “high” voltage state corresponds to the core voltage V CC  level and the “low” voltage corresponds to the ground reference. 
     During the reading mode, for a selected NVM cell that has a low electrical conductance, the voltage on the associated bit line remains at V R , as only very small leakage current flows through the selected NVM cell (e.g., bit line  201 ). The small leakage current is unable to discharge bit line  201  and affects the default states of data output terminals  203  and  204  (i.e., signals D and  D  are at “high” and “low” states, respectively). 
     At time t 1 , the voltage at the gate terminal of PMOS transistor MP 1  is set to “low” state (i.e., 0 volt) and PMOS transistor MP 1  begins to recharge the bit lines (e.g., bit line  201 ). The data output terminals  203  and  204  of read circuit  200  return to the default states (i.e., “high” and “low” voltage states for signals D and  D , respectively; see waveforms  304 ,  305 ,  306  and  307 ). Recharged, read circuit  200  is ready for the next read operation. 
     Thus, the present invention provides a read circuit that achieves low-power reading out of an NVM cell. 
       FIG. 4  shows circuit schematic for a NOR-type flash EEPROM array  400 , in accordance with one embodiment of the present invention.  FIG. 5  show simulation results that illustrate the operations of readout circuitry  420  included in the NOR-type flash EEPROM array  400  of  FIG. 4 , in accordance with one embodiment of the present invention. As shown in  FIG. 4 , flash EEPROM array  400  includes an array of flash EEPROM cells, selected by word lines, each word line activating a row of the flash EEPROM cells at a time. Each activated EEPROM cell provides its content on one of local bit lines  402 - 1  to  402 -N. Switch structure  401  connects one of the local bit lines  401 - 1  to  401 -N to a global bit line  403 , which is read by readout circuit  420 . Readout circuit  420  operates in substantially the same manner as read out circuit  200  discussed above. In flash EEPROM array  400 , a high conductance EEPROM cell has a low threshold voltage, and a low conductance EEPROM cell has a high threshold voltage. A suitable process for fabricating flash EEPROM array  400  is a 0.13 μm process. As shown in  FIG. 5 , during the read out period, output data signals D and  D  settles at its final signal values for those EEPROM cells with low threshold voltage (i.e., high electrical conductance) within several nanoseconds (see waveforms  501  in  FIG. 5 ). 
       FIG. 6  shows circuit schematic for a NAND-type flash EEPROM array  600 , in accordance with one embodiment of the present invention. As shown in  FIG. 6 , flash EEPROM array  600  includes an array of EEPROM cells, selected by word lines, each word line activating a row of EEPROM cells at a time. The EEPROM cells are also organized in columns as memory strings, with each memory string including a number of serially connected EEPROM cells served by one of local bit lines  602 - 1  to  602 -N. Each selected EEPROM cell controls the discharge of the corresponding one of local bit lines  602 - 1  to  602 -N to ground through NVM cells in its memory string. The unselected EEPROM cells are each biased to a high voltage to pass the bit line voltage and the ground voltage, respectively, to the drain terminal and the source terminal of the selected EEPROM cell. Therefore, if the selected EEPROM is high conductance (i.e., low threshold voltage), that EEPROM cell discharges its associated bit line. Conversely, if the selected EEPROM is low conductance (i.e., high threshold voltage), that EEPROM cell does not discharge its associated bit line. Switch structure  601  connects one of the local bit lines  601 - 1  to  601 -N to a global bit line  603 , which is read by readout circuit  620 . Readout circuit  620  operates in substantially the same manner as read out circuit  200  discussed above. 
       FIG. 7  shows circuit schematic for a NOR-type ROM array  700 , in accordance with one embodiment of the present invention. As shown in  FIG. 7 , ROM array  700  includes an array of ROM cells, selected by word lines, each word line addressing a row of the ROM cells at a time. The state of each addressed ROM cell can be read at a corresponding one of local bit lines  702 - 1  to  702 -N. Each ROM cell consists of a metal-semiconductor-oxide field effect transistor (MOSFET), which is either connected to the corresponding bit line or not connected to the corresponding bit line. Alternatively, connection of the MOSFET to the corresponding bit line may be controlled by a fuse which either connects the MOSFET to or keeps the MOSFET disconnected from the associated bit line. When the MOSFET is connected to its corresponding bit line, the voltage on its gate electrode renders the MOSFET conducting, thereby providing a discharge path from the bit line to the ground reference. Conversely, when the MOSFET is not connected to the bit line (i.e., a very high impedance path), the voltage V R  on the bit line is not discharged. Switch structure  401  connects one of the local bit lines  701 - 1  to  701 -N to a global bit line  703 , which is read by readout circuit  720 . Readout circuit  720  operates in substantially the same manner as read out circuit  200  discussed above. 
       FIG. 8  shows circuit schematic for a phase change memory (PCM) array  800 , in accordance with one embodiment of the present invention. As shown in  FIG. 8 , PCM array  800  includes an array of PCM cells, selected by word lines, each word line addressing a row of the PCM cells at a time. The state of each addressed PCM cell can be read at a corresponding one of local bit lines  802 - 1  to  802 -N. In a PCM cell as shown in  FIG. 8 , a phase change material such as Be 2 Sb 2 Te 5  (BST) is connected to a bit line through an access MOSFET. During a data write step, an amorphous phase of the phase change material (i.e., the high resistance or low conductance state) is created by heating up the phase change material by passing a high electrical current, followed by rapid cooling. A polycrystalline phase (i.e., low resistance state or high conductance state) is created by a mild electrical current heating, followed by a slow cooling step. When the word line addresses a MOSFET, the MOSFET connects the bit line to the phase change material. Depending on the programmed state of the phase change material, the voltage V R  on the bit line may discharge or may remain at voltage bias V R . Switch structure  801  connects one of the local bit lines  801 - 1  to  801 -N to a global bit line  803 , which is read by readout circuit  820 . Readout circuit  820  operates in substantially the same manner as read out circuit  200  discussed above. 
       FIG. 9  shows readout circuit  900 , in accordance with a second embodiment of the present invention.  FIG. 10  illustrates the operations of readout circuitry  900  of  FIG. 9 , in accordance with one embodiment of the present invention. In readout circuit  900  of  FIG. 9 , unlike in readout circuit  200 , where “Sensing Enable” signal is asserted at the gate electrode of P-type MOSFET MP 1  to initiate a read operation, the asserted “Sensing Enable” is asserted at the gate electrode of N-type MOSFET MN 1  to initiate the read operation. As shown in  FIG. 10 , the “Sensing Enable” signal is asserted by pulling the gate electrode of MOSFET MN 1  to a “low” voltage (waveform  1002  for  FIG. 10 ). Otherwise, at read out and standby operations, readout circuit  900  operates in substantially the same manner as read out circuit  200 . 
     The above detailed description is provided to illustrate the specific embodiments of the present invention and is not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is set forth in the accompanying claims.