Patent ID: 12243587

One of the major challenges in the development of artificial neural networks for high-performance information processing is a lack of adequate hardware technology. Indeed, practical neural networks rely on a very large number of synapses, enabling high connectivity between neurons, i.e., a very high computational parallelism. In principle, such complexity can be achieved with digital supercomputers or specialized graphics processing unit clusters. However, in addition to high cost, these approaches also suffer from mediocre energy efficiency as compared to biological networks, which consume much less energy primarily because they perform low-precision analog computation. CMOS analog circuits have been used for artificial neural networks, but most CMOS-implemented synapses have been too bulky given the high number of neurons and synapses.

Applicant previously disclosed an artificial (analog) neural network that utilizes one or more non-volatile memory arrays as the synapses in U.S. Patent Application Publication 2017/0337466A1, which is incorporated by reference. The non-volatile memory arrays operate as an analog neural memory and comprise non-volatile memory cells arranged in rows and columns. The neural network includes a first plurality of synapses configured to receive a first plurality of inputs and to generate therefrom a first plurality of outputs, and a first plurality of neurons configured to receive the first plurality of outputs. The first plurality of synapses includes a plurality of memory cells, wherein each of the memory cells includes spaced apart source and drain regions formed in a semiconductor substrate with a channel region extending there between, a floating gate disposed over and insulated from a first portion of the channel region and a non-floating gate disposed over and insulated from a second portion of the channel region. Each of the plurality of memory cells store a weight value corresponding to a number of electrons on the floating gate. The plurality of memory cells multiply the first plurality of inputs by the stored weight values to generate the first plurality of outputs.

Non-Volatile Memory Cells

Non-volatile memories are well known. For example, U.S. Pat. No. 5,029,130 (“the '130 patent”), which is incorporated herein by reference, discloses an array of split gate non-volatile memory cells, which are a type of flash memory cells. Such a memory cell210is shown inFIG.2. Each memory cell210includes source region14and drain region16formed in semiconductor substrate12, with channel region18there between. Floating gate20is formed over and insulated from (and controls the conductivity of) a first portion of the channel region18, and over a portion of the source region14. Word line terminal22(which is typically coupled to a word line) has a first portion that is disposed over and insulated from (and controls the conductivity of) a second portion of the channel region18, and a second portion that extends up and over the floating gate20. The floating gate20and word line terminal22are insulated from the substrate12by a gate oxide. Bitline24is coupled to drain region16.

Memory cell210is erased (where electrons are removed from the floating gate) by placing a high positive voltage on the word line terminal22, which causes electrons on the floating gate20to tunnel through the intermediate insulation from the floating gate20to the word line terminal22via Fowler-Nordheim (FN) tunneling.

Memory cell210is programmed by source side injection (SSI) with hot electrons (where electrons are placed on the floating gate) by placing a positive voltage on the word line terminal22, and a positive voltage on the source region14. Electron current will flow from the drain region16towards the source region14. The electrons will accelerate and become heated when they reach the gap between the word line terminal22and the floating gate20. Some of the heated electrons will be injected through the gate oxide onto the floating gate20due to the attractive electrostatic force from the floating gate20.

Memory cell210is read by placing positive read voltages on the drain region16and word line terminal22(which turns on the portion of the channel region18under the word line terminal). If the floating gate20is positively charged (i.e., erased of electrons), then the portion of the channel region18under the floating gate20is turned on as well, and current will flow across the channel region18, which is sensed as the erased or “1” state. If the floating gate20is negatively charged (i.e., programmed with electrons), then the portion of the channel region under the floating gate20is mostly or entirely turned off, and current will not flow (or there will be little flow) across the channel region18, which is sensed as the programmed or “0” state.

Table No. 1 depicts typical voltage and current ranges that can be applied to the terminals of memory cell210for performing read, erase, and program operations:

TABLE NO. 1Operation of Flash Memory Cell 210 of FIG. 2WLBLSLRead2-3V0.6-2V0VErase~11-13V0V0VProgram1-2V10.5-3μA9-10V

Other split gate memory cell configurations, which are other types of flash memory cells, are known. For example,FIG.3depicts a four-gate memory cell310comprising source region14, drain region16, floating gate20over a first portion of channel region18, a select gate22(typically coupled to a word line, WL) over a second portion of the channel region18, a control gate28over the floating gate20, and an erase gate30over the source region14. This configuration is described in U.S. Pat. No. 6,747,310, which is incorporated herein by reference for all purposes. Here, all gates are non-floating gates except floating gate20, meaning that they are electrically connected or connectable to a voltage source. Programming is performed by heated electrons from the channel region18injecting themselves onto the floating gate20. Erasing is performed by electrons tunneling from the floating gate20to the erase gate30.

Table No. 2 depicts typical voltage and current ranges that can be applied to the terminals of memory cell310for performing read, erase, and program operations:

TABLE NO. 2Operation of Flash Memory Cell 310 of FIG. 3WL/SGBLCGEGSLRead1.0-2V0.6-2V0-2.6 V0-2.6V0VErase−0.5 V/0 V0V0 V/−8 V8-12V0VProgram1V0.1-1μA8-11 V4.5-9V4.5-5V

FIG.4depicts a three-gate memory cell410, which is another type of flash memory cell. Memory cell410is identical to the memory cell310ofFIG.3except that memory cell410does not have a separate control gate. The erase operation (whereby erasing occurs through use of the erase gate) and read operation are similar to that of theFIG.3except there is no control gate bias applied. The programming operation also is done without the control gate bias, and as a result, a higher voltage is applied on the source line during a program operation to compensate for a lack of control gate bias.

Table No. 3 depicts typical voltage and current ranges that can be applied to the terminals of memory cell410for performing read, erase, and program operations:

TABLE NO. 3Operation of Flash Memory Cell 410 of FIG. 4WL/SGBLEGSLRead0.7-2.2V0.6-2V0-2.6V0 VErase−0.5 V/0 V0V11.5V0 VProgram1V0.2-3μA4.5V7-9 V

FIG.5depicts stacked gate memory cell510, which is another type of flash memory cell. Memory cell510is similar to memory cell210ofFIG.2, except that floating gate20extends over the entire channel region18, and control gate22(which here will be coupled to a word line) extends over floating gate20, separated by an insulating layer (not shown). The erase is done by FN tunneling of electrons from FG to substrate, programming is by channel hot electron (CHE) injection at region between the channel18and the drain region16, by the electrons flowing from the source region14towards to drain region16and read operation which is similar to that for memory cell210with a higher control gate voltage.

Table No. 4 depicts typical voltage ranges that can be applied to the terminals of memory cell510and substrate12for performing read, erase, and program operations:

TABLE NO. 4Operation of Flash Memory Cell 510 of FIG. 5CGBLSLSubstrateRead2-5 V0.6-2 V0 V0 VErase−8 to −10 V/0 VFLTFLT8-10 V/15-20 VProgram8-12 V3-5 V0 V0 V

The methods and means described herein may apply to other non-volatile memory technologies such as FINFET split gate flash or stack gate flash memory, NAND flash, SONOS (silicon-oxide-nitride-oxide-silicon, charge trap in nitride), MONOS (metal-oxide-nitride-oxide-silicon, metal charge trap in nitride), ReRAM (resistive ram), PCM (phase change memory), MRAM (magnetic ram), FeRAM (ferroelectric ram), CT (charge trap) memory, CN (carbon-tube) memory, OTP (bi-level or multi-level one time programmable), and CeRAM (correlated electron ram), without limitation.

In order to utilize the memory arrays comprising one of the types of non-volatile memory cells described above in an artificial neural network, two modifications are made. First, the lines are configured so that each memory cell can be individually programmed, erased, and read without adversely affecting the memory state of other memory cells in the array, as further explained below. Second, continuous (analog) programming of the memory cells is provided.

Specifically, the memory state (i.e., charge on the floating gate) of each memory cell in the array can be continuously changed from a fully erased state to a fully programmed state, and vice-versa, independently and with minimal disturbance of other memory cells. This means the cell storage is effectively analog or at the very least can store one of many discrete values (such as 16 or 64 different values), which allows for very precise and individual tuning of all the memory cells in the memory array, and which makes the memory array ideal for storing and making fine tuning adjustments to the synapsis weights of the neural network.

Neural Networks Employing Non-Volatile Memory Cell Arrays

FIG.6conceptually illustrates a non-limiting example of a neural network utilizing a non-volatile memory array of the present examples. This example uses the non-volatile memory array neural network for a facial recognition application, but any other appropriate application could be implemented using a non-volatile memory array based neural network.

S0 is the input layer, which for this example is a 32×32 pixel RGB image with 5 bit precision (i.e. three 32×32 pixel arrays, one for each color R, G and B, each pixel being 5 bit precision). The synapses CB1 going from input layer S0 to layer C1 apply different sets of weights in some instances and shared weights in other instances and scan the input image with 3×3 pixel overlapping filters (kernel), shifting the filter by 1 pixel (or more than 1 pixel as dictated by the model). Specifically, values for 9 pixels in a 3×3 portion of the image (i.e., referred to as a filter or kernel) are provided to the synapses CB1, where these 9 input values are multiplied by the appropriate weights and, after summing the outputs of that multiplication, a single output value is determined and provided by a first synapse of CB1 for generating a pixel of one of the feature maps of layer C1. The 3×3 filter is then shifted one pixel to the right within input layer S0 (i.e., adding the column of three pixels on the right, and dropping the column of three pixels on the left), whereby the 9 pixel values in this newly positioned filter are provided to the synapses CB1, where they are multiplied by the same weights and a second single output value is determined by the associated synapse. This process is continued until the 3×3 filter scans across the entire 32×32 pixel image of input layer S0, for all three colors and for all bits (precision values). The process is then repeated using different sets of weights to generate a different feature map of layer C1, until all the features maps of layer C1 have been calculated.

In layer C1, in the present example, there are 16 feature maps, with 30×30 pixels each. Each pixel is a new feature pixel extracted from multiplying the inputs and kernel, and therefore each feature map is a two dimensional array, and thus in this example layer C1 constitutes 16 layers of two dimensional arrays (keeping in mind that the layers and arrays referenced herein are logical relationships, not necessarily physical relationships—i.e., the arrays are not necessarily oriented in physical two dimensional arrays). Each of the 16 feature maps in layer C1 is generated by one of sixteen different sets of synapse weights applied to the filter scans. The C1 feature maps could all be directed to different aspects of the same image feature, such as boundary identification. For example, the first map (generated using a first weight set, shared for all scans used to generate this first map) could identify circular edges, the second map (generated using a second weight set different from the first weight set) could identify rectangular edges, or the aspect ratio of certain features, and so on.

An activation function P1 (pooling) is applied before going from layer C1 to layer S1, which pools values from consecutive, non-overlapping 2×2 regions in each feature map. The purpose of the pooling function P1 is to average out the nearby location (or a max function can also be used), to reduce the dependence of the edge location for example and to reduce the data size before going to the next stage. At layer S1, there are 16 15×15 feature maps (i.e., sixteen different arrays of 15×15 pixels each). The synapses CB2 going from layer S1 to layer C2 scan maps in layer S1 with 4×4 filters, with a filter shift of 1 pixel. At layer C2, there are 22 12×12 feature maps. An activation function P2 (pooling) is applied before going from layer C2 to layer S2, which pools values from consecutive non-overlapping 2×2 regions in each feature map. At layer S2, there are 22 6×6 feature maps. An activation function (pooling) is applied at the synapses CB3 going from layer S2 to layer C3, where every neuron in layer C3 connects to every map in layer S2 via a respective synapse of CB3. At layer C3, there are 64 neurons. The synapses CB4 going from layer C3 to the output layer S3 fully connects C3 to S3, i.e. every neuron in layer C3 is connected to every neuron in layer S3. The output at S3 includes 10 neurons, where the highest output neuron determines the class. This output could, for example, be indicative of an identification or classification of the contents of the original image.

Each layer of synapses is implemented using an array, or a portion of an array, of non-volatile memory cells.

FIG.7is a block diagram of an array that can be used for that purpose. Vector-by-matrix multiplication (VMM) array32includes non-volatile memory cells and is utilized as the synapses (such as CB1, CB2, CB3, and CB4 inFIG.6) between one layer and the next layer. Specifically, VMM array32includes an array of non-volatile memory cells33, erase gate and word line gate decoder34, control gate decoder35, bit line decoder36and source line decoder37, which decode the respective inputs for the non-volatile memory cell array33. Input to VMM array32can be from the erase gate and wordline gate decoder34or from the control gate decoder35. Source line decoder37in this example also decodes the output of the non-volatile memory cell array33. Alternatively, bit line decoder36can decode the output of the non-volatile memory cell array33.

Non-volatile memory cell array33serves two purposes. First, it stores the weights that will be used by the VMM array32. Second, the non-volatile memory cell array33effectively multiplies the inputs by the weights stored in the non-volatile memory cell array33and adds them up per output line (source line or bit line) to produce the output, which will be the input to the next layer or input to the final layer. By performing the multiplication and addition function, the non-volatile memory cell array33negates the need for separate multiplication and addition logic circuits and is also power efficient due to its in-situ memory computation.

The output of non-volatile memory cell array33is supplied to a differential summer (such as a summing op-amp or a summing current mirror)38, which sums up the outputs of the non-volatile memory cell array33to create a single value for that convolution. The differential summer38is arranged to perform summation of positive weight and negative weight.

The summed-up output values of differential summer38are then supplied to an activation function block39, which rectifies the output. The activation function block39may provide sigmoid, tanh, or ReLU functions. The rectified output values of activation function block39become an element of a feature map as the next layer (e.g. C1 inFIG.6), and are then applied to the next synapse to produce the next feature map layer or final layer. Therefore, in this example, non-volatile memory cell array33constitutes a plurality of synapses (which receive their inputs from the prior layer of neurons or from an input layer such as an image database), and summing op-amp38and activation function block39constitute a plurality of neurons.

The input to VMM array32inFIG.7(WLx, EGx, CGx, and optionally BLx and SLx) can be analog level, binary level, or digital bits (in which case a DAC is provided to convert digital bits to appropriate input analog level) and the output can be analog level, binary level, or digital bits (in which case an output ADC is provided to convert output analog level into digital bits).

FIG.8is a block diagram depicting the usage of numerous layers of VMM arrays32, here labeled as VMM arrays32a,32b,32c,32d, and32e. As shown inFIG.8, the input, denoted Inputx, is converted from digital to analog by a digital-to-analog converter31and provided to input VMM array32a. The converted analog inputs could be voltage or current. The input D/A conversion for the first layer could be done by using a function or a LUT (look up table) that maps the inputs Inputx to appropriate analog levels for the matrix multiplier of input VMM array32a. The input conversion could also be done by an analog to analog (A/A) converter to convert an external analog input to a mapped analog input to the input VMM array32a.

The output generated by input VMM array32ais provided as an input to the next VMM array (hidden level 1)32b, which in turn generates an output that is provided as an input to the next VMM array (hidden level 2)32c, and so on. The various layers of VMM array32function as different layers of synapses and neurons of a convolutional neural network (CNN). Each VMM array32a,32b,32c,32d, and32ecan be a stand-alone, physical non-volatile memory array, or multiple VMM arrays could utilize different portions of the same physical non-volatile memory array, or multiple VMM arrays could utilize overlapping portions of the same physical non-volatile memory array. The example shown inFIG.8contains five layers (32a,32b,32c,32d,32e): one input layer (32a), two hidden layers (32b,32c), and two fully connected layers (32d,32e). One of ordinary skill in the art will appreciate that this is merely an example and that a system instead could comprise more than two hidden layers and more than two fully connected layers.

Vector-by-Matrix Multiplication (VMM) Arrays

FIG.9depicts neuron VMM array900, which is particularly suited for memory cells310as shown inFIG.3and is utilized as the synapses and parts of neurons between an input layer and the next layer. VMM array900comprises memory array901of non-volatile memory cells and reference array902(at the top of the array) of non-volatile reference memory cells. Alternatively, another reference array can be placed at the bottom.

In VMM array900, control gate lines, such as control gate line903, run in a vertical direction (hence reference array902in the row direction is orthogonal to control gate line903), and erase gate lines, such as erase gate line904, run in a horizontal direction. Here, the inputs to VMM array900are provided on the control gate lines (CG0, CG1, CG2, CG3), and the output of VMM array900emerges on the source lines (SL0, SL1). In one example, only even rows are used, and in another example, only odd rows are used. The current placed on each source line (SL0, SL1, respectively) performs a summing function of all the currents from the memory cells connected to that particular source line.

As described herein for neural networks, the non-volatile memory cells of VMM array900, i.e., the memory cells310of VMM array900, may be configured to operate in a sub-threshold region.

The non-volatile reference memory cells and the non-volatile memory cells described herein are biased in weak inversion (sub threshold region):
Ids=Io*e(Vg−Vth/nVt=w*Io*e(Vg)/nVt,
wherew=e(−Vth)/nVt
where Ids is the drain to source current; Vg is gate voltage on the memory cell; Vth is threshold voltage of the memory cell; Vt is thermal voltage=k*T/q with k being the Boltzmann constant, T the temperature in Kelvin, and q the electronic charge; n is a slope factor=1+(Cdep/Cox) with Cdep=capacitance of the depletion layer, and Cox capacitance of the gate oxide layer; Io is the memory cell current at gate voltage equal to threshold voltage, Io is proportional to (Wt/L)*u*Cox*(n−1)*Vt2where u is carrier mobility and Wt and L are width and length, respectively, of the memory cell.

For an I-to-V log converter using a memory cell (such as a reference memory cell or a peripheral memory cell) or a transistor to convert input current into an input voltage:
Vg=n*Vt*log[Ids/wp*Io]
where, wp is w of a reference or peripheral memory cell.

For a memory array used as a vector matrix multiplier VMM array with the current input, the output current is:
Iout=wa*Io*e(Vg)/nVt,namely
Iout=(wa/wp)*Iin=W*Iin
W=e(Vthp−Vtha)/nVt
Here, wa=w of each memory cell in the memory array.
Vthp is effective threshold voltage of the peripheral memory cell and Vtha is effective threshold voltage of the main (data) memory cell. Note that the threshold voltage of a transistor is a function of substrate body bias voltage and the substrate body bias voltage, denoted Vsb, can be modulated to compensate for various conditions, on such temperature. The threshold voltage Vth can be expressed as:
Vth=Vth0+gamma(SQRT|Vsb−2*φF)−SQRT|2*φF|)
where Vth0 is threshold voltage with zero substrate bias, φF is a surface potential, and gamma is a body effect parameter.

A wordline or control gate can be used as the input for the memory cell for the input voltage.

Alternatively, the flash memory cells of VMM arrays described herein can be configured to operate in the linear region:
Ids=beta*(Vgs−Vth)*Vds;beta=u*Cox*Wt/L
W=α(Vgs−Vth)
meaning weight W in the linear region is proportional to (Vgs−Vth)

A wordline or control gate or bitline or sourceline can be used as the input for the memory cell operated in the linear region. The bitline or sourceline can be used as the output for the memory cell.

For an I-to-V linear converter, a memory cell (such as a reference memory cell or a peripheral memory cell) or a transistor operating in the linear region can be used to linearly convert an input/output current into an input/output voltage.

Alternatively, the memory cells of VMM arrays described herein can be configured to operate in the saturation region:
Ids=½*beta*(Vgs−Vth)2;beta=u*Cox*Wt/L
Wα(Vgs−Vth)2, meaning weightWis proportional to(Vgs−Vth)2

A wordline, control gate, or erase gate can be used as the input for the memory cell operated in the saturation region. The bitline or sourceline can be used as the output for the output neuron.

Alternatively, the memory cells of VMM arrays described herein can be used in all regions or a combination thereof (sub threshold, linear, or saturation) for each layer or multi layers of a neural network.

Other examples for VMM array32ofFIG.7are described in U.S. Pat. No. 10,748,630, which is incorporated by reference herein. As described in that application. a sourceline or a bitline can be used as the neuron output (current summation output).

FIG.10depicts neuron VMM array1000, which is particularly suited for memory cells210as shown inFIG.2and is utilized as the synapses between an input layer and the next layer. VMM array1000comprises a memory array1003of non-volatile memory cells, reference array1001of first non-volatile reference memory cells, and reference array1002of second non-volatile reference memory cells. Reference arrays1001and1002, arranged in the column direction of the array, serve to convert current inputs flowing into terminals BLR0, BLR1, BLR2, and BLR3 into voltage inputs WL0, WL1, WL2, and WL3. In effect, the first and second non-volatile reference memory cells are diode-connected through multiplexors1014(only partially depicted) with current inputs flowing into them. The reference cells are tuned (e.g., programmed) to target reference levels. The target reference levels are provided by a reference mini-array matrix (not shown).

Memory array1003serves two purposes. First, it stores the weights that will be used by the VMM array1000on respective memory cells thereof. Second, memory array1003effectively multiplies the inputs (i.e. current inputs provided in terminals BLR0, BLR1, BLR2, and BLR3, which reference arrays1001and1002convert into the input voltages to supply to wordlines WL0, WL1, WL2, and WL3) by the weights stored in the memory array1003and then adds all the results (memory cell currents) to produce the output on the respective bit lines (BL0-BLN), which will be the input to the next layer or input to the final layer. By performing the multiplication and addition function, memory array1003negates the need for separate multiplication and addition logic circuits and is also power efficient. Here, the voltage inputs are provided on the word lines WL0, WL1, WL2, and WL3, and the output emerges on the respective bit lines BL0-BLN during a read (inference) operation. The current placed on each of the bit lines BL0-BLN performs a summing function of the currents from all non-volatile memory cells connected to that particular bitline.

Table No. 5 depicts operating voltages and currents for VMM array1000. The columns in the table indicate the voltages placed on word lines for selected cells, word lines for unselected cells, bit lines for selected cells, bit lines for unselected cells, source lines for selected cells, and source lines for unselected cells. The rows indicate the operations of read, erase, and program.

TABLE NO. 5Operation of VMM Array 1000 of FIG. 10:WLWL -unselBLBL -unselSLSL -unselRead1-3.5V−0.5 V/0 V0.6-2 V0.6 V-2 V/0 V0 V0V(Ineuron)Erase~5-13V0 V0V0 V0 V0VProgram1-2V−0.5 V/0 V0.1-3uAVinh ~2.5 V4-10 V0-1V/FLT

FIG.11depicts neuron VMM array1100, which is particularly suited for memory cells210as shown inFIG.2and is utilized as the synapses and parts of neurons between an input layer and the next layer. VMM array1100comprises a memory array1103of non-volatile memory cells, reference array1101of first non-volatile reference memory cells, and reference array1102of second non-volatile reference memory cells. Reference arrays1101and1102run in row direction of the VMM array1100. VMM array is similar to VMM1000except that in VMM array1100, the word lines run in the vertical direction. Here, the inputs are provided on the word lines (WLA0, WLB0, WLA1, WLB2, WLA2, WLB2, WLA3, WLB3), and the output emerges on the source line (SL0, SL1) during a read operation. The current placed on each source line performs a summing function of all the currents from the memory cells connected to that particular source line.

Table No. 6 depicts operating voltages and currents for VMM array1100. The columns in the table indicate the voltages placed on word lines for selected cells, word lines for unselected cells, bit lines for selected cells, bit lines for unselected cells, source lines for selected cells, and source lines for unselected cells. The rows indicate the operations of read, erase, and program.

TABLE NO. 6Operation of VMM Array 1100 of FIG. 11WLWL -unselBLBL -unselSLSL -unselRead1-3.5V−0.5 V/0 V0.6-2V0.6 V-2 V/0 V~0.3-1 V0V(Ineuron)Erase~5-13V0 V0V0 V0VSL-inhibit(~4-8 V)Program1-2V−0.5 V/0 V0.1-3uAVinh ~2.5 V4-10V0-1V/FLT

FIG.12depicts neuron VMM array1200, which is particularly suited for memory cells310as shown inFIG.3and is utilized as the synapses and parts of neurons between an input layer and the next layer. VMM array1200comprises a memory array1203of non-volatile memory cells, reference array1201of first non-volatile reference memory cells, and reference array1202of second non-volatile reference memory cells. Reference arrays1201and1202serve to convert current inputs flowing into terminals BLR0, BLR1, BLR2, and BLR3 into voltage inputs CG0, CG1, CG2, and CG3. In effect, the first and second non-volatile reference memory cells are diode-connected through multiplexors1212(only partially shown) with current inputs flowing into them through BLR0, BLR1, BLR2, and BLR3. Multiplexors1212each include a respective multiplexor1205and a cascoding transistor1204to ensure a constant voltage on the bitline (such as BLR0) of each of the first and second non-volatile reference memory cells during a read operation. The reference cells are tuned to target reference levels.

Memory array1203serves two purposes. First, it stores the weights that will be used by the VMM array1200. Second, memory array1203effectively multiplies the inputs (current inputs provided to terminals BLR0, BLR1, BLR2, and BLR3, for which reference arrays1201and1202convert these current inputs into the input voltages to supply to the control gates (CG0, CG1, CG2, and CG3) by the weights stored in the memory array and then add all the results (cell currents) to produce the output, which appears on BL0-BLN, and will be the input to the next layer or input to the final layer. By performing the multiplication and addition function, the memory array negates the need for separate multiplication and addition logic circuits and is also power efficient. Here, the inputs are provided on the control gate lines (CG0, CG1, CG2, and CG3), and the output emerges on the bit lines (BL0-BLN) during a read operation. The current placed on each bitline performs a summing function of all the currents from the memory cells connected to that particular bitline.

VMM array1200implements uni-directional tuning for non-volatile memory cells in memory array1203. That is, each non-volatile memory cell is erased and then partially programmed until the desired charge on the floating gate is reached. If too much charge is placed on the floating gate (such that the wrong value is stored in the cell), the cell is erased and the sequence of partial programming operations starts over. As shown, two rows sharing the same erase gate (such as EG0 or EG1) are erased together (which is known as a page erase), and thereafter, each cell is partially programmed until the desired charge on the floating gate is reached.

Table No. 7 depicts operating voltages and currents for VMM array1200. The columns in the table indicate the voltages placed on word lines for selected cells, word lines for unselected cells, bit lines for selected cells, bit lines for unselected cells, control gates for selected cells, control gates for unselected cells in the same sector as the selected cells, control gates for unselected cells in a different sector than the selected cells, erase gates for selected cells, erase gates for unselected cells, source lines for selected cells, and source lines for unselected cells. The rows indicate the operations of read, erase, and program.

TABLE NO. 7Operation of VMM Array 1200 of FIG. 12CG -WL -BL -unsel sameCG -EG -SL -WLunselBLunselCGsectorunselEGunselSLunselRead1.0-2V−0.5 V/ 0 V0.6-2 V0 V0-2.6V0-2.6 V0-2.6 V0-2.6 V0-2.6 V0V0 V(Ineuron)Erase0V0 V0V0 V0V0-2.6 V0-2.6 V5-12 V0-2.6 V0V0 VProgram0.7-1V−0.5 V/0 V0.1-1uAVinh4-11V0-2.6 V0-2.6 V4.5-5 V0-2.6 V4.5-5V0-1 V(1-2 V)

FIG.13depicts neuron VMM array1300, which is particularly suited for memory cells310as shown inFIG.3, and is utilized as the synapses and parts of neurons between an input layer and the next layer. VMM array1300comprises a memory array1303of non-volatile memory cells, reference array1301or first non-volatile reference memory cells, and reference array1302of second non-volatile reference memory cells. EG lines EGR0, EG0, EG1 and EGR1 are run vertically while CG lines CG0, CG1, CG2 and CG3 and SL lines WL0, WL1, WL2 and WL3 are run horizontally. VMM array1300is similar to VMM array1400, except that VMM array1300implements bi-directional tuning, where each individual cell can be completely erased, partially programmed, and partially erased as needed to reach the desired amount of charge on the floating gate due to the use of separate EG lines. As shown, reference arrays1301and1302convert input current in the terminal BLR0, BLR1, BLR2, and BLR3 into control gate voltages CG0, CG1, CG2, and CG3 (through the action of diode-connected reference cells through multiplexors1314) to be applied to the memory cells in the row direction. The current output (neuron) is in the bit lines BL0-BLN, where each bit line sums all currents from the non-volatile memory cells connected to that particular bitline.

Table No. 8 depicts operating voltages and currents for VMM array1300. The columns in the table indicate the voltages placed on word lines for selected cells, word lines for unselected cells, bit lines for selected cells, bit lines for unselected cells, control gates for selected cells, control gates for unselected cells in the same sector as the selected cells, control gates for unselected cells in a different sector than the selected cells, erase gates for selected cells, erase gates for unselected cells, source lines for selected cells, and source lines for unselected cells. The rows indicate the operations of read, erase, and program.

TABLE NO. 8Operation of VMM Array 1300 of FIG. 13CG -WL -BL -unsel sameCG -EG -SL -WLunselBLunselCGsectorunselEGunselSLunselRead1.0-2V−0.5 V/0 V0.6-2 V0 V0-2.6V0-2.6 V0-2.6 V0-2.6 V0-2.6 V0V0 V(Ineuron)Erase0V0 V0V0 V0V4-9 V0-2.6 V5-12 V0-2.6 V0V0 VProgram0.7-1V−0.5 V/0 V0.1-1uAVinh4-11V0-2.6 V0-2.6 V4.5-5 V0-2.6 V4.5-5V0-1 V(1-2 V)

FIG.22depicts neuron VMM array2200, which is particularly suited for memory cells210as shown inFIG.2and is utilized as the synapses and parts of neurons between an input layer and the next layer. In VMM array2200, the inputs INPUT0. . . . , INPUTNare received on bit lines BL0, . . . BLN, respectively, and the outputs OUTPUT1, OUTPUT2, OUTPUT3, and OUTPUT4are generated on source lines SL0, SL1, SL2, and SL3, respectively.

FIG.23depicts neuron VMM array2300, which is particularly suited for memory cells210as shown inFIG.2and is utilized as the synapses and parts of neurons between an input layer and the next layer. In this example, the inputs INPUT0, INPUT1, INPUT2, and INPUT3are received on source lines SL0, SL1, SL2, and SL3, respectively, and the outputs OUTPUT0, . . . OUTPUTNare generated on bit lines BL0, . . . , BLN.

FIG.24depicts neuron VMM array2400, which is particularly suited for memory cells210as shown inFIG.2, and is utilized as the synapses and parts of neurons between an input layer and the next layer. In this example, the inputs INPUT0, . . . , INPUTMare received on word lines WL0, . . . , WLM, respectively, and the outputs OUTPUT0, . . . OUTPUTNare generated on bit lines BL0, . . . , BLN.

FIG.25depicts neuron VMM array2500, which is particularly suited for memory cells310as shown inFIG.3, and is utilized as the synapses and parts of neurons between an input layer and the next layer. In this example, the inputs INPUT0, . . . , INPUTMare received on word lines WL0, . . . , WLM, respectively, and the outputs OUTPUT0, . . . OUTPUTNare generated on bit lines BL0, . . . , BLN.

FIG.26depicts neuron VMM array2600, which is particularly suited for memory cells410as shown inFIG.4, and is utilized as the synapses and parts of neurons between an input layer and the next layer. In this example, the inputs INPUT0, . . . , INPUTNare received on vertical control gate lines CG0, . . . , CGN, respectively, and the outputs OUTPUT1and OUTPUT2are generated on source lines SL0and SL1.

FIG.27depicts neuron VMM array2700, which is particularly suited for memory cells410as shown inFIG.4and is utilized as the synapses and parts of neurons between an input layer and the next layer. In this example, the inputs INPUT0, . . . , INPUTNare received on the gates of bit line control gates2701-1,2701-2, . . . ,2701-(N−1), and 2701-N, respectively, which are coupled to bit lines BL0, . . . , BLN, respectively. Example outputs OUTPUT1and OUTPUT2are generated on source lines SL0and SL1.

FIG.28depicts neuron VMM array2800, which is particularly suited for memory cells310as shown inFIG.3, memory cells510as shown inFIG.5, and memory cells710as shown inFIG.7, and is utilized as the synapses and parts of neurons between an input layer and the next layer. In this example, the inputs INPUT0, . . . , INPUTMare received on word lines WL0, . . . , WLM, and the outputs OUTPUT0, . . . , OUTPUTNare generated on bit lines BL0, . . . , BLN, respectively.

FIG.29depicts neuron VMM array2900, which is particularly suited for memory cells310as shown inFIG.3, memory cells510as shown inFIG.5, and memory cells710as shown inFIG.7, and is utilized as the synapses and parts of neurons between an input layer and the next layer. In this example, the inputs INPUT0, . . . , INPUTMare received on control gate lines CG0, . . . , CGM. Outputs OUTPUT0, . . . , OUTPUTNare generated on vertical source lines SL0, . . . , SLN, respectively, where each source line SLiis coupled to the source lines of all memory cells in column i.

FIG.30depicts neuron VMM array3000, which is particularly suited for memory cells310as shown inFIG.3, memory cells510as shown inFIG.5, and memory cells710as shown inFIG.7, and is utilized as the synapses and parts of neurons between an input layer and the next layer. In this example, the inputs INPUT0, . . . , INPUTMare received on control gate lines CG0, . . . , CGM. Outputs OUTPUT0, . . . , OUTPUTNare generated on vertical bit lines BL0, . . . , BLN, respectively, where each bit line BLtis coupled to the bit lines of all memory cells in column i.

Long Short-Term Memory

The prior art includes a concept known as long short-term memory (LSTM). LSTM units often are used in neural networks. LSTM allows a neural network to remember information over predetermined arbitrary time intervals and to use that information in subsequent operations. A conventional LSTM unit comprises a cell, an input gate, an output gate, and a forget gate. The three gates regulate the flow of information into and out of the cell and the time interval that the information is remembered in the LSTM. VMMs are particularly useful in LSTM units.

FIG.14depicts an example LSTM1400. LSTM1400in this example comprises cells1401,1402,1403, and1404. Cell1401receives input vector x0and generates output vector h0and cell state vector c0. Cell1402receives input vector x1, the output vector (hidden state) h0from cell1401, and cell state c0from cell1401and generates output vector h1and cell state vector c1. Cell1403receives input vector x2, the output vector (hidden state) h1from cell1402, and cell state c1from cell1402and generates output vector h2and cell state vector c2. Cell1404receives input vector x3, the output vector (hidden state) h2from cell1403, and cell state c2from cell1403and generates output vector h3. Additional cells can be used, and an LSTM with four cells is merely an example.

FIG.15depicts an example implementation of an LSTM cell1500, which can be used for cells1401,1402,1403, and1404inFIG.14. LSTM cell1500receives input vector x(t), cell state vector c(t−1) from a preceding cell, and output vector h(t−1) from a preceding cell, and generates cell state vector c(t) and output vector h(t).

LSTM cell1500comprises sigmoid function devices1501,1502, and1503, each of which applies a number between 0 and 1 to control how much of each component in the input vector is allowed through to the output vector. LSTM cell1500also comprises tanh devices1504and1505to apply a hyperbolic tangent function to an input vector, multiplier devices1506,1507, and1508to multiply two vectors together, and addition device1509to add two vectors together. Output vector h(t) can be provided to the next LSTM cell in the system, or it can be accessed for other purposes.

FIG.16depicts an LSTM cell1600, which is an example of an implementation of LSTM cell1500. For the reader's convenience, the same numbering from LSTM cell1500is used in LSTM cell1600. Sigmoid function devices1501,1502, and1503and tanh device1504each comprise multiple VMM arrays1601and activation function blocks1602. Thus, it can be seen that VMM arrays are particular useful in LSTM cells used in certain neural network systems. The multiplier devices1506,1507, and1508and the addition device1509are implemented in a digital manner or in an analog manner. The activation function blocks1602can be implemented in a digital manner or in an analog manner.

An alternative to LSTM cell1600(and another example of an implementation of LSTM cell1500) is shown inFIG.17. InFIG.17, sigmoid function devices1501,1502, and1503and tanh device1504share the same physical hardware (VMM arrays1701and activation function block1702) in a time-multiplexed fashion. LSTM cell1700also comprises multiplier device1703to multiply two vectors together, addition device1708to add two vectors together, tanh device1505(which comprises activation function block1702), register1707to store the value i(t) when i(t) is output from sigmoid function block1702, register1704to store the value f(t)*c(t−1) when that value is output from multiplier device1703through multiplexor1710, register1705to store the value i(t)*u(t) when that value is output from multiplier device1703through multiplexor1710, and register1706to store the value o(t)*c˜(t) when that value is output from multiplier device1703through multiplexor1710, and multiplexor1709.

Whereas LSTM cell1600contains multiple sets of VMM arrays1601and respective activation function blocks1602, LSTM cell1700contains only one set of VMM arrays1701and activation function block1702, which are used to represent multiple layers in the example of LSTM cell1700. LSTM cell1700will require less space than LSTM1600, as LSTM cell1700will require ¼ as much space for VMMs and activation function blocks compared to LSTM cell1600.

It can be further appreciated that LSTM units will typically comprise multiple VMM arrays, each of which requires functionality provided by certain circuit blocks outside of the VMM arrays, such as a summer and activation function block and high voltage generation blocks. Providing separate circuit blocks for each VMM array would require a significant amount of space within the semiconductor device and would be somewhat inefficient. The examples described below therefore reduce the circuitry required outside of the VMM arrays themselves.

Gated Recurrent Units

An analog VMM implementation can be utilized for a GRU (gated recurrent unit) system. GRUs are a gating mechanism in recurrent neural networks. GRUs are similar to LSTMs, except that GRU cells generally contain fewer components than an LSTM cell.

FIG.18depicts an example GRU1800. GRU1800in this example comprises cells1801,1802,1803, and1804. Cell1801receives input vector x0and generates output vector h0. Cell1802receives input vector x1, the output vector h0from cell1801and generates output vector h1. Cell1803receives input vector x2and the output vector (hidden state) h1from cell1802and generates output vector h2. Cell1804receives input vector x3and the output vector (hidden state) h2from cell1803and generates output vector h3. Additional cells can be used, and an GRU with four cells is merely an example.

FIG.19depicts an example implementation of a GRU cell1900, which can be used for cells1801,1802,1803, and1804ofFIG.18. GRU cell1900receives input vector x(t) and output vector h(t−1) from a preceding GRU cell and generates output vector h(t). GRU cell1900comprises sigmoid function devices1901and1902, each of which applies a number between 0 and 1 to components from output vector h(t−1) and input vector x(t). GRU cell1900also comprises a tanh device1903to apply a hyperbolic tangent function to an input vector, a plurality of multiplier devices1904,1905, and1906to multiply two vectors together, an addition device1907to add two vectors together, and a complementary device1908to subtract an input from 1 to generate an output.

FIG.20depicts a GRU cell2000, which is an example of an implementation of GRU cell1900. For the reader's convenience, the same numbering from GRU cell1900is used in GRU cell2000. As can be seen inFIG.20, sigmoid function devices1901and1902, and tanh device1903each comprise multiple VMM arrays2001and activation function blocks2002. Thus, it can be seen that VMM arrays are of particular use in GRU cells used in certain neural network systems. The multiplier devices1904,1905,1906, the addition device1907, and the complementary device1908are implemented in a digital manner or in an analog manner. The activation function blocks2002can be implemented in a digital manner or in an analog manner.

An alternative to GRU cell2000(and another example of an implementation of GRU cell1900) is shown inFIG.21. InFIG.21, GRU cell2100utilizes VMM arrays2101and activation function block2102, which when configured as a sigmoid function applies a number between 0 and 1 to control how much of each component in the input vector is allowed through to the output vector. InFIG.21, sigmoid function devices1901and1902and tanh device1903share the same physical hardware (VMM arrays2101and activation function block2102) in a time-multiplexed fashion. GRU cell2100also comprises multiplier device2103to multiply two vectors together, addition device2105to add two vectors together, complementary device2109to subtract an input from 1 to generate an output, multiplexor2104, register2106to hold the value h(t−1)*r(t) when that value is output from multiplier device2103through multiplexor2104, register2107to hold the value h(t−1)*z(t) when that value is output from multiplier device2103through multiplexor2104, and register2108to hold the value h{circumflex over ( )}(t)*(1−z(t)) when that value is output from multiplier device2103through multiplexor2104.

Whereas GRU cell2000contains multiple sets of VMM arrays2001and activation function blocks2002, GRU cell2100contains only one set of VMM arrays2101and activation function block2102, which are used to represent multiple layers in the example of GRU cell2100. GRU cell2100will require less space than GRU cell2000, as GRU cell2100will require ⅓ as much space for VMMs and activation function blocks compared to GRU cell2000.

It can be further appreciated that GRU systems will typically comprise multiple VMM arrays, each of which requires functionality provided by certain circuit blocks outside of the VMM arrays, such as a summer and activation function block and high voltage generation blocks. Providing separate circuit blocks for each VMM array would require a significant amount of space within the semiconductor device and would be somewhat inefficient. The examples described below therefore reduce the circuitry required outside of the VMM arrays themselves.

The input to the VMM arrays can be an analog level, a binary level, a pulse, a time modulated pulse, or digital bits (in this case a DAC is needed to convert digital bits to appropriate input analog level) and the output can be an analog level, a binary level, a timing pulse, pulses, or digital bits (in this case an output ADC is needed to convert output analog level into digital bits).

In general, for each memory cell in a VMM array, each weight W can be implemented by a single memory cell or by a differential cell or by two blend memory cells (average of 2 cells).

In the differential cell case, two memory cells are needed to implement a weight W as a differential weight (W=W+−W−). In the two blend memory cells, two memory cells are needed to implement a weight W as an average of two cells.

FIG.31depicts VMM system3100. In some examples, the weights, W, stored in a VMM array are stored as differential pairs, W+ (positive weight) and W− (negative weight), where W=(W+)−(W−). In VMM system3100, half of the bit lines are designated as W+ lines, that is, bit lines connecting to memory cells that will store positive weights W+, and the other half of the bit lines are designated as W− lines, that is, bit lines connecting to memory cells implementing negative weights W−. The W− lines are interspersed among the W+ lines in an alternating fashion. The subtraction operation is performed by a summation circuit that receives current from a W+ line and a W− line, such as summation circuits3101and3102. The output of a W+ line and the output of a W− line are combined together to give effectively W=W+−W− for each pair of (W+, W−) cells for all pairs of (W+, W−) lines. While the above has been described in relation to W− lines interspersed among the W+ lines in an alternating fashion, in other examples W+ lines and W− lines can be arbitrarily located anywhere in the array.

FIG.32depicts another example. In VMM system3210, positive weights W+ are implemented in first array3211and negative weights W− are implemented in a second array3212, second array3212separate from the first array, and the resulting weights are appropriately combined together by summation circuits3213.

FIG.33depicts VMM system3300. the weights, W, stored in a VMM array are stored as differential pairs, W+(positive weight) and W− (negative weight), where W=(W+)−(W−). VMM system3300comprises array3301and array3302. Half of the bit lines in each of array3301and3302are designated as W+ lines, that is, bit lines connecting to memory cells that will store positive weights W+, and the other half of the bit lines in each of array3301and3302are designated as W− lines, that is, bit lines connecting to memory cells implementing negative weights W−. The W− lines are interspersed among the W+ lines in an alternating fashion. The subtraction operation is performed by a summation circuit that receives current from a W+ line and a W− line, such as summation circuits3303,3304,3305, and3306. The output of a W+ line and the output of a W− line from each array3301,3302are respectively combined together to give effectively W=W+−W− for each pair of (W+, W−) cells for all pairs of (W+, W−) lines. In addition, the W values from each array3301and3302can be further combined through summation circuits3307and3308, such that each W value is the result of a W value from array3301minus a W value from array3302, meaning that the end result from summation circuits3307and3308is a differential value of two differential values.

Each non-volatile memory cell used in the analog neural memory system is to be erased and programmed to hold a very specific and precise amount of charge, i.e., the number of electrons, in the floating gate. For example, each floating gate should hold one of N different values, where N is the number of different weights that can be indicated by each cell. Examples of N include 16, 32, 64, 128, and 256.

It is desirable to reduce latency in programming operations to increase the overall speed of operation of the artificial neural network.

SUMMARY OF THE INVENTION

Numerous examples are disclosed of programming multiple rows in an array in an artificial neural network as part of a single programming operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.1is a diagram that illustrates an artificial neural network.

FIG.2depicts a prior art split gate flash memory cell.

FIG.3depicts another prior art split gate flash memory cell.

FIG.4depicts another prior art split gate flash memory cell.

FIG.5depicts another prior art split gate flash memory cell.

FIG.6is a diagram illustrating the different levels of an exemplary artificial neural network utilizing one or more non-volatile memory arrays.

FIG.7is a block diagram illustrating a VMM system.

FIG.8is a block diagram illustrates an example artificial neural network utilizing one or more VMM systems.

FIG.9depicts another example of a VMM system.

FIG.10depicts another example of a VMM system.

FIG.11depicts another example of a VMM system.

FIG.12depicts another example of a VMM system.

FIG.13depicts another example t of a VMM system.

FIG.14depicts a prior art long short-term memory system.

FIG.15depicts an example cell for use in a long short-term memory system.

FIG.16depicts an example implementation of the cell ofFIG.15.

FIG.17depicts another example implementation of the cell ofFIG.15.

FIG.18depicts a prior art gated recurrent unit system.

FIG.19depicts an example cell for use in a gated recurrent unit system.

FIG.20depicts an example implementation t of the cell ofFIG.19.

FIG.21depicts another example implementation of the cell ofFIG.19.

FIG.22depicts another example of a VMM system.

FIG.23depicts another example of a VMM system.

FIG.24depicts another example of a VMM system.

FIG.25depicts another example of a VMM system.

FIG.26depicts another example of a VMM system.

FIG.27depicts another example of a VMM system.

FIG.28depicts another example of a VMM system.

FIG.29depicts another example of a VMM system.

FIG.30depicts another example of a VMM system.

FIG.31depicts another example of a VMM system.

FIG.32depicts another example of a VMM system.

FIG.33depicts another example of a VMM system.

FIG.34depicts another example of a VMM system.

FIG.35depicts an example of a prior art programming waveform.

FIGS.36A,36B,36C, and36Ddepict example programming waveforms for consecutive rows.

FIG.37depicts a block diagram of VMM system for consecutive row programming.

FIG.38depicts a ramp control circuit.

FIG.39depicts a ramp control circuit.

FIG.40depicts a ramp control circuit.

FIG.41depicts a ramp control circuit.

FIG.42depicts a high voltage level shifter and inverter.

FIG.43depicts a ramp-up control circuit and its exemplary waveforms.

FIG.44depicts a ramp-down control circuit and its exemplary waveforms.

DETAILED DESCRIPTION OF THE INVENTION

VMM System Architecture

FIG.34depicts a block diagram of VMM system3400. VMM system3400comprises VMM array3401, row decoder3402, high voltage row decoder3403, column decoders3404, bit line drivers3405, input circuit3406, output circuit3407, control logic3408, and bias generator3409. VMM system3400further comprises high voltage generation block3410, which comprises charge pump3411, charge pump regulator3412, and high voltage level generator3413. VMM system3400further comprises (program/erase, or weight tuning) algorithm controller3414, analog circuitry3415, control engine3416(that may include special functions such as arithmetic functions, activation functions, embedded microcontroller logic, without limitation), test control logic3417, and static random access memory (SRAM) block3418to store intermediate data such as for input circuits (e.g., activation data) or output circuits (neuron output data) or data for programming (such as input data for whole row or multiple rows).

The input circuit3406may include circuits such as a DAC (digital to analog converter), DPC (digital to pulses converter, digital to time modulated pulse converter), AAC (analog to analog converter, such as a current to voltage converter, logarithmic converter), PAC (pulse to analog level converter), or any other type of converters. The input circuit3406may implement one or more of normalization, linear or non-linear up/down scaling functions, or arithmetic functions. The input circuit3406may implement a temperature compensation function for input levels. The input circuit3406may implement an activation function such as ReLU or sigmoid.

The output circuit3407may include circuits such as an ADC (analog to digital converter, to convert neuron analog output to digital bits), AAC (analog to analog converter, such as a current to voltage converter, logarithmic converter), APC (analog to pulse(s) converter, analog to time modulated pulse converter), or any other type of converters. The output circuit3407may implement an activation function such as rectified linear activation function (ReLU) or sigmoid. The output circuit3407may implement one or more of statistic normalization, regularization, up/down scaling/gain functions, statistical rounding, or arithmetic functions (e.g., add, subtract, divide, multiply, shift, log) for neuron outputs. The output circuit3407may implement a temperature compensation function for neuron outputs or array outputs (such as bitline output) so as to keep power consumption of the array approximately constant, or to improve precision of the array (neuron) outputs, such as by keeping the IV slope approximately the same.

FIG.35depicts timing waveforms for prior art programming operation3500. Shown here are exemplary signals for HVSUP (the high voltage supply generated by high voltage generation block3410inFIG.34, which includes charge pump3411, and which HVSUP is particularly generated by high voltage level generator3413, and which is supplied to control gate lines, source lines, and/or bit lines of selected cells during a programming operation). In particular,FIG.35depicts CG (the voltage applied to control gate lines coupled to the selected cells during a programming operation), SL (the voltage applied to source lines coupled to the selected cells during a programming operation), EG (the voltage applied to erase gate lines coupled to the selected cells during a programming operation), and IPROG (the voltage generated by bias generator3409or analog circuitry3415and applied to bit lines coupled to the selected cells during a programming operation). These voltages are selected, for example, according to Tables 1-8 described previously for programming operations, without limitation. As can be seen, there is a significant ramp-up time, t1, and ramp-down time, t2, for HVSUP due to the high voltage of its peak value and the actions involved in generating that voltage, such as capacitance. The ramp-up time is, for example, affected by the charge pumping action of charge pump3411which takes time. The ramp-down time is, for example, affected by the time required to discharge the high voltage for all devices coupled to the high voltage, which includes large capacitance loading that causes the ramp-down time to be slow. There is a shorter ramp-up time t3 for CG, and a shorter ramp-down time t4 for CG.

Typical values for t1, t2, t3, t4, and t5 (to be described below) are 20-60 μs, 5-20 μs, 0.1-1 μs, 0.1-1 μs, and 0.1 μs, respectively. By contrast, the time required to program a single cell once the relevant voltages are in place is only 1 μs. As a result, it is common in the prior art to program an entire row of cells in a single high voltage cycle of HVSUP so that only a single ramp-up period and a single ramp-down period for HVSUP are invoked for the programming of the entire row. For example, a row might contain 2048 cells, and each word might contain 128 bits, meaning that each row contains 16 words. Thus, in the example ofFIG.35, Word0, . . . , Word15 (16 words total) are programmed during a single cycle of HVSUP, such that a single ramp-up of t1 and a ramp-down of t2 are incurred for the programming of each set of 16 words. An additional delay of t5 is incurred between programming different rows of cells.

FIG.36Adepicts timing waveforms for programming operation3600. Again, waveforms are shown for HVSUP, CG, SL, EG, and IPROG. Here, K consecutive rows of cells are programmed for each cycle of HVSUP, where K>1. For example, K=2, K=3, or K=4.

First, the ramp-up period of t1 is incurred such as from charge pumping action of charge pump3411and from charging up all capacitance loading of all devices coupled to high voltage generation block3410. After the high voltage (HVSUP) is stable, the HVSUP is then switched into one or more lines coupled to the array terminals (CG, WL, SL, EG, and/or BL). For example, HVSUP can be applied to a CG, SL, or EG line, and other voltages (lower than HVSUP) can be applied to the remaining lines according to Table Nos. 1-8, above. The high voltages applied to CG, SL, EG, and BL optionally can be four different voltages. The low voltages applied to CG, SL, EG, and BL optionally can be four different voltages, the same voltage (such as ground) or any variation in between. Second, the program current (IPROG) is enabled for a tPROG (e.g., 0.5-1 μs) for each word for 16 words of a row as example. Instead of HVSUP ramping down after each row programming, HVSUP stays high for a next row programming, and instead, a circuit switches CG, SL, and EG off and then back on. Switching CG, SL, and EG off and then back on requires a time period equal to t4+t3+t5, where t4 is the ramp-down time required for the slowest of CG, SL, and EG, t3 is the ramp-up period for the slowest of CG, SL, and EG, and t5 provides a gap to ensure the ramp-up circuitry does not mistakenly begin ramping up CG, SL, and EG before it has completed its ramp-down. Notably, t3 is much smaller than t1, and t4 is much smaller than t2. Third, a second row of cells is programmed without ramping down HVSUP. Fourth, the ramp-down period of t2 is incurred. For example, typical numbers are t1˜10-60 μs, t3˜0.1-1 μs, t4˜0.1˜1 μs, t5<0.1 μs, t2˜2=10 μs. Using this same sequence, K rows are programmed for each constant high period of HVSUP, where K>1. In the example shown, the ramp-up and ramp-down slopes of CG, SL, and EG are steeper than the ramp-up and ramp-down slope, respectively, of HVSUP.

FIG.36Bdepicts timing waveforms for programming operation3620, which is similar to programming operation3600inFIG.36Aexcept there is no t5, meaning there is no delay between the end of the ramp-down period of t4 and the ramp-up period of t3 and the CG signal is switched without delay from a first CG line for a first row to a second CG line for a second row. This reduces the programming time compared to programming operation3600.

FIG.36Cdepicts timing waveforms for programming operation3640, which is similar to programming operation3600ofFIG.36Aexcept t4 and t3 overlap, meaning that the CG signal is switched from a first CG line for a first row to a second CG line for a second row while the ramp-down period of t4 is occurring. This reduces the programming time compared to programming operation3620, although there is an increased risk that the ramp-down of CG does not progress to the point where it will be considered by “low” before it ramps-up again.

FIG.36Ddepicts timing waveforms for programming operation3660, which is similar to programming operation3600ofFIG.36A, except that t3 begins before t4 ends, meaning that the CG signal that is applied to a second CG line of a second row ramps up before, or at the same time, that the CG signal that is applied to a first CG line of a first row ramps down. This reduces the programming time compared to programming operation3640, at the expense of not having the option of using a change in state of CG to trigger any action.

Thus, programming operations3600,3620,3640, and3660are faster than programming operation3500for the programming of two or more consecutive rows. This amount can be significant, on the order of ˜3-10× improvement in programming time

FIG.37to41depict circuitry for implementing programming operations3600,3620,3640, and3660.

FIG.37depicts VMM system3700, which comprises VMM array3401, row decoder3402, high voltage row decoder3403, and high voltage generation block3410as inFIG.34. VMM system3700further comprises high voltage predecoder3701, which implements the switching off and on sequence for CG, SL, and EG discussed previously with respect toFIG.36, and which receives HVSUP as a voltage input.

The ramp-up and ramp-down functions for HVSUP as shown inFIGS.36A-36Dcan be implemented in HV generation block3410, HV predecoder3701or the row decoder3402.FIGS.38-41depict circuits that can be used to implement the ramp-up and ramp-down functions.

FIG.38depicts ramp control circuit3800-i, bias circuit3810, bias circuit3820, and decoder3830, where i ranges from 0 to N, where there are N+1 rows of cells in array3401. Ramp control circuit3800-icomprises PMOS transistor3801and NMOS transistor3802, and can cause a signal Vout (e.g., CG, EG, SL array terminals, pre-decoded High Voltage Signals) to ramp up towards HVSUP from ground or to ramp down to ground. A first terminal of PMOS transistor3801receives HVSUP, a gate of PMOS transistor3801receives a first control signal, PBIAS[N:0] (which optionally is a current-controlled signal), and a second terminal of PMOS transistor3801is the Vout node. A first terminal of NMOS transistor3802is coupled to the Vout node, a gate of NMOS transistor3802receives a second control signal, NBIAS[N:0](which optionally is a current-controlled signal), and a second terminal of NMOS transistor3802is coupled to ground.

An instantiation of ramp control circuit3800-iis coupled to each control gate line, source line, and erase gate line to perform the switching-in and switching-off operations of programming operation3600,3620,3640, and3660. In the example shown, the suffix [N:0] indicates that there are N+1 different instantiations. The instantiations of ramp control circuit3800are coupled together in a decoding current mirror configuration.

Decoder3830receives a row address and generates enable signals, EN[N:0], complements of the enable signals, ENB[N:0], such that N+1 EN signals and N+1 ENB signals are generated, one each for each of the instantiations of ramp control circuit3800.

Bias circuit3810comprises primary circuit3815and N+1 instantiations of sub-circuit3816-i, where i ranges from 0 to N. Primary circuit3815comprises current source3811and NMOS transistor3812and generates the current iNthat will be mirrored to generate NBIAS[N:0]. Each sub-circuit3816-icomprises switch3813controlled by EN[N:0] from decoder3830and switch3814controlled by ENB[N:0] from decoder3830.

Bias circuit3820comprises primary circuit3825and N+1 instantiations of sub-circuit3826-i, where i ranges from 0 to N. Primary circuit3825comprises current source3821and PMOS transistor3822and generates the current ipthat will be mirrored to generate PBIAS[N:0]. Each sub-circuit3826-icomprises switch3823controlled by EN[N:0] from decoder3830and switch3824controlled by ENB[N:0] from decoder3830.

In ramp control circuit3800-i, a high voltage on NBIAS [N:0] causes a ramp down, and low voltage on PBIAS [N:0] causes a ramp up. When PBIAS is asserted to switch on PMOS transistor3801, and NBIAS is de-asserted, Vout is pulled up to HVSUP, thus performing a switch on function. When NBIAS is asserted to switch on NMOS transistor3802, and PBIAS is de-asserted, Vout is pulled down to ground, thus performing a switch off function. Ramp control circuit3800here is shown for Vout, which can be used to provide the CG, SL, or EG voltages inFIGS.36A-36D. Due to the use of the current mirrors in bias circuit3810and bias circuit3820, all N+1 instantiations receive identical currents on PBIAS [N:0] and identical currents on NBIAS [N:0] when the corresponding row is enabled by the row address provided to decoder3830.

FIG.39depicts ramp control circuit3900-i, where i ranges from 0 to N, where there are N+1 rows of cells in array3401. Ramp control circuit3900-icomprises PMOS transistors3901and3902and NMOS transistors3903and3904. An instantiation of ramp control circuit3900-iis coupled to each control gate line, source line, and erase gate line to perform the switching-in and switching-off operations of programming operation3600. In the example shown, the suffix [N:0] indicates that there are N+1 different instantiations. A high voltage on NBIAS [N:0] and NCASCODE causes a ramp down, and a low voltage on PBIAS [N:0] and PCASCODE causes a ramp up.

A first terminal of PMOS transistor3901receives HVSUP and a gate of PMOS transistor3901receives a first control signal, PBIAS[N:0] (which optionally is a current-controlled signal). A first terminal of PMOS transistor3902is coupled to a second terminal of PMOS transistor3901, a gate of PMOS transistor3902receives a second control signal, PCASCODE, and a second terminal of PMOS transistor3902is coupled to the Vout node (e.g., CG, EG, SL array terminals, pre-decoded High Voltage Signals). A first terminal of NMOS transistor3903is coupled to the Vout node and a gate of NMOS transistor3903receives a third control signal, NCASCODE. A first terminal of NMOS transistor3904is coupled to a second terminal of NMOS transistor3903, a gate of NMOS transistor3904receives a fourth control signal, NBIAS [N:0] (which optionally is a current-controlled signal), and a second terminal of NMOS transistor3904is coupled to ground.

PMOS transistor3902and NMOS transistor3903perform a cascoding function to isolate PMOS transistor3901and NMOS transistor3904from Vout. When PCASCODE and PBIAS for row N are asserted, PMOS transistors3901and3902are turned on, and with NCASCODE and NBIAS for row N de-asserted, Vout is pulled up to HVSUP, thus performing a switch on function. When NCASCODE and NBIAS for row N are asserted, NMOS transistors3903and3904are turned on, and with PCASCODE and PBIAS for row N de-asserted, Vout is pulled down to ground, thus performing a switch off function. Ramp control circuit3900here is shown for Vout, which can be used to provide the CG, SL, or EG voltages inFIGS.36A-36D. Optionally, the PBIAS [N:0] signals for the N+1 instantiations of ramp control circuit3900-I can be provided by bias circuit3820inFIG.38, such that all N+1 instantiations receive identical currents on PBIAS [N:0], and the NBIAS [N:0] signals for the N+1 instantiations of ramp control circuit can be provided by bias circuit3810inFIG.38, such that all N+1 instantiations receive identical currents on NBIAS [N:0]. Optionally, decoder3830inFIG.38also is used in these configurations to provide EN[N:0] and EN[N:B].

FIG.40depicts ramp control circuit4000-i, where i ranges from 0 to N. Ramp control circuit4000-icomprises PMOS transistors4001and4002, NMOS transistor4003, and current bias sources4004and4005. Current bias source4005comprises a first terminal coupled to the high voltage generation block. PMOS transistor4001comprises a first terminal coupled to a second terminal of current bias source4005, a second terminal, and a gate to receive a first control signal, PBIAS[N:0] (which optionally is a current-controlled signal). PMOS transistor4002comprises a first terminal coupled to the second terminal of PMOS transistor4001, a second terminal coupled to the Vout node (e.g., CG, EG, SL array terminals, pre-decoded High Voltage Signals), and a gate to receive a second control signal, GP[N:0]. NMOS transistor4003comprises a first terminal coupled to the Vout node, a second terminal, and a gate to receive a third control signal, NBIAS[N:0] (which optionally is a current-controlled signal). Current bias source4004is coupled between the second terminal of NMOS transistor4004and ground.

An instantiation of ramp control circuit4000is coupled to each control gate line, source line, and erase gate line to perform the switching-in and switching-off operations of programming operation3600,3620,3640, and3660. In the example shown, the suffix [N:0] indicates that there are N+1 different instantiations. A high voltage on NBIAS [N:0] causes a ramp down, and a low voltage on PBIAS [N:0] and GP[N:0] causes a ramp up. When PBIAS and GP for row N are asserted, PMOS transistors4001and4002are turned on, and with NBIAS for row N de-asserted, Vout is pulled up to HVSUP, where the slope of the ramp can be controlled by the amount of current provided by current bias source4005, thus performing a switch on function. When NBIAS is asserted, NMOS transistor4003is turned on, and with PBIAS and GP for row N de-asserted, Vout is pulled down to ground, where the slope of the ramp down can be controlled by the amount of current provided by current bias source4004, thus performing a switch off function. Ramp control circuit4000here is shown for Vout, and Vout can be used for CG, SL, or EG inFIGS.36A-36D.

Optionally, the PBIAS [N:0] signals for the N+1 instantiations of ramp control circuit3900-I can be provided by bias circuit3820inFIG.38, such that all N+1 instantiations receive identical currents on PBIAS [N:0], and the NBIAS [N:0] signals for the N+1 instantiations of ramp control circuit can be provided by bias circuit3810inFIG.38, such that all N+1 instantiations receive identical currents on NBIAS [N:0]. Optionally, decoder3830inFIG.38also is used in these configurations to provide EN[N:0] and EN[N:B].

FIG.41depicts ramp control circuit4100-i, where i ranges from 0 to N. Ramp control circuit4100-I comprises PMOS transistors4101,4102, and4103; NMOS transistors4104and4105; and current bias source4106and current bias source4107. An instantiation of ramp control circuit4100-iis coupled to each control gate line, source line, and erase gate line to perform the switching-in and switching-off operations of programming operation3600.

Current bias source4107comprises a first terminal coupled to HVSUP. PMOS transistor4101comprises a first terminal coupled to a second terminal of current bias source4107, a second terminal, and a gate to receive a first control signal, GPIAS[N:0] (which optionally is a current-controlled signal). PMOS transistor4102comprises a first terminal coupled to the second terminal of PMOS transistor4101, a second terminal, and a gate to receive a second control signal, GP[N:0]. PMOS transistor4103comprises a first terminal coupled to the second terminal of PMOS transistor4103, a second terminal coupled to the Vout node, and a gate to receive a third control signal, PCASCODE. NMOS transistor4104comprises a first terminal coupled to the Vout node, a second terminal, and a gate to receive a fourth control signal, NCASCODE. NMOS transistor4105comprises a first terminal coupled to the second terminal of the NMOS transistor4104, a second terminal, and a gate to receive a fifth control signal, NBIAS[N:0] (which optionally is a current-controlled signal). Current bias source4106is coupled between the second terminal of NMOS transistor4105and ground.

In the example shown, the suffix [N:0] indicates that there are N+1 different instantiations. A high voltage on NBIAS [N:0] causes a ramp down, and a low voltage on PBIAS [N:0] and GP[N:0] causes a ramp up. PMOS transistor4103and NMOS transistor4104perform a cacoding function. GP[N:0] performs another pre-decoded signal function. When GPIAS, GP and PCASCODE for row N are asserted, PMOS transistors4101,4102and4103are turned on, and with NBIAS and NCASCODE for row N de-asserted, Vout is pulled up to CGSUP, where the slope of the ramp can be controlled by the amount of current provided by current bias source4107, thus performing a switch on function. When NBIAS and NCASCODE are asserted, NMOS transistors4104and4105are turned on, and with PBIAS, GP and PCASCODE for row N de-asserted, Vout is pulled down to ground which is accelerated by current bias source4107, where the slope of the ramp down can be controlled by the amount of current provided by current bias source4106, thus performing a switch off function. Ramp control circuit4100here is shown for Vout, which can be used for CG, SL, or EG inFIGS.36A-36D.

Optionally, the PBIAS [N:0] signals for the N+1 instantiations of ramp control circuit3900-I can be provided by bias circuit3820inFIG.38, such that all N+1 instantiations receive identical currents on PBIAS [N:0], and the NBIAS [N:0] signals for the N+1 instantiations of ramp control circuit can be provided by bias circuit3810inFIG.38, such that all N+1 instantiations receive identical currents on NBIAS [N:0]. Optionally, decoder3830inFIG.38also is used in these configurations to provide EN[N:0] and EN[N:B].

FIG.42depicts high voltage level shifter and inverter4200, which comprises current bias source4201; PMOS transistors4202and4204; NMOS transistors4203,4205,4207, and4208; and current bias source4206. The inputs, IN and INB (logic voltage levels, where INB is the inverse of IN), are received, and the output (high voltage level), OUT, is generated, at the drain of NMOS transistor4208. A low value for IN results in a high value for OUT, and a high value for IN results in a low value for OUT, where the high values of IN and OUT are different voltage levels.

In some detail, when IN is high, NMOS transistor4208is on, pulling OUT to ground, and PMOS transistor4204is turned off and NMOS transistor4205is turned on, such that OUT is pulled low by NMOS transistor4205, which in turn turns on PMOS transistor4202and turns off NMOS transistor4203, latching the high voltage level shifter.

When IN is low, INB is high, PMOS transistor4204is on, NMOS transistor4205is off, and OUT is pulled high to HVSUP, which in turn turns off PMOS transistor4202and turns on NMOS transistor4203. The current bias sources4201and4206respectively control the slope of the ramping of the output voltages.

FIG.43depicts ramp up control circuit4300, which comprises NMOS transistors4301,4302, and4305; capacitor4303, which has a capacitance Cap; and current bias4304, which draws current Ibias. Ramp up control circuit4300provides ramp up control to VHV (e.g., HVSUP, CG, EG, SL array terminals, pre-decoded High Voltage Signals). The ramp up control circuit4300can be applied at the HV generation block3410, HV predecoder3701, or row decoders3402. VHV is the ramp controlled high voltage node. NMOS transistor4301is always on based on VDD being applied to its gate. VNCTRLU is a control bias signal that turns on transistor4302, which will sink current from HVSUP to control the ramp rate of the VHV. ENB is high when itis desired for VNCTRLU to be low, as NMOS transistor4305will pull down VNCTRLU to ground. When VNCTRLU is low, NMOS transistor4302will be off. ENB goes low when it is desired for VHV to ramp up to a final voltage. When ENB goes low, NMOS transistor4305turns off. Charge will begin to accumulate on the lower plate of capacitor4303to produce a voltage on VCNTRLU to control the ramp rate of the VHV. The ramp-up rate is equal to Cap*VHV/Ibias.FIG.4310shows the waveforms for the ramp up control circuit4300. The voltage at node VHV is initially kept at a low voltage level before ramping (circuit not shown).

FIG.44depicts ramp-down control circuit4400, which comprises NMOS transistors4403,4404,4405,4406, and4407; current bias source4402, which draws current Ibias; and capacitor4408, which has a capacitance, Cap. Ramp-down control circuit4400is switchably coupled during a programming operation to the CG, SL, or EG lines of the selected row (where it is switched from a first selected row to a second selected row during the operation). Ramp-down control circuit4400can be implemented in HV generation block3410, HV predecoder3701, or row decoder3402. VHV is the controlled ramp down high voltage node (e.g., HVSUP, CG, EG, SL array terminals, pre-decoded High Voltage Signals). NMOS transistors4403and4407are always on based on VDD being applied to their respective gates. When EN is high, ENB will be low, and NMOS transistor4404will be on and NMOS transistor4405will be off. In this state, VCNTRLD will be at a bias voltage to control the ramp down rate of the VHV. When EN goes low, ENB will go high, and transistor4405will turn on and VCNTRLD will be pulled to ground. The ramp-down rate is equal to Cap*VHV/Ibias.FIG.4410shows the waveforms for ramp down control circuit4400.

It should be noted that, as used herein, the terms “over” and “on” both inclusively include “directly on” (no intermediate materials, elements or space disposed therebetween) and “indirectly on” (intermediate materials, elements or space disposed therebetween). Likewise, the term “adjacent” includes “directly adjacent” (no intermediate materials, elements or space disposed therebetween) and “indirectly adjacent” (intermediate materials, elements or space disposed there between), “mounted to” includes “directly mounted to” (no intermediate materials, elements or space disposed there between) and “indirectly mounted to” (intermediate materials, elements or spaced disposed there between), and “electrically coupled” includes “directly electrically coupled to” (no intermediate materials or elements there between that electrically connect the elements together) and “indirectly electrically coupled to” (intermediate materials or elements there between that electrically connect the elements together). For example, forming an element “over a substrate” can include forming the element directly on the substrate with no intermediate materials/elements therebetween, as well as forming the element indirectly on the substrate with one or more intermediate materials/elements there between.