Threshold gating device design method, threshold gating device and dynamic memory

Abstract

The invention discloses a threshold gating device design method, a threshold gating device and a dynamic memory, and relates to the technical field of memorizers, and the method comprises the following steps: testing a holding voltage, a low resistance state resistance and a threshold voltage of a threshold gating function layer; determining a target holding voltage of the threshold gating device according to the threshold voltage; determining the sum of the resistances of the upper electrode and the lower electrode of the threshold gating device according to the holding voltage of the threshold gating function layer, the low-resistance-state resistance and the target holding voltage of the threshold gating device; and selecting materials and film parameters of the upper electrode and the lower electrode according to the sum of the resistances of the upper electrode and the lower electrode of the threshold gating device so as to design the threshold gating device meeting the target holding voltage. According to the threshold gating device, the target holding voltage of the threshold gating device can be adjusted by changing the sum of the resistances of the upper electrode and the lower electrode, and the threshold gating device meeting different holding voltages is designed.

Description

Technical Field

[0001] The present application relates to the field of memory technology, and in particular to a threshold gating device design method, a threshold gating device and a dynamic memory. Background Art

[0002] The bidirectional threshold conversion device OTS (Ovonic Threshold switch) can realize threshold transition under the control of an external electrical signal. The specific process is as follows: when an electrical signal is applied to the gate device unit and exceeds the threshold voltage, the material changes from a high resistance state to a low resistance state, and the device is in the on state; at this time, the applied electrical signal is reduced, and when the signal is less than the holding voltage, the material changes from a low resistance state to a high resistance state, and the device is in the off state. If the transistors in DRAM are replaced by threshold conversion devices, since the threshold conversion device is a two-port device, the peripheral circuit consumption is less, and it is easy to stack in three dimensions, the storage density can be greatly improved. The holding voltage of the threshold selection device will determine the write operation window of the DRAM, but the academic community has always believed that the holding voltage is an inherent characteristic of the threshold selection device and there is no way to adjust it, thus limiting the improvement of DRAM performance. Summary of the invention

[0003] The purpose of the present application is to provide a threshold gating device design method, a threshold gating device, and a dynamic memory, which can be used to design a threshold gating device that meets different holding voltages.

[0004] To achieve the above objectives, this application provides the following solutions:

[0005] In a first aspect, the present application provides a threshold gating device design method, comprising:

[0006] Testing the holding voltage, low-resistance resistance and threshold voltage of the threshold selection functional layer;

[0007] determining a target holding voltage of a threshold gating device according to the threshold voltage;

[0008] Determine the sum of the resistances of the upper electrode and the lower electrode of the threshold gating device according to the holding voltage of the threshold gating functional layer, the low-resistance resistance and the target holding voltage of the threshold gating device;

[0009] The material and film parameters of the upper electrode and the lower electrode are selected according to the sum of the resistances of the upper electrode and the lower electrode of the threshold gating device to design a threshold gating device that meets the target holding voltage.

[0010] Optionally, a target holding voltage of the threshold gating device is less than or equal to a threshold voltage of the threshold gating functional layer.

[0011] Optionally, the expression for determining the sum of the resistances of the upper electrode and the lower electrode of the threshold gating device is:

[0012] R=(V Hold -V hold )*R _low / V hold ;

[0013] Where R is the sum of the resistances of the upper electrode and the lower electrode; V Hold is the target holding voltage for the threshold-gated device; V hold is the holding voltage of the threshold selection functional layer; R _low It is the low-resistance state resistance of the threshold selection functional layer.

[0014] Optionally, the material of the upper electrode and the lower electrode includes at least one of C, Al, Co, Ni, Pt, Au, Ag, Cu, Wu, and Pd.

[0015] In a second aspect, the present application provides a threshold gating device designed based on the above-mentioned threshold gating device design method, comprising: a threshold gating functional layer, an upper electrode and a lower electrode; one end of the threshold gating functional layer is connected to the upper electrode, and the other end of the threshold gating functional layer is connected to the lower electrode;

[0016] The threshold gating functional layer has two states: a high resistance state and a low resistance state; when the voltage across the threshold gating functional layer exceeds the threshold voltage of the threshold gating functional layer, the threshold gating functional layer switches from the high resistance state to the low resistance state; when the voltage across the threshold gating functional layer in the low resistance state is lower than the holding voltage of the threshold gating functional layer, the threshold gating functional layer switches from the low resistance state to the high resistance state.

[0017] Optionally, when the threshold gating functional layer is in a high-resistance state, the threshold voltage of the threshold gating device is the same as the threshold voltage of the threshold gating functional layer.

[0018] Optionally, the material of the threshold gating functional layer includes OTS-type materials, conductive bridge-type materials and metal-insulator transition-type materials.

[0019] Optionally, the OTS-based materials include: a Se-based compound system, a Te-based compound system and a S-based compound system.

[0020] In a third aspect, the present application provides a dynamic memory, comprising: a threshold gating device and a capacitor connected in series; the threshold gating device is the above-mentioned threshold gating device; the end of the threshold gating device not connected to the capacitor is the input end of the dynamic memory; the end of the capacitor not connected to the threshold gating device is the output end of the dynamic memory.

[0021] Optionally, the lower limit of the write operation window of the dynamic memory is the threshold voltage of the threshold gating device, and the upper limit is the sum of the threshold voltage of the threshold gating device and the target holding voltage.

[0022] According to the specific embodiments provided in this application, this application discloses the following technical effects:

[0023] The present application provides a threshold gating device design method, a threshold gating device and a dynamic memory, wherein the holding voltage, low-resistance resistance and threshold voltage of the threshold gating functional layer are tested; the target holding voltage of the threshold gating device is determined according to the threshold voltage; the sum of the resistances of the upper electrode and the lower electrode of the threshold gating device is determined according to the holding voltage, low-resistance resistance and the target holding voltage of the threshold gating device; the material and film parameters of the upper electrode and the lower electrode are selected according to the sum of the resistances of the upper electrode and the lower electrode of the threshold gating device to design a threshold gating device that meets the target holding voltage. The present invention can predetermine the target holding voltage required for the threshold gating device, and then calculate the threshold gating device that meets the corresponding target holding voltage in combination with the holding voltage and low-resistance resistance of the threshold gating functional layer. Obviously, different target holding voltages of the threshold gating device can be achieved by the sum of the resistances of different upper and lower electrodes. Then, the target holding voltage of the threshold gating device can be adjusted by changing the sum of the resistances of the upper and lower electrodes, and a threshold gating device that meets different holding voltages can be designed. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0025] Figure 1 A schematic flow chart of a threshold gating device design method provided in Example 1 of the present application;

[0026] Figure 2 A schematic diagram of the structure of a threshold gating device provided in Example 2 of the present application;

[0027] Figure 3 The total holding voltage V of the control threshold gating device provided in Example 2 of the present application Hold Schematic diagram of the expression of ;

[0028] Figure 4 A schematic diagram of the change in current through the threshold gating device after a triangular wave is applied to both ends of the threshold gating device provided in Example 2 of the present application;

[0029] Figure 5In Example 1 provided for Embodiment 2 of the present application, after a triangular wave with a peak value of 6V and a duration of 240ns is applied to both ends of the threshold gating device, the current change simulation diagram of the threshold gating device and the holding voltage V of the threshold gating device are compared. Hold Schematic diagram;

[0030] Figure 6 In Example 2 provided for Embodiment 2 of the present application, after a triangular wave with a peak value of 6V and a duration of 240ns is applied to both ends of the threshold gating device, the current change simulation diagram of the threshold gating device and the holding voltage V of the threshold gating device are compared. Hold Schematic diagram;

[0031] Figure 7 A schematic diagram of the structure of a 1S1C dynamic memory composed of threshold selection devices provided in Example 3 of the present application.

[0032] Reference numerals: 1 - upper electrode; 2 - threshold selection functional layer; 3 - lower electrode. DETAILED DESCRIPTION

[0033] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0034] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0035] Example 1

[0036] like Figure 1 As shown, this embodiment provides a threshold gating device design method, including the following steps S1 to S4.

[0037] Step S1, testing the holding voltage V of the threshold selection functional layer 2 hold , low resistance resistor R _low and threshold voltage V th .

[0038] Step S2, according to the threshold voltage V th Determine the target holding voltage V for threshold-gated devices Hold Wherein, the target holding voltage V Hold Less than or equal to the threshold voltage V of the threshold selection functional layer 2 th .

[0039] Step S3, determining the sum of the resistances of the upper electrode 1 and the lower electrode 3 of the threshold gating device according to the holding voltage of the threshold gating functional layer 2, the low-resistance resistance and the target holding voltage of the threshold gating device.

[0040] Step S4, according to the sum of the resistances of the upper electrode 1 and the lower electrode 3 of the threshold gating device, select appropriate materials for the upper electrode 1 and the lower electrode 3, and then adjust the preparation process to obtain film parameters that meet the resistance requirements, so that the sum of the resistances of the upper electrode 1 and the lower electrode 3 meets the result calculated in step S3, and finally design a threshold gating device that meets the target holding voltage. Among them, the film parameters include the preparation parameters of the film, as well as the doping, thickness, element composition, etc. of the film itself, and the control means are conventional control means, the purpose of which is to make the film reach the target resistance.

[0041] As an optional implementation, the expression for determining the sum of the resistances of the upper electrode 1 and the lower electrode 3 of the threshold gating device is:

[0042] R=(V Hold -V hold )*R _low / V hold ;

[0043] Where, R is the sum of the resistances of the upper electrode 1 and the lower electrode 3; V Hold is the target holding voltage for the threshold-gated device; V hold is the holding voltage of the threshold selection functional layer 2; R _low is the low resistance state resistance of the threshold selection functional layer 2.

[0044] As an optional embodiment, the material of the upper electrode 1 and the lower electrode 3 includes at least one of C, Al, Co, Ni, Pt, Au, Ag, Cu, Wu, and Pd to adjust the sum of the upper and lower electrode resistances.

[0045] In this embodiment, a method for adjusting the holding voltage of a threshold gating device is provided, which expands the application scenarios of the threshold gating device. Specifically, the holding voltage V of the threshold gating device can be adjusted by adjusting the sum of the resistances of the upper and lower electrodes. Hold The adjustment breaks the view that "the holding voltage of the threshold selection device is an inherent characteristic of the threshold selection device and there is no way to adjust it". The adjustability of the holding voltage of the threshold selection device can improve DRAM performance.

[0046] Example 2

[0047] like Figure 2As shown, this embodiment provides a threshold gating device, which is designed based on the threshold gating device design method described in Example 1, including: a threshold gating functional layer 2, an upper electrode 1 and a lower electrode 3; one end of the threshold gating functional layer 2 is connected to the upper electrode 1, and the other end of the threshold gating functional layer 2 is connected to the lower electrode 3.

[0048] As an optional implementation, the material of the threshold gating functional layer 2 includes OTS-type materials, conductive bridge-type materials and metal-insulator transition-type materials.

[0049] As an optional implementation, the OTS-type material is composed of sulfur compounds, and its main material systems include three categories: Se-based compound systems, Te-based compound systems, and S-based compound systems.

[0050] The threshold gating functional layer 2 has two states: high resistance state and low resistance state; when the voltage across the threshold gating functional layer 2 exceeds the threshold voltage of the threshold gating functional layer 2, the threshold gating functional layer 2 switches from the high resistance state to the low resistance state; when the voltage across the threshold gating functional layer 2 in the low resistance state is lower than the holding voltage of the threshold gating functional layer 2, the threshold gating functional layer 2 switches from the low resistance state to the high resistance state. By using this characteristic of the threshold gating functional layer 2, the total holding voltage V of the threshold gating device can be adjusted by adjusting the sum of the resistances of the upper and lower electrodes. Hold , improving the operating window of its 1S1C dynamic memory.

[0051] Among them, the holding voltage V Hold =V hold +I hold *R, where R is the sum of the resistances of the upper electrode 1 and the lower electrode 3, Figure 3 shows the holding voltage V of the threshold gate device Hold There is a positive linear relationship between the sum of the resistances of the upper and lower electrodes, R. The greater the sum of the resistances of the upper and lower electrodes, the greater the effect on V Hold The more obvious the positive regulatory effect is. hold =V hold / R _low , where R _low is the low resistance value of the threshold selection functional layer 2. R value range 0≤R≤R max , where R max To make V Hold Equal to V th By adjusting the resistance of R, the holding voltage V of the threshold gate device can be adjusted. Hold Make adjustments.

[0052] Further preferably, the threshold gating device has a threshold voltage V TH, because when the threshold gating functional layer 2 is in a high-resistance state, the current passing through the threshold gating device is very small, so the voltage division between the upper electrode 1 and the lower electrode 3 can be ignored, so the threshold voltage V of the threshold gating device TH The threshold voltage V of the threshold selection functional layer 2 th Basically consistent, and not affected by the resistance of the upper electrode 1 and the lower electrode 3.

[0053] Apply a voltage across the threshold gate device such as Figure 4 The voltage waveform shown. At the initial moment, the applied voltage is less than the threshold voltage V of the threshold selection functional layer 2 th , the threshold selection functional layer 2 is in a high-resistance state, and a very small current flows through the circuit at this time. Figure 4 The figure shows that there is no current flowing through the circuit, ignoring the weak current in the circuit when the threshold gate functional layer 2 is in a high-resistance state; when the voltage rises to a value greater than the threshold voltage V of the threshold gate functional layer 2 th When the threshold gate function layer 2 switches from a high resistance state to a low resistance state, the current in the circuit suddenly increases and changes with the applied voltage; when the voltage drops to less than the holding voltage V of the threshold gate function layer 2 hold When , the threshold selection functional layer 2 switches from a low resistance state to a high resistance state, and the current in the circuit suddenly decreases, almost to the point where no current flows.

[0054] Since the applied voltage rises to V th There is approximately no current flowing through the previous circuit, so the total threshold voltage V of the threshold selection device TH The threshold voltage V of the threshold selection functional layer 2 th equal, that is, adjusting the sum of the resistances of the upper and lower electrodes will not affect V TH size; and when the applied voltage drops to V hold There is a large on-state current in the previous circuit, so the total holding voltage V Hold Equal to the holding voltage V of the threshold gate functional layer 2 hold The sum of the resistance voltage of the upper and lower electrodes, that is, V Hold It can be controlled by adjusting the sum of the resistances of the upper and lower electrodes. The specific control method is as follows:

[0055] When it is necessary to increase the holding voltage V of the threshold gate device Hold When the voltage across the threshold gating functional layer 2 is equal to V hold When the on-state current in the circuit is I hold At this time, the resistance voltage division of the upper and lower electrodes increases, making the total holding voltage V of the threshold selection device Hold Increase.

[0056] When it is necessary to reduce the holding voltage V of the threshold gate device Hold When the voltage across the threshold gating functional layer 2 is equal to V hold When the on-state current in the circuit is I hold At this time, the resistance voltage of the upper and lower electrodes decreases, making the total holding voltage V of the threshold gating device Hold Reduce.

[0057] The following example further illustrates V Hold It can be controlled by adjusting the sum of the resistances of the upper and lower electrodes.

[0058] Example 1:

[0059] A threshold gating device, wherein the threshold voltage of the threshold gating function layer 2 is 3V, the holding voltage is 1.2V, the on-state current under the holding voltage is 6mA, and the sum of the resistances of the upper and lower electrodes is adjusted to 100Ω. A triangular wave with a peak value of 6V and a duration of 240ns is applied to both ends of the threshold gating device: in the initial state, the threshold gating function layer 2 is in a high-resistance state, and the current passing through is very small. At this time, the voltage across the threshold gating function layer 2 is approximately equal to the applied voltage across the threshold gating device; when the voltage across the threshold gating function layer 2 increases to 3V, the threshold gating function layer 2 switches from a high-resistance state to a low-resistance state, the current increases, and the resistances of the upper and lower electrodes begin to divide; when Vs gradually decreases, so that the voltage across the threshold gating function layer 2 decreases to 1.2V, the on-state current passing through the circuit is 6mA, the upper and lower electrodes are divided by 0.6V, and the total holding voltage V of the threshold gating device is Hold =1.8V, such as Figure 5 shown.

[0060] Example 2:

[0061] A threshold gating device, wherein the threshold voltage of the threshold gating function layer 2 is 3V, the holding voltage is 1.2V, the on-state current under the holding voltage is 6mA, and the sum of the resistances of the upper and lower electrodes is adjusted to 200Ω. A triangular wave with a peak value of 6V and a duration of 240ns is applied to both ends of the threshold gating device: in the initial state, the threshold gating function layer 2 is in a high-resistance state, and the current passing through is very small. At this time, the voltage across the threshold gating function layer 2 is approximately equal to the applied voltage Vs across the threshold gating device; when the voltage across the threshold gating function layer 2 increases to 3V, the threshold gating function layer 2 switches from a high-resistance state to a low-resistance state, the current increases, and the resistances of the upper and lower electrodes begin to divide; when Vs gradually decreases, so that the voltage across the threshold gating function layer 2 decreases to 1.2V, the on-state current passing through the circuit is 6mA, the upper and lower electrodes are divided by 1.2V, and the total holding voltage V of the threshold gating device is Hold =2.4V, such as Figure 6 shown.

[0062] In the present embodiment, a threshold selection device is provided, and the holding voltage of the threshold selection device can be changed by adjusting the sum of the resistances of the upper and lower electrodes. Therefore, the required holding voltage of the threshold selection device can be flexibly adjusted according to needs, thereby realizing the controllability of the holding voltage of the threshold selection device, breaking the view that "the holding voltage of the threshold selection device is an inherent characteristic of the threshold selection device and there is no way to adjust it". The adjustability of the holding voltage of the threshold selection device can improve DRAM performance.

[0063] Example 3

[0064] like Figure 7 As shown, this embodiment provides a dynamic memory, including: a threshold gating device and a capacitor connected in series; the threshold gating device is the threshold gating device described in Example 2; the end of the threshold gating device not connected to the capacitor is the input end of the dynamic memory; the end of the capacitor not connected to the threshold gating device is the output end of the dynamic memory.

[0065] Among them, the lower limit of the write operation window of the dynamic memory is the threshold voltage of the threshold selection device, and the upper limit is the sum of the threshold voltage of the threshold selection device and the target holding voltage. Therefore, the write operation window of the dynamic memory can be increased by increasing the sum R of the resistances of the upper electrode 1 and the lower electrode 3.

[0066] The two examples in Embodiment 2 are used as examples to illustrate that the write window of the dynamic memory can be increased by increasing the sum R of the resistances of the upper electrode 1 and the lower electrode 3. A threshold gating device, the threshold voltage of the threshold gating functional layer 2 is 3V, the holding voltage is 1.2V, the on-state current is 6mA under the holding voltage, and the sum of the resistances of the upper and lower electrodes is 100Ω. A triangular wave with a peak value of 6V and a duration of 240ns is applied to both ends of the threshold gating device. When the applied voltage is gradually reduced so that the voltage across the threshold gating functional layer 2 is reduced to 1.2V, the on-state current passing through the circuit is 6mA, the upper and lower electrode voltage is 0.6V, and the total holding voltage V of the threshold gating device is 1.2V. Hold =1.8V. For the 1S1C dynamic memory composed of this threshold selection device, the lower limit of its write operation window is V TH , the upper limit is V TH +V Hold Therefore, the operating window of 1S1C dynamic memory is 1.8V.

[0067] Now adjust the sum of the resistances of the upper and lower electrodes to 200Ω, and still apply a triangle wave with a peak value of 6V and a duration of 240ns to both ends of the threshold gating device. When the applied voltage is gradually reduced, so that the voltage across the threshold gating functional layer 2 is reduced to 1.2V, the on-state current through the circuit is 6mA, the upper and lower electrodes are divided by 1.2V, and the total holding voltage V of the threshold gating device is Hold =2.4V. For the 1S1C dynamic memory composed of this threshold selection device, the lower limit of its write operation window is V TH , the upper limit is V TH +V Hold Therefore, the write operation window of the 1S1C dynamic memory is 2.4V; compared with the write operation window before the sum of the resistances of the upper and lower electrodes is adjusted, the space is increased by 0.6V.

[0068] In this embodiment, the holding voltage of the threshold selection device can be changed by adjusting the sum of the resistances of the upper and lower electrodes, and the write operation window of the 1S1C dynamic memory formed by the threshold selection device can be further improved, thereby improving the anti-interference ability of the dynamic memory to noise and voltage fluctuations, and also helping to improve the reliability and fault tolerance of the memory and reduce the bit error rate.

[0069] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0070] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A threshold gating device design method, characterized in that: include: Testing the holding voltage, low-resistance resistance and threshold voltage of the threshold selection functional layer; determining a target holding voltage of a threshold gating device according to the threshold voltage; Determine the sum of the resistances of the upper electrode and the lower electrode of the threshold gating device according to the holding voltage of the threshold gating functional layer, the low-resistance resistance and the target holding voltage of the threshold gating device; The material and film parameters of the upper electrode and the lower electrode are selected according to the sum of the resistances of the upper electrode and the lower electrode of the threshold gating device to design a threshold gating device that meets the target holding voltage.

2. The threshold gating device design method according to claim 1, characterized in that: The target holding voltage of the threshold gating device is less than or equal to the threshold voltage of the threshold gating functional layer.

3. The threshold gating device design method according to claim 1, characterized in that: The expression for determining the sum of the resistances of the upper electrode and the lower electrode of the threshold gating device is: R=(V Hold -V hold )*R _low / V hold ; Where R is the sum of the resistances of the upper electrode and the lower electrode; V Hold is the target holding voltage for the threshold-gated device; V hold is the holding voltage of the threshold selection functional layer; R _low It is the low-resistance state resistance of the threshold selection functional layer.

4. The threshold gating device design method according to claim 1, characterized in that: The material of the upper electrode and the lower electrode includes at least one of C, Al, Co, Ni, Pt, Au, Ag, Cu, Wu, and Pd.

5. A threshold gating device designed based on the threshold gating device design method according to any one of claims 1 to 4, characterized in that: include: A threshold gating functional layer, an upper electrode and a lower electrode; one end of the threshold gating functional layer is connected to the upper electrode, and the other end of the threshold gating functional layer is connected to the lower electrode; The threshold gating functional layer has two states: a high resistance state and a low resistance state; when the voltage across the threshold gating functional layer exceeds the threshold voltage of the threshold gating functional layer, the threshold gating functional layer switches from the high resistance state to the low resistance state; when the voltage across the threshold gating functional layer in the low resistance state is lower than the holding voltage of the threshold gating functional layer, the threshold gating functional layer switches from the low resistance state to the high resistance state.

6. The threshold gating device according to claim 5, characterized in that: When the threshold gating functional layer is in a high-resistance state, the threshold voltage of the threshold gating device is the same as the threshold voltage of the threshold gating functional layer.

7. The threshold gating device according to claim 5, characterized in that: The material of the threshold gating functional layer includes OTS-type materials, conductive bridge-type materials and metal-insulator transition-type materials.

8. The threshold gating device according to claim 7, characterized in that: The OTS materials include: a Se-based compound system, a Te-based compound system and a S-based compound system.

9. A dynamic memory, characterized in that: include: A threshold gating device and a capacitor connected in series; the threshold gating device is the threshold gating device described in any one of claims 5 to 8; the end of the threshold gating device not connected to the capacitor is the input end of the dynamic memory; the end of the capacitor not connected to the threshold gating device is the output end of the dynamic memory.

10. The dynamic memory according to claim 9, characterized in that: The lower limit of the write operation window of the dynamic memory is the threshold voltage of the threshold selection device, and the upper limit is the sum of the threshold voltage of the threshold selection device and the target holding voltage.

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