Based on the classical circuit theory, there are four basic circuit physical quantities, i.e., current (i), voltage (v), charge (q), and magnetic flux (o). According to these four basic physical quantities, six mathematical relationships can be derived theoretically, while defining three basic circuit components (resistance R, capacitance C, inductance L). 1971, Prof. Tsai Shao-tang, based on the theoretical derivation of the relationship between the four basic electrical physical quantities of voltage, current, charge and magnetic flux, put forward the fourth type of basic circuit components - Memristor ( Memristor, which represents the correlation between magnetic flux and electric charge.
Fig:Relationship between the four passive components and between the four electrical variables
Structural Characteristics of Memristors
Memristor is a two-terminal device and has a simple Metal/Di-electric/Metal "sandwich" structure, as shown in the figure below, generally consists of a top electrode, an insulating dielectric layer and a bottom electrode. The upper and lower metal layers are used as electrodes, the upper metal layer is used as the top electrode and the lower metal layer is used as the bottom electrode, the metal is usually a traditional metal monomer, such as Ni, Cu, etc. The middle dielectric layer is usually composed of binary transition metal oxides, such as HfO2, WOx, etc., and it can also be made up of some complex structural materials, such as IGzO, etc., and these dielectrics generally have high impedance.
Its expression formula for d = M (q) d q, where M (q) for the memory resistance value, said the magnetic flux () with the cumulative charge (q) rate of change, and the resistance has the same magnitude. The difference is that the internal physical state of the ordinary resistor does not change, its resistance value usually remains unchanged, while the resistance value of the memory resistor is not a fixed value, it is associated with the magnetic flux, current has a certain degree of correlation, and electrical excitation stops, the resistance value will not return to the initial value, but to stay in the previous value, i.e., with the characteristics of the "memory resistance".
Figure:Internal diagram of the amnesia structure
Resistance mechanism and material properties of memristors
Memristive devices have two typical resistance states, high resistance state (HRS) and low resistance state (LRS), the high resistance state has a very high resistance value, usually a few kΩ to a few MΩ, and the low resistance state has a lower resistance value, usually a few hundred Ω. Initially, i.e., without any electrical excitation, the memristive device is in a high-resistance state, and under electrical excitation, it will switch between the two resistance states. For a new memristor device, before switching between the high and low resistive states, it needs to undergo an electrical activation process, which is usually characterized by high voltage and current limitation to prevent the device from breakdown. Memristor from high resistance to low resistance state of the transition for the set (SET) process, from low resistance to high resistance state of the transition for the reset (RESET) process. When the polarity of the voltage applied to the SET process and RESET process is the same, it is called unipolar resistive behavior, and when the polarity of the voltage applied to the SET process and RESET process is different, it is called bipolar resistive behavior.
Fig:Unipolar resistive behavior and bipolar resistive behavior
The selection of amnesia materials is an extremely important step in the construction of amnesia devices, and its material system usually includes dielectric layer materials and electrode materials, and different combinations of the two can make the amnesia device have different resistance mechanisms and performances. Since the model of TiO2-based memristor was proposed by HP Labs, more and more new materials have been found to be suitable for memristors, mainly including organic materials, oxide materials, sulfur compounds, and electrode materials with different activities.
Table:Comparison of typical performance parameters of amnesia with different dielectric materials
At present, the metals that can be used as electrode materials for memristors are generally divided into two categories: one is metal materials, including active metals Cu, Ag, Ru, etc., inert metals Pt, Pd, Au, W, etc.; the other is compound materials, including oxides such as SrRuO3, LaAlO3, ITO, IZO, etc., and nitrides such as TaN, TiN, etc. The electrode materials assembled based on different electrode materials often have different resistive mechanisms and electrochemical properties. Memristors assembled based on different electrode materials often have different resistance mechanisms and electrochemical properties.
Fig: Model diagram of Ru/TazOs/Pt amnistors in different resistance states and I-V curves at different temperatures
As a kind of resistive switch, the size of the memristor can be reduced to less than 2 nm, the switching speed can be controlled within 1 ns, the switching times can be more than 2 × 107, in addition to the lower operating power consumption compared with the existing electronic components. Memristor simple Metal/Dielectric/Metal structure, as well as low operating voltage, and with the traditional CMOS process compatibility and many other advantages, has been applied to a variety of fields, digital circuits, analog circuits, artificial intelligence and neural networks, memory and other areas play an important role. The high and low resistance of the device can be used to represent the binary "0" or "1", the transition time of different resistance states is as small as nanoseconds, the low operating voltage leads to low power consumption, and relative to the MOS structure, it is not subject to the limitations of the characteristics of the size, which is very suitable for use as a high-density The memory is also often referred to as resistive variable memory (RRAM) because it is not constrained by feature size compared to MOS structures.
Figure:Typical Amnesia Picture
Table:Parameter benchmarking of amnesia under development against conventional memories
Current-voltage characteristics and classification of memristors
The resistive behavior of the memristor is mainly reflected in its I-V curve diagram. Memristor devices composed of different kinds of materials differ in many details and can be classified into two kinds according to the change of resistance value with the change of applied voltage or current, which are linear memristor LM (linear memristor) and non-linear memristor NLM (non-linear). memristor).
Linear memristor voltage or current will not occur sudden change, that is, its resistance value with the change of the applied electrical signal is continuously changed. Linear memristors are bipolar devices, i.e., when the input signal is positive, the resistance value decreases, and when the input signal is negative, the resistance value increases.
Fig:Schematic diagram of I-V characteristic curve of amnesia at different frequencies
Nonlinear memristor has a good threshold characteristics, it exists a critical voltage, the input voltage does not reach the critical voltage, the resistance value is basically unchanged, the current through the device also changes little, when the input voltage reaches the critical voltage, the resistance value will be changed abruptly, the current flowing through the device will undergo a drastic change (increase or decrease). Based on the polarity of the voltage added in the process of setting and resetting, the nonlinear memristor is divided into unipolar device UM (Unipolar Memristor) and bipolar device BM (Bipolar Memristor).
Fig.:Schematic diagram of I-V curve of the device
Memristor basic performance research test
Evaluation of memristive devices generally includes DC, pulse and AC characterization tests to analyze the memristive characteristics of the device under the corresponding DC, pulse and AC effects, as well as measurements of non-electrical characteristics such as retention and stability of the memristive device. Generally, the main tests are shown in the table below.
DC l-V Characterization
Different polarity, different size of the voltage (current) excitation will make the memory resistor resistance changes, DC l-V characteristics directly reflect the device in different voltage (current) excitation resistance changes, is the basic means to characterize the electrical properties of the device. Through the DC characteristics test curve can be initially studied memory resistor device resistance characteristics and threshold voltage/current characteristics, and observe its l-V, R-V and other characteristics curve.
AC l-V and C-V Characterization Tests
Since the resistance value of ideal memristor varies with the change of the charge flowing through it, the traditional DC I-V scanning is used to test the output with a step-like signal, and during the DC characteristic test, its inrush current and inrush pulse produce a large change in the instantaneous charge flowing through the memristor, and the resistance value has a large impact, so the l-V curve derived from the traditional DC scanning does not truly reflect the characteristics of the memristor.
Pulse Characterization and Holding Power Test
The pulse characterization of the memristor includes the testing of the multi-resistive state characteristics, resistive state switching rate and switching amplitude, and resistive state switching durability of the test samples.
The multi-resistance characteristics characterize the multi-resistance characteristics of the memristor in different operation modes, which directly reflects the non-linear resistance characteristics of the memristor. Resistive switching rate and switching amplitude characterize the ease of switching of the memristor in different resistive states. Keeping the excitation pulse amplitude certain, the smaller the minimum pulse width that can make the memristor resistive state change, the higher its resistive switching rate is, and vice versa, the lower it is; Keeping the excitation pulse width certain, the smaller the minimum pulse amplitude that can make the memristor resistive state change, the easier it is to change the memristor resistive state. Resistive state switching durability, by selecting the appropriate pulse, measuring the memory device under the action of the pulse resistive state switching back and forth a number of times, the size of this parameter reflects the device's resistance to change the stability.
Memristor Basic Performance Test Solution
The whole set of test system is based on Purcell S/P/CP series of high-precision digital source meters (SMU), together with the probe stage, low-frequency signal generators, oscilloscopes, and special host computer software, etc., which can be used for basic parameter testing of memristors, medium-speed impulse performance testing, and AC characterization, and is suitable for the research on the new material system and the special cyber-physical mechanism.
Purcells high precision digital source meters (SMUs) play an extremely important role in semiconductor characterization and measurement. It has higher accuracy than ordinary ammeters and voltmeters, and is highly sensitive to weak voltage and small current signals. In addition, as the requirements for sensitivity, speed, remote voltage detection and four-quadrant output in the measurement process continue to increase, the traditional programmable power supply is difficult to cope with. Purcell S/P/CP series of high-precision digital source meter (SMU) for memory resistor as an excitation source to generate voltage or current scanning test signals, and real-time test samples corresponding to the current or voltage feedback value, combined with a special test software, you can real-time output of DC or pulse l-V characteristic curve.
S Series High Precision DC Source Meter
S series source meter is the first localized source meter with high precision, large dynamic range and digital touching, which is built by Purcells for many years, integrating voltage and current input/output and measurement, with the maximum voltage of 300V and the maximum current of 1A, and supporting four-quadrant operation, which is suitable for the DC l-V characteristic test in the scientific research and testing stage of memory resistor.
Table:Main technical specifications of Purcell S series source meters
P Series High Accuracy Pulse Source Meter
Р Series Pulse Source Meter is a new high-precision, large dynamic, digital touch source meter on the basis of DC source meter, bringing together voltage, current input and output and measurement and other functions, the maximum output voltage up to 300v, the maximum pulse output current up to 10A, support for four-quadrant work.
Table:Main technical specifications of Purcell P series source meter
CP Series Pulse Constant Voltage Source
Purcell CP series pulse constant voltage source is a narrow pulse width, high precision, wide range plug-in pulse constant voltage source introduced by Wuhan Purcell Instrumentation. The device supports narrow pulse voltage output, and synchronization of output voltage and current measurement; support for multi-device triggering to achieve the device's pulse l-V scanning, etc.; support for the output pulse timing adjustment, can be output complex curves. Its main features include: large pulse current, up to 10A; narrow pulse width, as low as 100ns; support for DC, pulse two voltage output modes; support for linear, logarithmic, and customized scanning mode. The product can be applied to memory resistor and material research and testing.
Figure:CP Series Pulse Constant Voltage Source
Table:CP Series Pulse Constant Pressure Source Main Specifications
Wuhan Purcells has been focusing on the development of electrical performance test instruments and systems in the field of power devices, RF devices, memristors and third-generation semiconductors. Based on the advantages of core algorithms and system inheritance and other technological platforms, Purcells has taken the lead in independently developing high-precision digital source meters, pulsed source meters, pulsed high-current source meters, high-speed data acquisition cards, pulsed constant-voltage sources and other instrumentation products, as well as the complete set of test systems. These products are widely used in the scientific research and testing of various cutting-edge materials and devices. Purcell offers a variety of different configurations to meet different customer needs.
For more information about the system construction program and test line connection guide, welcome to call us!
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