Continuously-Variable Programming in Ionic-Conducting Devices Displaying Differential Negative Resistance
Due to their ability to achieve a wide range of resistance states, ion-conducting devices are ideal for use in artificial neural networks  and in threshold logic gates where they have been both successfully simulated  and implemented at the circuit level . One of the potential types of ion-conducting device is a unipolar device that exhibits differential negative resistance, the IC-DNR device [3, 4]. This device has the most promise for successful application in systems requiring good control over the programmed resistance state since it can be programmed easily within a continuous range of resistances simply by using a varying voltage amplitude pulse with single polarity . In this work, we have characterized the programming conditions of an IC-DNR device comprised of layers of Ge40Se60 and Ag2Se. Electrical characterization tests were performed with an Agilent B1500A equipped with a fast waveform generation unit which allows for impedance matching during measurement. In a continuously variable device, such as the IC-DNR device, it is critical that device characterization is performed with a clean pulse containing minimal reflections (typically seen when a fast pulse is applied to a non-matched load). Since the reflected signals will also program the device, their presence in the test precludes accurate device characterization. Our results show that while the devices do not each achieve the same resistance state with the same programming conditions, a response that is typical of the amorphous materials used in these devices, they do exhibit repeatability in programming trends. As an example, a programming pulse may program a device to a lower resistance in each device, within a certain percentage of the original resistance state, but the starting and ending resistance values could be different. With the device resistance programming control exhibiting well-defined trends, the device has applicability in systems where fine control over resistance is required, but where application of multiple pulses to achieve that resistance are acceptable. Experiments performed on the IC-DNR devices include pulse width and amplitude programming tests, data retention, and endurance.