Electrically variable or programmable capacitors based on the unique properties of thin perovskite films are undergoing development. These capacitors show promise of overcoming two important deficiencies of prior electrically programmable capacitors:
- Unlike in the case of varactors, it is not necessary to supply power continuously to make these capacitors retain their capacitance values. Hence, these capacitors may prove useful as components of nonvolatile analog and digital electronic memories.
- Unlike in the case of ferroelectric capacitors, it is possible to measure the capacitance values of these capacitors without changing the values. In other words, whereas readout of ferroelectric capacitors is destructive, readout of these capacitors can be nondestructive.
A capacitor of this type is a simple two-terminal device. It includes a thin film of a suitable perovskite as the dielectric layer, sandwiched between two metal or metal oxide electrodes (for example, see Figure 1). The utility of this device as a variable capacitor is based on a phenomenon, known as electrical-pulse-induced capacitance (EPIC), that is observed in thin perovskite films and especially in those thin perovskite films that exhibit the colossal magnetoresistive (CMR) effect. In EPIC, the application of one or more electrical pulses that exceed a threshold magnitude (typically somewhat less than 1 V) gives rise to a nonvolatile change in capacitance. The change in capacitance depends on the magnitude duration, polarity, and number of pulses. It is not necessary to apply a magnetic field or to cool the device below (or heat it above) room temperature to obtain EPIC. Examples of suitable CMR perovskites include Pr1–xCaxMnO3, La1–xCaxMnO3, La1–xSrxMnO3, and Nb1–xCaxMnO3.
Figure 2 is a block diagram showing an EPIC capacitor connected to a circuit that can vary the capacitance, measure the capacitance, and/or measure the resistance of the capacitor. A pulse generator applies voltage pulses to change the capacitance. If desired, after each pulse, the capacitance and resistance can be measured by use of an inductance-capacitance-resistance multimeter or an impedance/ gain analyzer. Also if desired, the DC resistance can be measured by applying a current of ≈1 μA and measuring the resulting voltage drop between the electrodes by use of a high-internal-resistance voltmeter. The magnitude of the AC test potential applied by the multimeter or analyzer and/or the magnitude of the DC test potential is kept below 50 mV — well below the threshold magnitude — so as not to change the capacitance unintentionally.
The threshold potential depends on a number of factors, including the composition and thickness of the perovskite film and the details of the process used to fabricate the device. The change in capacitance caused by a given pulse can be wholly or partly reversed by reversing the polarity of the pulse: that is, a pulse with one polarity causes the capacitance to decrease, and a pulse of the opposite polarity causes the capacitance to increase. The sign of the change in capacitance in relation to polarity of a pulse depends on the aforementioned factors and on additional factors, including the capacitance- change history of the device, the amplitude and duration of the pulse. After each change, the capacitance value is stable: It remains the same after repeated measurements using a signal much smaller than a capacitance-changing pulse.