a team of researchers from technion (the israel institute of technology in haifa) has published a report about a new capacitor that features a metal-insulator-semiconductor (mis) diode structure and that has strong frequency dispersion, which makes it highly tunable using illumination, according to an article from the american institute of physics (aip). 

researchers at technion have developed a novel diode structure for tuning
capacitance with illumination. (wikimedia commons)

researchers believe that this device could enhance wireless capability for information processing and telecommunications. the mis diode structure opens the potential for optoelectronic circuits as light-sensitive capacitors in remote sensing circuits.

the mis diode structure is similar to a metal-insulator-metal (mim) diode that are common in today’s electronics and like mim diodes the new structure’s capacitance is independent of its supply voltage, which offers more predictable performance. the advantage for the mis diode structure is that its capacitance is tunable by light.

one of the researchers noted, “the excitation of trap states enhances the internal polarization. second, it increases the minority carrier density (due to photo generation) and reduces the depletion region width. this change modifies the capacitance.” 

the article explained, “the researchers created three mis structures, fabricated on a bulk silicon substrate, based on a multilayer dielectric stack, which consisted of a thin thermal silicon dioxide film and a hafnium oxide layer. the two layers were separated by strontium fluoride (srf₂) sublayers in which ferrum (fe, iron) or cobalt (co) nanoparticles were embedded.”

electronic polarization is caused by the fluoridation-oxidation process of the iron atoms, which causes a gradient in the in the valence state of iron ions on the active layer.

the article added, “four additional structures were prepared for comparison: two lacked the srf₂ sublayers and one of them was prepared without the iron film. the other two structures contained srf₂: one did not have cobalt and the second included a one-nanometer co layer. the comparison with other mis capacitors that contained the metal nanoparticles with or without the srf₂ sublayers led to the unequivocal conclusion that only devices consisting of the combination of fe and srf₂ turn the mis structure into a photo-sensitive mim-like structure.”

the research was recently published in the journal of applied physics. the abstract stated:

“this paper describes a metal-insulator-semiconductor (mis) capacitor with flat capacitance voltage characteristics and a small quadratic voltage capacitance coefficient. the device characteristics resemble a metal-insulator-metal diode except that here the capacitance depends on illumination and exhibits a strong frequency dispersion.

“the device incorporates fe nanoparticles (nps), mixed with srf₂, which are embedded in an insulator stack of sio₂ and hfo₂. positively charged fe ions induce dipole type traps with an electronic polarization that is enhanced by photogenerated carriers injected from the substrate and/or by inter nanoparticle exchange of carriers.

“the obtained characteristics are compared with those of five other mis structures: two based on fe nps, one with and the other without srf₂ sublayers. additionally, devices contain co nps embedded in srf₂ sublayers, and finally, two structures have no nps, with one based on a stack of sio₂ and hfo₂ and the other which also includes srf₂.

“only structures containing fe nps, which are incorporated into srf₂, yield a voltage independent capacitance, the level of which can be changed by illumination. these properties are essential in radio frequency/analog mixed signal applications.”