A theoretical model based on the band bending theory was developed for explaining the field-emission mechanism of wide-band gap semiconductors (WBGSs). It was shown that the maximum value of the band bending, which is nearly linearly proportional to the band gap of WBGSs, may amount to a few eV. Furthermore, the calculated field-emission energy distribution combined with the band bending analyzed on cubic boron nitride (c-BN) as typical one of WBGSs, indicated that the electron emission originates from the conduction band minimum resulting from the band bending. These results present a perspective to explain the field-emission mechanism, in which it is considered that the band bending, as well as the negative electron affinity, is of equal importance to the excellent field emission performances of WBGSs.

Negative differential conductance (NDC) at 300 K in n+-nn+-GaAs/AlAs superlattice structures biased perpendicularly to the layers is demonstrated, and shown to be strongly enhanced at microwave frequencies close to the inverse transit time of electrons. The deduced electron velocities are in fair agreement with those independently determined in undoped superlattices where NDC was inhibited by the electric field nonuniformity. From the analysis of the experimental data, we show that NDC is a bulk superlattice effect, not related to ‘‘quantum defects,’’ e.g., enlarged barriers.

There has been a considerable interest on forming ambipolar organic thin film transistors (OTFTs) because such devices are advantageous for integrated circuits, such as lower power consumption, design and fabrication simplification, and better immunity. Most recently, Shi et al. observed a substantial mobility improvement in ambipolar OTFTs based on the heterojunction formed between copper-hexadecafluoro-phthalocyanine (F 16 CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T). Specifically, the hole and electron mobility are improved by 3 and 12 folds from the bulk values, respectively. We examined the interface formation between F 16 CuPc and BP2T using ultraviolet photoemission (UPS) and inverse photoemission spectroscopy (IPES). It is observed that in F 16 CuPc/BP2T the heterojunction is characterized by band bending in both materials, while in BP2T/F 16 CuPc the band bending is confined in BP2T only. For F 16 CuPc/BP2T, the band bending of BP2T and F 16 CuPc are 0.40 and 0.35 eV, respectively. The band bending region is ~15 nm in both materials, from which the Debye lengths of the materials can be deduced. The combination of the band bending and finite Debye lengths offers an explanation to the observed improvement and thickness dependence of the mobility in OTFTs based on such heterojunctions.