Threshold-Voltage Extraction Methods for Atomically Deposited Disordered ZnO Thin-Film Transistors

Abstract

In this paper, we present a threshold-voltage extraction method for zinc oxide (ZnO) thin-film transistors (TFTs). Bottom-gate atomic-layer-deposited ZnO TFTs exhibit typical n-type enhancement-mode transfer characteristics but a gate-voltage-dependent, unreliable threshold voltage. We posit that this obscure threshold voltage is attributed to the localized trap states of ZnO TFTs, of which the field-effect mobility can be expressed as a gate-bias-dependent power law. Hence, we derived the current–voltage relationship by dividing the drain current with the transconductance to rule out the gate-bias-dependent factors and successfully extract the reliable threshold voltage. Furthermore, we investigated the temperature-dependent characteristics of the ZnO TFTs to validate that the observed threshold voltage was genuine. Notably, the required activation energies from the low-temperature measurements displayed an abrupt decrease at the threshold voltage, which was attributed to the conduction route change from diffusion to drift. Thus, we conclude that the reliable threshold voltage of accumulation-mode ZnO TFTs can be determined using a gate-bias-dependent factor-removed current–voltage relationship with a low-temperature analysis.

Subthreshold Conduction of Disordered ZnO-Based Thin-Film Transistors

“…At the low-gate-bias regime ( V gs < 15 V), as the gate bias increased, it increased rapidly to ~10 13 states eV −1 cm 2 , whereas, at the high-gate-bias regime ( V gs > 15 V), it slowly increased to ~10 14 states eV −1 cm 2 . As reported elsewhere [ 39 ], these abrupt changes in the trap DOS of the ZnO TFTs are attributed to the conduction route change from diffusion to drift, of which the V gs of 15 V can be defined as the threshold voltage of the disordered ZnO TFTs. Notably, when the interface trap density ( D it ) is deduced using the relation of C it = q 2 D it [ 23 ] and compared with the DOS, as in orange in Figure 5 b, the crossover point of the 15 V can be observed, which is strongly regarded as the threshold voltage.…”

“…Furthermore, to gain insights into the subthreshold conduction of the disordered ZnO TFTs, we investigated and carefully analyzed the temperature-dependent current–voltage characteristics of the ZnO TFTs from 180 to 300 K. Figure 5 a shows the temperature-dependent transfer characteristics of the ZnO TFTs. By assuming that the gate-dependent activation energy ( E a ) is closely related to the energetic difference between the Fermi level and conductive states, the gate-dependent activation energy can be extracted using the Meyer–Neldel rule of I ( V gs ) = I 0 exp(− E a / kT ), as in the inset of Figure 5 a [ 22 , 39 ]. From the gate-dependent activation energy, the areal density of states, DOS, g ( E ) can be deduced using the relation of g ( E ) = qC i ( dE a / dV gs ) −1 , as in Figure 5 b [ 40 , 41 ].…”

Subgap states in aluminium- and hydrogen-doped zinc-oxide thin-film transistors

“…Thus, charge carriers in disordered semiconductor-based TFTs are thought to be mainly transported by the diffusion of carriers below the threshold voltage through the deep trap states, and by the drift of carriers above the threshold voltage through the shallow trap states. [50][51][52] Hence, if the deep trap states change to the subgap states through doping in our study, the charge transport near the threshold voltage will be enhanced, leading to significant threshold voltage shifts. However, to our best knowledge, these subgap states have been scarcely reported in ZnO, although they have been reported in indium-and gallium-doped ZnO (IGZO).…”