On the threshold voltage and channel conductance of polycrystalline silicon thin-film transistors

Abstract

A model for the grain-boundary barrier height of undoped polycrystalline silicon thin-film transistors is developed based on a rodlike structure of the grains with a square cross section and a Gaussian energy distribution of the trapping states at the grain boundaries. An analytical expression for the threshold voltage is derived in terms of the distribution parameters of the grain-boundary trapping states, the grain size, and the gate oxide thickness. Comparison between the developed model and the experimental drain current versus gate voltage data has been made and excellent agreement was obtained. The key parameters affecting the threshold voltage and the channel conductance of the transistor were investigated by computer stimulation. The threshold voltage is mainly affected by the grain size and the gate oxide thickness. For the improvement of the channel conductance, besides the passivation of the grain-boundary trapping states, the increase of the grain size and mainly the scaling down of the gate oxide thickness are the key factors.

Explicit calculation for grain boundary barrier height in polysilicon TFTs based on quasi-two-dimensional approach

“…4, the grain size L g has significant effects on the potential barrier. Threshold voltage determined as the gate voltage at which the peak of w b locates [7,16], increases with deceasing L g . Furthermore, in the subthreshold region, when L g is increased, the barrier height increases as well as its peak, because y d ¼ L g =2 and more trapped charges are needed for equilibrium according to (6).…”

“…The minimum is solved by (12) when V gs ¼ V th . To roughly estimate the maximum, we solve it using (21) by setting N D % 0 and V gs approaching its maximum, where the deep-state density in (16) is neglected and the solution of (21) can be obtained without using w gb00 . It can be clearly seen from Fig.…”

“…5, it is clear that the calculated results are in good agreement with the measured ones in the above-threshold region. However, in the subthreshold region, mainly due to the influence of the leakage current, it is difficult to measure the experimental GB barrier height [16].…”

A new analytical threshold voltage model for the doped polysilicon thin-film transistors

“…Furthermore, it needs to introduce a ''critical" grain size to differentiate between large grain size and small grain size. Based on a Gaussian distribution of DOS, Dimitriadis and Tassis [7] defined the threshold voltage as the gate voltage when the barrier height is maximum. However, empirical expressions are yet introduced to take into account the grain size and the gate oxide thickness; and the substrate doping concentration is still not included in the model.…”

A New Observation of the Elliot Curve Waveform in Charge Pumping of Poly-Si TFTs

“…First, the poly-Si TFT channel consists of many grains. Their V tp and V tn depend on the grain size and D t and vary in several volts [15]. Generally, the mentioned device V tp (or V tn ) is associated with the accumulation (or inversion) of the entire channel, i.e., close to the maximum V tp (or V tn ) among all grains.…”