What does Mobility Say About TFT Performance?
As the equation in the page header shows, mobility is a measure of how easily a charge moves through a metal or semiconductor. Mobility is defined as the speed of a charge through the metal or semiconductor in proportion to the electric field energy generated to move the charge. Mobility is a key factor in defining the switching speed and drain-source current of a thin film transistor. Generally speaking, greater mobilities indicate the potential for scaling down the TFT footprint while maintaining performance, or increasing performance at a given physical footprint.
While there are many types of mobility defined at the material level for specific operating conditions, at the transistor level field effect mobility (µ(FE)) is the accepted standard performance metric.
In this equation, dI(ds)/dV(g) represents the transconductance (Gm) of the TFT; L/W defines the semiconductor channel dimensions; C(i) is the accumulation capacitance at peak Gm; and V(ds) is the drain-source voltage used to define Gm. Gm is a direct function of the electric field energy stored in the gate capacitance (the E term in 𝜐/E).
The thin film transistor industry has focused on the 𝜐 term in mobility to improve TFT performance. Primarily this manifests in the transition from amorphous silicon and low-temperature polysilicon semiconductors to amorphous compound metal oxide semiconductors. Researchers have focused on alloys such as indium gallium zinc oxide (IGZO), indium zinc oxide (IZO) and variations on these materials.
Before Amorphyx, work on improving amorphous metal oxide TFT mobility ignored the impact of the E term in the mobility equation. Material scientists assumed that improving the semiconductor’s 𝜐 was the only path to increased field effect mobility.
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