Important Organic TFT Parameters for Display Applications

When considering the usefulness of OTFTs, the transistor parameter that has received by far the greatest attention has been the field-effect mobility J,pE. It is commonly cited as a performance metric for comparing different OTFT materials and fabrication methods. But the field-effect mobility is only one of several important OTFT parameters. In this section, we discuss the field-effect mobility as a design parameter, and in following sections we discuss other important parameters. Other treatments of the application of OTFTs to active matrix displays have been reviewed in the literatme [2,3]. 

In the standard metal-oxide semiconductor field-effect transistor (MOSFET) drain-current equations, is a proportionality factor that relates the drain current /p, to the gate and drain voltages and V[)s, respectively the threshold voltage V, the channel width W and length L and the gate dielectric capacitance per unit area Cox. The standard drain-current equation for an -channel device in the linear region of operation - V) is [4] 

The field-effect mobility is distinct from the more physically fundamental carrier mobility, which is only one factor in the field-effect mobility. In this chapter, when we refer to the mobility, we are referring to the field-effect mobility, since we will not have occasion to refer to the carrier mobility. We often assume for simplicity that mobility can be treated as constant, but experimentally the mobility is found to depend on V s and V s- Therefore, for extracting mobility from measurements and for accurate simulations, mobility must be treated as a voltage-dependent quantity that is derived from small-signal measurements in the linear region, for very small drain voltages (Vps Vos-Vt), 

FIGURE 6.4,1 Measured voltage-dependent mobility in the saturation region for a penta-cene OTFT with Si02 gate dielectric. The mobility is plotted only for data points in the 

FIGURE 6.4.2 Measured linear-region drain-current data (cnrved line, = -0.1 V) and the tangent at the point of maximnm slope for the same pentacene OTFT as in Fignre 6.4.1. The tangent allows the threshold voltage to be calcnlated as +6.5 V. 

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