Gate voltage dependent mobility

Access to contact resistance was first sought by modeling. Figure 1.11 shows equivalent circuits used for that purpose. Note that the bottom circuit includes head-to-toe diodes to account for non-linear contact resistance [35]. The model developed by Necliudov and coworkers also assumed a gate voltage-dependent mobility (this point will be discussed in more detail below). 

For an ohmic contact resistance (Vc = RC1d) and d I, Eq. (16) reduces to Eq. (14). While the two previous analyses [35, 36] assumed constant contact resistance and gate voltage-dependent mobility, Street and coworkers make the assumption that the mobility is constant and find a non-ohmic contact resistance that varies with gate voltage. 

Charge transport in the accumulation channel is described by the percolation model [24] based on thermally activated tunneling of holes between localized states in an exponential density of states, described in Section 13.2.2. In the accumulation regime this Variable Range Hopping (VRH) model yields a gate-voltage dependent field-effect mobility of the form ... 

To estimate the transistor requirements for our display, we make use of a transistor model (Detcheverry 2000) that includes a gate-voltage-dependent field-effect mobility ... 

G. Horowitz, M. E. Hajlaoui, and R. Hajlaoui, Temperature and gate voltage dependence of hole mobility in polycrystalline oligofhiophene thin film transistors , Journal of Applied Physics 87, 4456 (2000). 

The following expression is obtained for the temperature- and gate voltage-dependent mobility... 

An important outcome of the MTR model is that in the case of an energy distributed DOS, mobility is gate voltage dependent. The mechanism at work is schematically pictured in Figure 2.2.9. 

FIGURE 2.2.9 Gate voltage dependent mobility indnced by an energy distributed density of traps. 

It is worth pointing out that in transistors made with single crystal, the mobility is found to be very seldom gate voltage dependent [35], which indirectly confirms that the Vq dependence originates from localized levels associated with chemical and physical defects. 

Gilles Horowitz, Riadh Hajlaoui, Denis Fichou, and Ahmed El Kassmi. Gate voltage dependent mobility of oligothiophene field-effect transistors. Journal of Applied Physics, 85(6) 3202-3206, 1999. 

Sensitivity of an OFET Sensor Gate Voltage Dependence and Contributions of Mobility and Threshold Voltage Changes... 

Here, Ob is a prefactor for the conductivity, a an effective overlap parameter that intervenes in the hopping of charges from site to site and Be the critical number for the onset of percolation, q is the elemental charge and s the permittivity of the semiconductor. A major outcome of the model is that it predicts a gate voltage dependent mobility that follows a power law, a behavior that has been indeed observed in polymeric transistors. 

Gate Voltage Dependent Mobility and Parameter Extraction... 

Both the hopping transport and MTR model predict a gate voltage dependent mobility. This has dramatic consequences on parameter extraction, because Eqs. (6) and (7) are no longer valid. For this reason, it is of importance to recall here a few rules that should be followed for a safe parameter extraction ... 

It should always be kept in mind that in the saturation regime, the potential in the channel substantially varies from source to drain (see Eqs. 1 and 3). A direct consequence of gate voltage dependent mobility is therefore that parameters extracted in this regime correspond to uncontrolled average values. For this reason, extraction in the linear regime should always be preferred. 

It is also worth pointing out that, once the threshold voltage is known, the gate voltage dependent mobility can be safely extracted from Eq. (18) without any further derivation that would introduce additional uncontrollable terms [22]. 

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