Gate dependent mobility

If conducted in the linear regime, the details of both the gate dependent mobility and contact resistance can be deduced in addition to the aforementioned parameters. This will be discussed later. 

Contributions of Gate Dependent Mobility and Contact Resistance... 

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. 

Using Eq. (1) as the basic definition for field-effect mobility, it was found that in organic FETs, as in a-Si FETs, the mobility depends on the induced charge [1], The Qind dependence is experimentally observed as a gate-bias dependent mobility, because ... 

OFETs with spin coated film of H2TPP from a chloroform solution as active semiconductor layers, Al as gate electrode, and Cd as source-drain electrodes were fabricated by Checcoli and co-workers in 2003 [100]. This device exhibited zero-bias hole transfer mobility of 0.007 cm2 V-1 s-1 with a threshold of —7.5 V and field-dependent mobility, as high as 0.012 cm2 V-1 s-1. 

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

Figure 14-25. Arrhenius plot of the temperature-dependent mobility of 8T evaporated film Data were recorded at various gate voltages and corrected for the contact series resistance (taken from Ref. [124]).

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

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... 

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. 

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]. 

Here, is an effective overlap parameter that characterizes the tunneling of chaiges from one site to the other (it has the same meaning as a in Eq. (14.60)). T0 is the characteristic temperature of the exponential distribution and a0 and Be are adjustable parameters connected to the percolation theory. Bc is the critical number of bonds reached at percolation onset. For a three-dimensional amorphous system, Bc rs 2.8. Note that the model predicts a power law dependence of the mobility with gate voltage. 

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