Drain conductance

Two of the most important FET configurations make use of a-Si H as semiconductor material for the source - drain conduction channel. One uses a c-Si silicon substrate as gate material (Neudeck and Malhotra, 1975,1976 Matsumura and Nara, 1980 Thompson et al., 1982 Powell et al., 1981 Hayama and Matsumura, 1980 Matsumura et al., 1980 Abdulrida and Allison, 1983), the other uses glass only as a supporting substrate (Le-Comber et al., 1979, 1981 Tuan et al., 1982 Matsumura et al., 1981 Ishibashi and Matsumura, 1982 Lloyd et al., 1983 Nara and Matsumura, 1982 Matsumura and Hayama, 1980), and both employ the a-Si H technology for the semiconductor channel. Merely to give an idea of the differences, schematics of the two types of devices are shown in Fig. 15a,b. The transfer characteristics are reported and discussed in the previously mentioned literature. 

Assuming that the drain current /p is reduced due to the voltage drop over the contact resistance Rq, an estimate for the contact resistances and the mobility in the channel region can be obtained from drain conductance and transconductance g = [35]. Based on a drain current according... 

A polymer FET sensor comprises three terminals with the analyte solution directly contacting the thin gate dielectric to form the gate contact, which gates the source-drain conductivity. 

In the case of a phototransistor, the gate contact is transparent to light. This allows photons to pass through the gate and dielectric layer into the semiconductive layer below. The photons can generate hole-electron pairs which split and contribute to current flowing between the source and drain, thereby modulating the source-drain conductivity. 

For small-signal circuits the output resistance Vg of the MOSFET (Malik, 1995 Sedra and Smith, 1991) is important in limiting the gain of amplifiers. This resistance is related to the small-signal drain conductance... 

Fig. 13 Measured output characteristics (a) and drain conductance (b) for a P3HT transistor with L = 1 pm and w = 500pm and the bUayer contact as drain and interchanged contacts. Figure taken from [55]...

Fig. 14 Simulated output characteristics and drain conductance (a) and potential profiles from source to drain 1 nm below the interface to the insulator for different drain voltages (b) for Vos = —6 V and different source- and drain work functions...

In Fig. 16 the simulated output characteristics and drain conductances are depicted at a gate voltage Vgs = -6 V for different contact conditions. It can be seen that a nonlinear drain current increase and the resulting maximum of the drain conductance at finite drain voltage occur always if at least one contact has a work function

drain current is lower when the double layer contact acts as source. From these results one can conclude that the low chromium work function causes the nonlinear increase of the drain current. But, in contrast to the measurements, the maximum of the drain conductance occurs at almost the same drain voltage for the double layer contact as source or as drain. Additional simulations... 

The characteristics of the short-channel transistor are shown in Fig. 19. The transfer characteristics are depicted in Fig. 19a and the output characteristics and the drain conductance gd in Fig- 19b. Due to the surface modification, the hole mobility is enhanced in the channel (i.e., parallel to the interface) and it is lower perpendicular to the interface which is not of interest for the transistor. From the transfer characteristics, the mobility in the channel has been determined in both the active and the saturation regions. The value extracted from the active region is 1.6 X 10 cm V s at a drain voltage of -IV. From the saturation region one obtains 2.8 x 10 cm V s at —8V. The extraction of different... 

Fig. 19 Transfer characteristics on a Hnear and logarithmic scale for different drain voltages and gate voltage sweep directions (a) and output characteristics and drain conductance for different gate voltages (b) for the TPD(4M)-MEH-PPV transistor with L = 1 pm and w = 250pm...

For comparison, one can write the drain conductance in the linear region as... 

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