Drain-source current

The carriers in tire channel of an enhancement mode device exhibit unusually high mobility, particularly at low temperatures, a subject of considerable interest. The source-drain current is carried by electrons attracted to tire interface. The ionized dopant atoms, which act as fixed charges and limit tire carriers mobility, are left behind, away from tire interface. In a sense, tire source-drain current is carried by tire two-dimensional (2D) electron gas at tire Si-gate oxide interface. 

In a MESFET, a Schottky gate contact is used to modulate the source-drain current. As shown in Figure 14-6b, in an //-channel MESFET, two n+ source and drain regions are connected to an //-type channel. The width of the depletion layer, and hence that of the channel, is modulated by the voltage applied to the Schottky gate. In a normally off device (Fig. 14-9 a), the channel is totally depleted at zero gate bias, whereas it is only partially depleted in a normally on device (Fig. 14-9 b). 

The operation principle of these TFTs is identical to that of the metal-oxide-semiconductor field-effect transistor (MOSFET) [617,618]. When a positive voltage Vg Is applied to the gate, electrons are accumulated in the a-Si H. At small voltages these electrons will be localized in the deep states of the a-Si H. The conduction and valence bands at the SiN.v-a-Si H interface bend down, and the Fermi level shifts upward. Above a certain threshold voltage Vth a constant proportion of the electrons will be mobile, and the conductivity is increased linearly with Vg - Vih. As a result the transistor switches on. and a current flows from source to drain. The source-drain current /so can be expressed as [619]... 

Source air pollution sampling, 26 673 Source-based macromolecular nomenclature, 17 403 Source-drain current, 22 136 Source gases, 13 456... 

Replacing the respective variables in Eq. (4.3) using the Eqs. (4.5), (4.6), (4.7) and (4.8), a temperature-dependent MOS transistor model is obtained. This temperature-dependent model provides a term for the source-drain current depending on the source-gate voltage, the source-drain voltage, and the temperature. 

The last step in the construction of the MOSFET-heater model includes the description of an appropriate heating process. Due to the source-drain current flow, the membrane is heated by resistive Joule heating in the channel region. By assuming that all electric power dissipated in the device is converted into heat, the corresponding heating power is ... 

Combining Eq. (4.9) with the lumped-model Eq. (3.24), one gets an expression for the temperature on the membrane depending on the source drain-current ... 

For correct function of the ISFET, a sufficiently large gate voltage, Vq, must be applied between the leads to the reference electrode and to the substrate, so that a sufficiently large potential difference is formed between the surface and the interior of the substrate for formation of the n-type conductive channel at the insulator/substrate interface. This channel conductively connects drain 1 and source 2, which are connected with the substrate by a p-n transition. On application of voltage Vj between the drain and the source, drain current /p begins to pass. Under certain conditions the drain current is a linear function of the difference between Vq and the Volta potential difference between the substrate and the membrane. 

A polyacetylene field-effect transistor has been described622 but the response time is slow, apparently because the carrier mobility is low. An FET has been made from polythiophene but source-drain currents were less than 20 nA for drain voltages up to 50 V. The hole mobility was very low, calculated to be 2 x 10 5 cm2 V-1 s 1 623). 

The electrical current flows from the source, via the channel, to the drain. However, the channel resistance depends on the electric field perpendicular to the direction of the current and the potential difference over the gate oxide. Should this surface be in contact with an aqueous solution, any interactions between the silicon oxide gate and ions in solution will affect the gate potential. Therefore, the source-drain current is influenced by the potential at the Si02/aqueous solution interface. This results in a change in electron density within the inversion layer and a measurable change in the drain current. This means we have an ion-selective FET (an ISFET), since the drain current can be related to ion concentration. Usually these are operated in feedback mode, so that the drain current is kept constant and the change of potential compared to a reference electrode is measured. 

The n+ underlay was used with the source and drain contacts to provide good electron-injecting properties. This caused the reproducibility of the contacts to increase, and the ON current obtained in this way increased significantly. For example, devices with n+ contacts consistently produced source-drain currents a factor of about three higher than the best currents obtained without the n+ layers. 

Figure 10 illustrates the short-term dc stability of the devices (Mackenzie et al, 1983). The output characteristics of one FET are shown for 10 successive scanning cycles of VG from —10 V to +45 V and back. The gate voltage was scanned in both directions at a rate of about 0.5 V sec-1. The traces show remarkably little drift or hysteresis in fact, the maximum variation in VG is approximately 0.4 V for a given value of source-drain current. 

Two methods have been used to determine Both rely on the following simple expression for the source-drain current (e.g., Muller and... 

Fig. 12. Square root of source-drain current plotted against source-drain voltage, with gate and drain connected together, for various temperatures. [From Mackenzie el al. (1983).]...

Fig. 14. Variation of activation energy of the source-drain current with gate voltage (a) early devices without n+ contacts (b) intermediate stage of the optimization (c) optimized devices described in the paper by Mackenzie et al. (1983).

In most cases, however, the FET will saturate and operate over the major part of the charging cycle of the LCD as a current-limiting device. In this case, the transistor characteristics can no longer be specified by a minimum ON resistance. Rather we now need to specify the minimum source-drain current /SD. V(t) depends on /SD as... 

Fig. 12. Source-drain current in an intermittently illuminated a-Si H FET. (The transistor is an experimental device using thermally grown Si02 as a gate dielectric in order to minimize...

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