Source-drain channel

Once again typical FET action characteristics well related to those displayed in Figure 8.48 are clearly reproduced [81,175,209]. Moreover, the source-drain channel current increases with increasing temperatures at a fixed source-drain voltage and gate voltage. On the basis of equations (8.2) and (8.3), this increased source-drain current was found to result principally from enhanced mobility with temperature. The increased carrier density with raised temperatures is also responsible for increment of the source-drain current. We will return to this point at the end of this section. 

Figure 1.3 The resonant gate field effect transistor, one of the first MEMS devices. A released metal cantilever beam forms the gate electrode over the diffused source-drain channel. The input signal is applied to the input force plate, which causes the cantilever beam to vibrate, modulating the current through the transistor. Maximum vibration occurs at the resonant frequency of the cantilever beam, enabling the device to act as a high-Q electromechanical filter. (Reprinted with permission from IEEE Trans. Electron Devices, The resonant gate transistor, H.C. Nathanson, W.E. Newell, R.A. Wickstrom and J.R. Davis Jr., 1967 IEEE.)...

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. 

Fig. 9. Fabrication sequence for an oxide-isolated -weU CMOS process, where is boron and X is arsenic. See text, (a) Formation of blanket pod oxide and Si N layer resist patterning (mask 1) ion implantation of channel stoppers (chanstop) (steps 1—3). (b) Growth of isolation field oxide removal of resist, Si N, and pod oxide growth of thin (<200 nm) Si02 gate oxide layer (steps 4—6). (c) Deposition and patterning of polysihcon gate formation of -source and drain (steps 7,8). (d) Deposition of thick Si02 blanket layer etch to form contact windows down to source, drain, and gate (step 9). (e) Metallisation of contact windows with W blanket deposition of Al patterning of metal (steps 10,11). The deposition of intermetal dielectric or final...

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 second short-channel effect is a drain field-dependent mobility, which occurs for source-drain fields above I05 V/cm, in agreement with similar phenom-... 

The most common a-Si H TFT structure is the so-called inverted staggered transistor structure [40], in which silicon nitride is used as the gate insulator. A schematic cross section is shown in Figure 74. The structure comprises an a-Si H channel, a gate dielectric (SiN.v), and source, drain, and gate contacts. 

The thickness of the active layer is about 100-300 nm, while the source-drain distance (channel length) amounts to a few micrometers. The channel length is determined by the current requirements and usually exceeds 10 /xm. Other manufacturing schemes as well as alternative stmctures are described elsewhere [619, 621]. Technology developments for the next generation TFTs that are to be used for high-resolution displays have been summarized by Katayama [627]. 

These equations describe an unheated transistor and were verified for a device with no backside etching (no membrane). The modelling parameters were provided by the manufacturer, whereas the value of the threshold voltage was taken from wafer map data. The channel length modulation parameter. A, had to be extracted from measurement data. The discrepancy between simulated and measured source-drain saturation current, fsd,sat> for a transistor embedded in the bulk silicon was less than 1%, which confirmed the vaHdity of the model assumptions. 

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 ... 

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. 

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. 

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