Flow Field-Effect Transistor

Field-effect flow control Flow field-effect transistor Fluidic amphflcation and oscillation Fluidic logic... 

These are also known as flow field-effect transistors (flow FETs). Fluidic transistors make use of the surface charge conditions to control the flow in a microfluidic chip. They are generally used with electrokinetically based microflows to modulate the zeta potential at the shear plane. Fluidic... 

Flow field-effect transistor Field-effect flow control Fluerics Fluidic logic Fluidic amplification and oscillation... 

The invention of the germanium transistor in 1947 [I, 2] marked the birth of modem microelectronics, a revolution that has profoundly influenced our current way of life. This early device was actually a bipolar transistor, a structure that is mainly used nowadays in amplifiers. However, logical circuits, and particularly microprocessors, preferentially use field-effect transistors (FETs), the concept of which was first proposed by Lilicnficld in 1930 [3], but was not used as a practical application until 1960 [4]. In a FET, the current flowing between two electrodes is controlled by the voltage applied to a third electrode. This operating mode recalls that of the vacuum triode, which was the building block of earlier radio and TV sets, and of the first electronic computers. 

A novel development of the use of ion-selective electrodes is the incorporation of a very thin ion-selective membrane (C) into a modified metal oxide semiconductor field effect transistor (A) which is encased in a non-conducting shield (B) (Fig. 15.4). When the membrane is placed in contact with a test solution containing an appropriate ion, a potential is developed, and this potential affects the current flowing through the transistor between terminals Tt and T2. 

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]... 

Sun, B. Sirringhaus, H. 2006. Surface tension and fluid flow driven self-assembly of ordered ZnO nanorod films for high-performance field effect transistors. /. Am. Chem. Soc. 128 16231-16237. 

The measurement of changes of the surface potential Vo at the interface between an insulator and a solution is made possible by incorporating a thin film of that insulator in an electrolyte/insulator/silicon (EIS) structure. The surface potential of the silicon can be determined either by measuring the capacitance of the structure, or by fabricating a field effect transistor to measure the lateral current flow. In the latter case, the device is called an ion-sensitive field effect transistor (ISFET). Figure 1 shows a schematic representation of an ISFET structure. The first authors to suggest the application of ISFETs or EIS capacitors as a measurement tool to determine the surface potential of insulators were Schenck (15) and Cichos and Geidel (16). 

A particular type of biosensor can be developed by putting a membrane in contact with the semi-conducting layer of a field effect transistor. If the membrane incorporates an enzyme adapted to transform a particular analyte (Fig. 19.8), reaction of that enzyme will modify the polarity at the surface of the insulating layer. This will in turn modify the conduction between the source and the collector of the field effect transistor. The current flowing through these two electrodes (source and collector) serves as the signal. 

Figure 15-28 Operation of a field effect transistor, (a) Nearly random distribution of holes and electrons in the base in the absence of gate potential. ( >) Positive gate potential attracts electrons that form a conductive channel beneath the gate. Current can flow through this channel between source and drain.

The base of the field effect transistor in Figure 15-28 is constructed of p-Si with two n-type regions called source and drain. An insulating surface layer of Si02 is overcoated by a conductive metal gate between source and drain. The source and the base are held at the same electric potential. When a voltage is applied between source and drain (Figure 15-28a), little current flows because the drain-base interface is a /injunction in reverse bias. 

Successful operation of potentiometric chemosensors opened up the possibility for the fabrication of chemical field-effect transistors (chemFETs) and ion-selective field-effect transistors (ISFETs). A sensing element in these devices, i.e. the MIP film loaded with the molecular, neutral or ionic, respectively, imprinted substance is used to modify surface of the transistor gate area. Apparently, any change in the potential of the film due to its interactions with the analyte alters the current flowing between the source and drain. 

---