Vacuum triode

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. 

We will see later that the characteristics of vacuum triodes are replicated in npn junction transistors, albeit by a very different mechanism. One can operate very well with vacuum tubes, but they are more costly and difficult to manufacture and are prone to relatively early failure, mainly due to the tube becoming "gassy" with the evaporation of W from the filament or from vacuum breakdown, due to the prolonged heating of the vacuum tube. The typical vacuum tube duty cycles (a few thousand hours) are much inferior to the duty cycles of transistors. 

It was established, that possibility of the control appears due to essential differences in conductivity of polymer layers. As in a vacuum triode, the conducting layer corresponds to an electronic cloud between grid and cathode. 

Developments such as that of the vacuum triode, cathode ray tube and, later, the transistor and the solid-state integrated-circuit device, had an enormous influence on the design and construction of instruments. Electronic systems can respond with lightning speed to an almost-zero signal. The glass ion-sensitive electrode was discovered in the early 1900 s, but its appearance in the pH meter had to wait for developments in electronics. 

FIGURE 3.42 Scheme and sample for vacuum triode (left) and for transistor (middle and right) see (Further Readings on Quantum Electronics 1958-2000 ElyperPhysics, 2003). 

In addition to the conventional ionization gauge, whose electrode structure resembles that of a common triode, there are various ionization vacuum gauge systems (Bayard-Alpert system, Bayard-Alpert system with modulator, extractor system) which more or less suppress the two effects, depending on the design, and are therefore used for measurement in the high and ultrahigh vacuum range. Today the Bayard-Alpert system is usually the standard system. 

Triode ionization vacuum gauge for high vacuum... 

Plot of plate current It, versus plate voltage Et, for various fixed values of the grid voltage Ec in a vacuum tube triode (common-cathode or grounded-cathode circuit). The grid is back-biased, the plate is forward-biased. Note that the curves are essentially the same, but are displaced along the plate voltage axis and resemble those of Fig. 9.8. In actual triodes the amplification factor varies and is not just simply the n — 5 used here. Adapted from Terman [5]. 

Characteristics of a typical common-cathode or grounded-cathode vacuum tube triode (curves of plate current /b versus plate voltage Eb, for different values of grid voltage Eg — Ec). The horizontal line connects points of constant plate current lb, and the vertical arrows show how Eq. (9.3.7) is applied to obtain the transconductance. Note that changing Eg only displaces the IV curve but, to first approximation, does not distort it. Adapted from Mandl [6]. 

When an np rectifier is connected, through a shared p region, to a pn rectifier, we have a npn junction "triode" transistor, or bipolar junction transistor (BJT). This transistor can amplify signals, just as does the vacuum-tube triode, but by a totally different mechanism. 

Common-base, common-emitter, and common-collector circuits for a bipolar npn transistor (A, B, C, respectively), and the equivalent grounded-grid, grounded-cathode, and grounded-plate circuits for vacuum-tube triodes (A corresponds to A, B to B, and C to C). Adapted from Terman [5]. 

The FET scheme depends, as in the vacuum-tube triode, on the relative size and spacing of three electrodes the source (S), the gate (G), and the drain (D). 

Vacuum fluorescent displays (VFDs) are strongly related to flat-panel CRTs. Electrons are ejected from a cathode source, traverse a vacuum and then strike a pattern of triodes with individual anodes covered in red, green and blue phosphorescent material. However, the operating voltages, e.g. 12 V, and power consumption are much lower than those found for CRTs and PDFs. 

Because of their fragility and occasional unreliability, the point-contact electrodes were eventually replaced with three layers of adjacent semiconducting surfaces, each of which corresponded to an element in the triode vacuum tube the emitter layer (for the heated filament which is the source of electrons), the base (for the grid that controls the electron flow), and the collector, for the triode plate that receives the electrons. The areas where the layers join one another are called junctions, and transistors made in this way are known as junction transistors. 

The sensor consisted of a vacuum tube containing a filament, grid and anode, very similar in form to the thermionic triode valve. An adjustable leak was arranged to feed a portion of the column eluent into the gauge which was operated under reduced pressure. The sensor was fitted with its own pumping system and vacuum gauge and the usual necessary cold traps. Helium was used as a carrier gas and the grid collector-electrode was set at +18 V with respect to the cathode... 

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