Junction transistors

The large-scale reproducible manufacture of minute, electronically-stable, single-crystal transistor junctions is a triumph of the elegant techniques of solid-state chemical synthesis. The sequence of steps is illustrated in diagrams (i)-(v). 

FET Field-Effect Transistor. Junction-FETs exploit the change of the... 

Digital electronics got its start in 1906, when American inventor Lee de Forest constructed the triode vacuum tube. Large, slow, and power-hungry as they were, vacuum tubes were nevertheless used to construct the first analogfue and digital electronic computers. In 1947, American physicist William Shockley and colles ues constructed the first semiconductor transistor junction, which quickly developed into more advanced silicon-germanium junction transistors. 

Producing Junction Transistors. Junction transistors can be produced during the original crystal growing process for the silicon (or germanium) crystal by adding known n-type and petype impurities to the molten semiconductor as the solid crystal is slowly pulled from the melt. 

Transistors—junction, MOSFET (electrical characteristics, mechanics of operation) (Chapter 18)... 

The bipolar junction transistor (BIT) consists of tliree layers doped n-p-n or p-n-p tliat constitute tire emitter, base and collector, respectively. This stmcture can be considered as two back-to-back p-n junctions. Under nonnal operation, tire emitter-base junction is forward biased to inject minority carriers into tire base region. For example, tire n type emitter injects electrons into a p type base. The electrons in tire base, now minority carriers, diffuse tlirough tire base layer. The base-collector junction is reverse biased and its electric field sweeps tire carriers diffusing tlirough tlie base into tlie collector. The BIT operates by transport of minority carriers, but botli electrons and holes contribute to tlie overall current. 

Figure C2.16.8. Schematic energy band diagram for an n-p-n bipolar junction transistor. Positions of quasi-Fenni levels and bias voltages are indicated.

Semiconductors are materials that are characterized by resistivities iatermediate between those of metals and of iasulators. The study of organic semiconductors has grown from research on conductivity mechanisms and stmcture—property relationships ia soHds to iaclude appHcations-based research on working semiconductor junction devices. Organic materials are now used ia transistors, photochromic devices, and commercially viable light-emitting diodes, and the utility of organic semiconductors continues to iacrease. 

In addition to its use as a rectifier, the p—n junction (26) is the fundamental building block for bipolar, junction EFT (fFET), and MOSFET transistors. A thorough understanding of p—n junctions explains much of transistor behavior. The theory (5) of the p—n junction and its role in bipolar transistors was presented within a year of the discovery of the point-contact transistor. 

As Figure 10 shows, the n—p—n bipolar junction transistor (BJT) may be regarded as two back-to-back p—n junctions separated by a thin base region (26,32,33). If external voltages are applied so that the base-emitter (BE) junction is forward biased and the base-coUector (BC) junction is reverse biased, electrons injected into the base from the emitter can travel to the base-coUector junction within their lifetime. If the time for minority carrier electrons to... 

Eig. 10. The n—p—n transistor biased ia its active region, where 7 = current, (------) indicate depletion regions at the p—n junctions, and S is the electric field ... 

A high gain transistor requires a nearly equal to 1. In the absence of collector junction breakdown, a is the product of the base transport factor and emitter efficiency. The base transport factor, a, is the fraction of the minority current (electrons for an n—p—n transistor) that reaches the collector. 

The frequency response or switching speed of the bipolar transistor is governed by the same processes which control the speed of thep—n junction, the capacitance associated with the movement of charge into and out of the depletion regions. To achieve high frequencies the dimensions of the active areas and parasitic circuit elements must be reduced. The two critical dimensions are the width of the emitter contact and the base thickness, W. The cutoff frequency,, is the frequency at which = 57 / - b /t > where is the emitter-to-coUector delay time and is the sum of the emitter... 

To achieve the lowest possible delay a bipolar switching transistor developed by IBM minimizes parasitic resistances and capacitances. It consists of self-aligned emitter and base contacts, a thin intrinsic base with an optimized collector doping profile, and deep-trench isolation (36). Devices must be isolated from each other to prevent unwanted interactions in integrated circuits. While p—n junctions can be used for isolation, IBM s approach etches deep trenches in the siUcon wafer which are filled with Si02 to provide electrical insulation. 

Figure 6.16 Circuit symbol for a two-junction transistor or power Darlington...

The thyristor is a semiconductor device made of germanium or silicon wafers and comprises three or more Junctions, which can be switched from the OFF state to the ON state or vice versa. Basically it is a ptipn junction, as shown in Figure 6.20(a) and can be considered as composed of two transistors with npn and pnpjunctions, as illustrated in Figure 6.20(b). It does not turn ON when it is forward biased, unlike a diode, unless there is a gate firing pulse. Thyristors are forced commutated (a technique... 

Secondary Ion Mass Spectroscopy (SIMS). When the p-n junction and the GaAs/GaAlAs heterojunction are not coincident, carrier recombination occurs, reducing the current and the performance of fabricated heterojunction bipolar transistors. 

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

A transistor, or n-p-n junction, is built up of two n-type regions of Si separated by a thin layer of weakly p-type (Fig. e). When the emitter is biased by a small voltage in the forward direction and the collector by a larger voltage in the reverse direction, this device acts as a triode amplifier. The relevant energy level diagram is shown schematically in Fig. f... 

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