Junctions and Transistors

The p-n junction, located at the juncture between p-type and n-type material, is the basic building block for semiconductor devices. At the transition between n- and p-type materials, a potential gradient is formed to separate the regions with high and low electron and hole concentrations. 

A piece of Si with joined n-type and p-type regions, each with 1016 donors or acceptors, is shown in Fig. 9.1. In thermal equilibrium, the Fermi level, EP, is constant across the transition from n- to p-type materials. The magnitude of the difference between conduction band edges (i.e., the barrier height eV0) is given by 

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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. 

Electrodeposition of metals on semiconductor surfaces has been used by Allongue et al. to form nearly ideal Schottky barriers on GaAs [6.176] and InP [6.177], and to stabilize photoelectrodes with ultrathin and transparent metal films [6.174, 6.178, 6.179]. Selective metal deposition has also been performed to reveal p-n junctions and transistors on silicon chips [6.180]. 

One common use for semiconducting materials is in integrated-circuit components. In our discussion of rectifying junctions and transistors—two of these components—we detailed their electrical characteristics and mechanics of operation, per the following concept map ... 

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. 

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 ... 

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. 

There are several methods to investigate the charge states of a semiconductor electrode, for example high-frequency resistometry (HFR) [Otl]. Below a transistor-like set-up, as shown in the inset of Fig. 3.2, will be discussed because it shows in an exemplary way the similarities and differences of solid-state junctions and liquid junctions. 

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, aT, is the fraction of the minority current (electrons for an n-p—n transistor) that reaches the collector. ocT 1 — W2 /2L, where W is the base width, is the distance between emitter and collector junctions and Lg is the minority carrier diffusion length in the base. High gain transistors require a thin base as well as a long minority carrier lifetime for a large Lg. Because aT is >0.995 in modem transistors, there is little room for improvement. The emitter efficiency, the fraction of emitter current due to minority carriers injected into the base instead of the emitter,... 

The regions located immediately to either side of the junction are known as depletion regions because there are fewer current carriers (electrons or empty levels) in these regions. Applying an external electric field across such a junction disturbs the equilibrium and the consequences of this are exploited in LEDs and transistors. In LEDs, which are discussed in Chapter 8, the voltage is applied so that the //-type semiconductor is negative relative to the p-type. An important feature of most transistors is a voltage applied in the reverse direction, that is, the /3-type is positive with respect to the p-lype. 

Junction transistors (Figs. 9.18A and 9.18B) can be synthesized in two ways (1) the grown-junction pnp transistor or (2) the fused-junction transistor... 

The total current in the emitter IE, not too far from the emitter-to-base junction, consists of two contributions the electron current (drift current) 1 e, which proceeds under forward bias toward the emitter-to-base junction, and the much smaller hole current (a diffusion current), which originates in the base and proceeds in the opposite direction, but decays exponentially, as the distance from the junction increases. The total current in the collector, close to the base-to-collector junction, consists of electrons l c (a large fraction of 1 e) that have somehow evaded capture within the base and proceed against reverse bias in the collector region. The rest of the electron current in the collector is what in pn diodes is called reverse saturation current frs, and here it is called collector current with zero emitter current Iqo = hs- l c is the "useful" electron flow in the transistor. 

The emitter current IE is not independent of the collector voltage Vc -Figure 9.23 shows that Vc affects the thickness of the base-to-collector junction and thus influences the diffusion time for currents to cross the base. Transistor designers wish to bring the ratio Ic/h as close to unity as possible. Four factors tend to make h differ from IE ... 

Electrolytic etching has been used to reveal p-n junctions (43) as well as to remove n- or p-type material preferentially from diodes and transistors (28). These processes make use of the rectifying barrier of p-n junctions as well as the hole depletion effect at the surface of n-type germanium and silicon. 

Even though the field-effect transistor did not come into widespread use until the 1960 s, its invention predated both the junction and point-contact transistors by many years. As it is normal with many innovations, its practical realization was delayed until adequate materials and technologies were available for its fabrication. We can even say that the Thin Film Transistor (TFT) was the first solid-state amplifier ever patented. The basic principle of the field-effect transistor (what we call now JFET) was proposed by the first time by Julius Edgar Lilienfeld as early as 1925 and patented in 1930 Ref (1) (see Fig. la) where an adaptation of the cross section of the... 

The Josephson junction is one such ultrafast superconducting switching device. Josephson junctions, which until recently operated only at liquid-helium temperature, are traditionally made of niobium-tin or niobium-germanium and are really simple connections between superconductors. They can do everything vacuum tubes and transistors do, but a lot faster. 

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