Schottky emitters

In principle, energy-analyzer systems can be designed such that their electron-optical properties do not limit the energy resolution attainable, i. e. their intrinsic energy resolution is much better than the energy width of the primary electron beam, which is of the order of approximately 1.5-2.5 eV for a tungsten hairpin cathode, approximately 1 eV for a LaBg cathode, approximately 0.7 eV for a Schottky field emitter, and 0.3-0.5 eV for a pure cold-field emitter. 

Let us now consider the charge state of the electrode. The emitter is positively biased. A p-type silicon electrode is therefore under forward conditions. If the logarithm of the current for a forward biased Schottky diode is plotted against the applied potential (Tafel plot) a linear dependency with 59 meV per current decade is observed for moderately doped Si. The same dependency of 1EB on VEB is observed at a silicon electrode in HF for current densities between OCP and the first current peak at JPS, as shown in Fig. 3.3 [Gal, Otl]. Note that the slope in Fig. 3.3 becomes less steep for highly doped substrates, which is also observed for highly doped Schottky diodes. This, and the fact that no electrons are detected at the collector, indicates that the emitter-base interface is under depletion. This interpretation is sup-... 

If VEB is increased, IEB increases and the current density at the electrode eventually becomes equal to JPS. It has been speculated that this first anodic current peak is associated with flat-band condition of the emitter-base junction. However, data of flat-band potential of a silicon electrode determined from Mott-Schottky plots show significant scatter, as shown in Fig. 10.3. However, from C-V measurement it can be concluded that all PS formation occurs under depletion conditions independent of type and density of doping of the Si electrode [Otl]. 

Hot electron spin transistors are hybrid metal/semiconductor devices that rely on spin-dependent transport of hot (nonthermalized) electrons rather than electrons near the Fermi level. The spin-valve transistor (SVT) was the first example of this new class of spintronic devices [128, 129], It has a three-terminal structure consisting of a metallic spin-valve base that is sandwiched between two semiconductor substrates, serving as the emitter and the collector, respectively. The electrons in this device are transported perpendicular to the spin-valve layers at energies just above the collector Schottky barrier height. 

Contrary to the MTT the SVT operates at constant hot electron energy that is determined by the collector Schottky barrier height ( 0.8 eV). As a result, the transfer ratio and the MC are only weakly dependent on the emitter/base bias voltage. From Eq. 17 it then follows that the SVT collector current increases almost linearly with the emitter current. It has recently been shown that collector currents exceeding 40 pA and large MC of around 400% can be obtained with SVT devices when large emitter currents are used [156],... 

The spin transistor as represented in Fig. 34 is a vertical spin transistor. Figure 35 is a schematic picture of a lateral spin transistor as originally proposed by Datta and Das [179]. In this case, a iron emitter injects spins into a 2D electron gas. A Schottky gate can rotate the spin polarization by the Rashba effect, and another iron analyzer detects transmitted spin polarized current. 

Two classifications of field emitters are used in electron microscopes, cold and Schottky sources (Table 7.2). Whereas the former operates through electron... 

The surface of the sp -carbon film was examined as a cold emitter of electrons in a normal applied electric field. Figure 11.18 shows the volt-current characteristic of a vacuum diode with a flat sp -carbon cathode at room temperature. The figure is plotted in the Schottky coordinates log(7) — where U is the applied voltage. The distance between the anode and cathode was about 0.3 mm. As can be seen in Figure 11.18, the electron emission from the cathode can be described by the Schottky law ... 

Two classifications of field emitters are used in electron microscopes, cold and Schottky sources (Table 7.2). Whereas the former operates through Fowler-Nord-heim electron tunneling " from a cathode wire held at room temperature, the latter features thermionic emission from a Zr02-coated sharpened W filament at 1,800 K. In both cases, an electrical field draws electrons from the narrow filament tip into an... 

The detection of photon- or chemically induced electronic excitation became possible with metal-insulator-metal (MIM) tunnel junctions as well as with Schottky devices. In this case, excited carriers are detected that have enongh energy to overcome either a tunnel or a Schottky barrier. Therefore, the metal film acts as a substrate for the reaction, as a photon-adsorbing layer, and as an emitter of hot carriers. There have been many experimental attempts to elucidate the nature of hot carriers using the MIM junction structure [1, 36 6]. It was found that hot electrons injected in MIM structures influence the surface reactivity [47-49]. 

Similarly, the field emitter can be adapted by reducing the work function. The practical device is called the Schottky field emission gun, but it is really a field-enhanced thermionic emitter. The sharp tip is coated with zirconium oxide. The field is reduced by flattening the tip and acts to reduce the potential barrier, but the electrons are thermally excited over the low barrier, not sucked through it. 

Heterojunction diodes can behave as homojunction diodes or Schottky barriers. They are used, for example, to control direction of carrier injection, induce electron or hole gas layers, and control energy gap on one side of the jrmction. Bipolar junction transistors consist of three layers of semiconductor with alternating doping type where the center layer of the three is relatively thin. The three layers are, respectively, the emitter, base, and collector. A small current emitter to base allows a large current emitter to collector when properly biased. Field-effect transistors have three regions, source, channel, and drain as well as a gate, which controls the conductivity of the channel connecting source to drain. 

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