Double Schottky barrier

Diagrams of electron depletion for oxide grains and the resistance of contact between grains, (a) Space charge layer model, (b) double Schottky barrier model, (c) regional and volume depletion model, (d) surface conductive grains contact model. 

The double Schottky barrier model (Fig. 1.4(b)) also turned out to be completely misleading. It focused attention on the electron transport path running through the centers of contacting grains. In reality, however, there... 

Under the influence of deep acceptor states on grain boundary, the double Schottky barrier is generated as shown in Figure 2.1.2. According to Poisson s equation, its barrier height is given by... 

FIGURE 2.1.2 Band model of double Schottky barrier at grain boundary. 

In the temperature region III (T > Tn), the electrons that overcome the double Schottky barrier increase with temperature, and resistance decreases from maximum resistance. 

The ZnO grains are n-type semiconductors. The intergranular traps are formed in this grain boundary due to the presence of Bi203 or PriOs and transition metal oxides, which cause the double Schottky barrier. 

When a voltage V is applied across the double Schottky barrier, the barrier height (j) decreases with increase of the voltage. A small fraction of electrons can penetrate the depletion region. These electrons, which are accelerated in the depletion region, can excite the valence electrons to conduction band. Therefore, holes recombine with the trapped electrons at the grain boundary, and decrease the barrier height. 

Fig. 2.20 Double Schottky barrier in polycrystalline oxide layers (a) physical model (b) energy zone diagram (c) grain size influence on mechanism of polycrystalline MOX layer conductance. The filled and empty areas indicate high and low resistances, respectively [Idea from Yamazoe (1991) and Yamazoe and Miura (1992)]...

It is evident that the double layers at the grain boundaries constitute Schottky barriers which are similar in some respects to those formed in VDR resistors. In accord with this it is found that the resistivity-temperature relation of PTC material is voltage sensitive. The low-temperature resistivity may be reduced by a factor of 4 by an increase in applied field from 1 to 80kVm-1, and the ratio of maximum to minimum resistivities, above and below Tc, may be reduced from five to three orders of magnitude. 

This mcclianism is not so efrcctivc in polar semiconductors. The conversion of empty hybrids to doubly occupied hybrids on a GaAs surface would require the double occupation of a gallium hybrid, which is unfavorable because of the polar energy. Indeed, recent experiments (Chye, Babalola, Sukegawa, and Spicer, 1975) indicate that the I crmi level is not pinned on surfaces of GaP at the vacuum. Nonetheless, Schottky barriers can arise at GaP- metal interfaces. Metal-induced surface states" have been proposed as a mechanism (discussed in Section 18-1 ) but the barriers could well arise simply from incorporation of metal atoms in the semiconductor or vice versa. 

Cryogenically cooled detectors employ the low-noise GaAs Schottky barrier Mott diodes. Between 140 and 220 GHz they exhibit 400 K noise equivalent temperature at a lower limit junction temperature of 20 K, below which the performance degrades. The noise temperature is around 1000 K at 300 K junction temperature. Sensitivity of a Schottky barrier mixer diode ranges from about 2.75 VmW" to 1 VmW over the range 90-325 GHz.In comparison the helium-cooled InSb bolometer used by the present authors (Section 3.4.1) can provide double sideband noise temperatures of 200-300 K in the region 100-300 GHz and sensitivity of 5-6 VmW . ... 

Thus, if the semiconductor corresponds essentially to an insulator of the Schottky barrier type, use of Mott-Schottky plots will allow the determination of the capacitance of the inner layer. Utilization of impedance measurements with different frequencies may give rise to the possibility of determining the double layer capacity separate from the inner layer. In this way a map of the double layer, an estimation of the Helmholtz potential difference and the potential difference (pd) in the space charge region may be obtained. The pd in the Helmholtz layer is, however, not only given by the charge on the surface of the polymer, but also by the potential difference due to aggregated layers which form within it, and in particular the solvent dipole layer (70). 

Insofar as this concentration of surface states is high (and this means more than 1 % of the total sites of the surface), the Schottky part of the barrier—the amount of potential within the space charge region—tends to decrease, and the potential drop at the Helmholtz part of the double layer will increase. 

Simmons treated tunneling and Schottky emission through very thin films with a double-image force barrier. With thin enough films the current depended on the work function of the positive electrode, as in Standley and Maissel s work. 

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