Semiconductors reverse bias

The simplest and most widely used model to explain the response of organic photovoltaic devices under illumination is a metal-insulaior-metal (MIM) tunnel diode [55] with asymmetrical work-function metal electrodes (see Fig. 15-10). In forward bias, holes from the high work-function metal and electrons from the low work-function metal are injected into the organic semiconductor thin film. Because of the asymmetry of the work-functions for the two different metals, forward bias currents are orders of magnitude larger than reverse bias currents at low voltages. The expansion of the current transport model described above to a carrier generation term was not taken into account until now. 

FIGURE 3.47 The structure of a p-n junction allows an electric current to flow in only one direction, (a) Reverse bias the negative electrode is attached to the p-type semiconductor and current does not flow, (b) Forward bias the electrodes are reversed to allow charge carriers to be regenerated. 

A variety of colors, such as green, amber, and red (and infrared), can be obtained with different semiconductor materials without the need for a filter (see Ch. 13, Table 13.4). A LED (or photodiode) device may consist of multiple diodes in an array operating in the reverse-bias mode. Patterns of light showing symbols, letters, or numbers can thus be produced with different colors obtained by doping the semiconductor material by CVD or ion implantation. 

The Schottky barriers were excellent diodes for films annealed at 600 °C, with turn on voltages of 0.6-0.8V and minimal reverse bias leakage.48 However, many of the contacts on the as-deposited films gave large reverse bias currents and nearly ohmic responses. This behavior is indicative of degeneracy of the semiconductor because of a high carrier density resulting from native defects. The improvement in the diode behavior of the annealed films is attributed to enhanced crystallinity and reduction of defects. 

Migration of Ions (especially sodliam) Into unprotected semiconductors results In the degradation of Junction characteristics such as reverse bias leakage currents so that the chip no longer performs to specifications and eventually falls. 

As shown in Fig. 3.13(b) and 3.13(c) when ratio n/nsfl is less than or greater than 1 the system is in non-equilibrium resulting in a net current, with the electron transfer kinetics at the semiconductor-electrolyte interface largely determined by changes in the electron surface concentration and the application of a bias potential. Under reverse bias voltage, Vei > 0 and ns,o > ns as illustrated in Fig. 3.13(b), anodic current will flow across the interface enabling oxidized species to convert to reduced species (reduction process). Similarly, under forward bias, Ve2 < 0 and ns > ns,o as illustrated in Fig. 3.13(c), a net cathodic current will flow. 

Fig. 3.13 Semiconductor-electrolyte interface (a) at equilibrium, (b) under reverse bias (c) under forward bias. Arrows denote direction of current flow [reduction reaction ox + e red], (d) Electron transfer mediated through surface states.

CdO, a degenerate n-type semiconductor, was chemically deposited on single-crystal p-type Si [40]. The junction showed clear diode behavior, and, although no photovoltaic effect was observed, photocurrent was generated under reverse bias. From the spectral response of the photocurrent, almost all of the current generation occurred in the Si. 

The sample, a reverse-biased p-n or metal-semiconductor junction, is placed in a capacitance bridge and the quiescent capacitance signal nulled out. The diode is then repetitively pulsed, either to lower reverse bias or into forward bias, and the transient due to the emission of trapped carriers is analyzed. As discussed in the preceding section, for a single deep state with JVT Nd the transient is exponential with an initial amplitude that gives the trap concentration, and a time constant, its emission rate. The capacitance signal is processed by a rate window whose output peaks when the time constant of the input transient matches a preset value. The temperature of the sample is then scanned (usually from 77 to 450°K) and the output of the rate window plotted as a function of the temperature. This produces a trap spectrum that peaks when the emission rate of carriers equals the value determined by the window and is zero otherwise. If there are several traps present, the transient will be a sum of exponentials, each having a time... 

Figure 8. Schematic representations of p-n junctions and corresponding energy band diagrams under various conditions (a) uniformly doped p-type and n-type semiconductors before junction is formed, (b) thermal equilibrium, (c) forward bias, and (d) reverse bias. Abbreviations are defined as follows Ec, electron energy at conduction band minimum E, , electron energy at valence band minimum IF, forward current Vf, forward voltage Vr, reverse voltage ...

Figure 6 Schottky junction between a metal and an n-type semiconductor (a) before contact (b) after contact, without bias (c) forward bias (d) reverse bias.

Historically, this is the material which really sparked interest in the solar photoelectrolysis of water. Early papers on TiCh mainly stemmed from the applicability of TiCh in the paint/pigment industry255 although fundamental aspects such as current rectification in the dark (in the reverse bias regime) shown by anodically formed valve metal oxide film/ electrolyte interfaces was also of interest (e.g., Ref. 52). Another driver was possible applications of UV-irradiated semiconductor/electrolyte interfaces for environmental remediation (e.g., Refs. 256, 257). 

The depletion layer profile contains information about the density of states distribution and the built-in potential. The depletion layer width reduces to zero at a forward bias equal to and increases in reverse bias. The voltage dependence of the jimction capacitance is a common method of measuring W V). Eq. (9.9) applies to a semiconductor with a discrete donor level, and 1 is obtained from the intercept of a plot of 1/C versus voltage. The 1/C plot is not linear for a-Si H because of the continuous distribution of gap states-an example is shown in Fig. 4.16. The alternative expression, Eq. (9.10), is also not an accurate fit, but nevertheless the data can be extrapolated reasonably well to give the built-in potential. The main limitation of the capacitance measurement is that the bulk of the sample must be conducting, so that the measurement is difficult for undoped a-Si H. 

Zviagin and Liutovich (11) found similar minimum values for p-type Si as we did for the Ge samples. The theoretical curve of the Russian authors is calculated on the assumption that the minority carriers are depleted. This is possible for a p-type semiconductor only in the case of cathodic polarization. Since the Russian authors did not take into account the possibility of enrichment of the minority carriers, they did not get a distinct minimum of the theoretical capacity-potential curve. We found the minimum for n-type Ge under reverse bias, i. e., under anodic current. This result is to be expected (in contrast to a common rectifier) as long as the resistance across the phase boundary (R ) is high compared to the recombination rate or the rate orformation of free carriers. It is to be expected, in other words, as long as the electrochemical potential of the free carriers remains nearly constant across the space charge up to the surface. The Russian authors point out that the measured capacity is not equal to the space charge capacity, but should be related to it. This relationship is indicated by the measured frequency dependence of the measured impedances. It is in agreement with our assumption that the... 

We have also realized interdigital metal-semiconductor-metal (MSM) contacts. We found a strong increase of the current through the stmcture by illuminating it with UV-light. If the stability under reverse bias of regular Schottky diodes (see Fig. 7) is achieved for interdigital stmctures we are confident that MSM stmctures are well suited for the realization of solar-blind UV-detectors. 

Figure 6 An energy diagram of the charge-transfer process at an n-type semiconductor/metal interface when an external potential (F) is applied across the semiconductor electrode. The applied potential changes the electric potential difference between the semiconductor surface and the bulk region. This perturbs the concentration of electrons at the surface of the semiconductor (ns), and a net current flows through the semiconductor/metal interface. The forward reaction represents the transfer of electrons from the semiconductor to the metal and the reverse reaction represents the injection of electrons into the semiconductor from the metal. The width of the arrows indicates schematically the relative magnitude of the current, (a) The reverse bias condition for an n-type semiconductor (V > 0). The forward reaction rate is reduced relative to its equilibrium value, while the reverse reaction rate remains constant. A net positive current exists at the electrode surface, (b) The forward bias condition (V < 0), the forward reaction rate increases compared to its equilibrium value, while the reverse reaction rate remains unaffected. A net negative current exists at the electrode surface...

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