Junction capacitance charging time

Next the junction capacitance charging time is the time required to change the voltage across the junction capacitance due to current flow through the bulk semiconductor regions (an RC time constant) and is given by ... 

If a semiconductor element with negative differential conductance is operated in a reactive circuit, oscillatory instabilities may be induced by these reactive components, even if the relaxation time of the semiconductor is much smaller than that of the external circuit so that the semiconductor can be described by its stationary I U) characteristic and simply acts as a nonlinear resistor. Self-sustained semiconductor oscillations, where the semiconductor itself introduces an internal unstable temporal degree of freedom, must be distinguished from those circuit-induced oscillations. The self-sustained oscillations under time-independent external bias will be discussed in the following. Examples for internal degrees of freedom are the charge carrier density, or the electron temperature, or a junction capacitance within the device. Eq.(5.3) is then supplemented by a dynamic equation for this internal variable. It should be noted that the same class of models is also applicable to describe neural dynamics in the framework of the Hodgkin-Huxley equations [16]. 

As discussed previously, the junction capacitance at high frequency depends on the thermal emission and capture times of trapped gap-state charge. In effect, this imposes an energy cutoff such that states at deeper than E cannot respond to the applied ac voltage. As discussed in Part II, this energy cutoff is given by... 

One can use the PM technique with pulse excitations and study the time evolution of the spectra or the decay of the total oscillator strength of the bands. In this way one can obtain information about electronic relaxation processes. However, in this chapter we restrict ourselves to steady-state PM spectroscopy. We also note that instead of using illumination, the state occupation can be changed by changing the bias of a junction, as used in deep-level transient spectroscopy junction capacitance techniques. This method, dubbed charge-induced absorption, or CIA [44J, has already been used in conducting polymers and is discussed in another chapter of this book. 

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... 

How can such problems be counterbalanced Since a large capacitance of a semiconductor/electrolyte junction will not negatively affect the PMC transient measurement, a large area electrode (nanostructured materials) should be selected to decrease the effective excess charge carrier concentration (excess carriers per surface area) in the interface. PMC transient measurements have been performed at a sensitized nanostructured Ti02 liquidjunction solar cell.40 With a 10-ns laser pulse excitation, only the slow decay processes can be studied. The very fast rise time cannot be resolved, but this should be the aim of picosecond studies. Such experiments are being prepared in our laboratory, but using nanostructured... 

An alternative approach that was used in the past was to treat the photoelectrochemical cell as a single RC element and to interpret the frequency dispersion of the "capacitance" as indicative of a frequency dispersion of the dielectric constant. (5) In its simplest form the frequency dispersion obeys the Debye equation. (6) It can be shown that in this simple form the two approaches are formally equivalent (7) and the difference resides in the physical interpretation of modes of charge accumulation, their relaxation time, and the mechanism for dielectric relaxations. This ambiguity is not unique to liquid junction cells but extends to solid junctions where microscopic mechanisms for the dielectric relaxation such as the presence of deep traps were assumed. 

Definite collection volumes can be achieved in PIN diodes, where an undoped zone I of an intrinsic semiconductor separates the p- and -regions (Fig. 4.95). Since no space charges exist in the intrinsic zone, the bias voltage applied to the diode causes a constant electric field, which accelerates the carriers. The intrinsic region may be made quite wide, which results in a low capacitance of the p n junction and provides the basis for a very fast and sensitive detector. The limit for the response time is, however, also set by the transit time r = ru/uth of the carriers in the intrinsic region, which is... 

Traps and recombination centers can have dramatic effects on the performance of devices. Normally these defects are a problem. In a defect free diode or bipolar junction transistor the speed of the device can be limited by the time to remove charge from the base. Because recombination centers reduce this time, they can, in theory, make a transistor switch faster. However, the other losses of performance due to recombination generally outweigh the gain in recombination speed. There are also other ways of accelerating devices that do not involve degrading them (see Chapter 6). Traps slow device switching and can lead to transient capacitance effects because... 

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