Generation Lifetime

Before discussing DLTS, it is appropriate to talk about some device parameters that are affected by these impurities. The main parameters are the recombination and generation lifetimes because they affect junction leakage current, device switching speed, light emitting diode efficiency and a number of other device performance indicators. 

In contrast to r measurements, in which the decay of excess carriers is monitored the generation lifetime is determined from the reverse-biased pn junction leakage current or from the pulsed MOS capacitor (22.) latter and the more popular of the two, an MOS-C is pulsed into deep depletion and the capacitance is monitored as a function of time. An appropriate analysis of the C-t response yields t. ... 

Applying these considerations to generation lifetime we would expect a drop in lifetime outside of the DZ (fig. 17). This in fact is observed. Fig.18 shows lifetime-measurements on a wafer, which after precipitation was slightly wedge shaped polished in order to vary the depth of the DZ. Subsequently MOS capacitors had been built and the lifetime measured. The increase in lifetime is due to the DZ. 

Figure 18. Generation lifetime of capacitors with varying distances to the precipitated area of the wafer.

Fig.21 shows an example for the generation lifetime distribution across a wafer. Note the high resolution of the plot. It reflects the homogeneity of the critical electrical parameters across wafers representing the state of the art of growing and shaping techniques. 

Figure 21. MOS generation lifetime map. Each point represents a MOS capacitor. The average lifetime is 500 psec, the standard deviation is 70 psec.

There have been few studies on the lifetime of dielectric elastomer transducers and fewer still that consider lifetime of dielectric elastomer generators. Plante and Dubowsky [32] smdied failures in acrylic materials and identified factors to predict performance limitations. Kornbluh et al. [33] did report some generator lifetime results, which will be highlighted here. The requirement for long lifetime can have... 

Deep levels influence a variety of device parameters. For example, in minority carrier devices they influence the recombination and generation lifetimes. The lifetime in turn controls junction currents and refresh times in dynamic random access memories. For this reason we discuss lifetimes. Defects... 

We will classify the carrier lifetimes into two broad categories recombination lifetimes and generation l etimes [60]. The recombination lifetime, applies when there are excess carriers in the semiconductor and recombination dominates. The generation lifetime, x, obtains when thermal generation dominates. Occasionally the lifetimes are further subdivided. For example, it is usefiil to distinguish between low-level and high-level injection lifetimes. But such finer subdivision is not done here. For a more detailed treatment of lifetime concepts and their measurements, the reader is referred to ref.61. 

Multiphonon generation is the inverse of multiphonon recombination. Multiphonon generation is a thermally activated process. Electrons are diermally excited from G-R centers into the conduction band and holes are similarly thermally excited from G-R centers into the valence band. The key feature of SRH generation is thermal excitation. A thermally activated process is very dependent on the temperature and on the activation energy. We define a generation lifetime as [60]... 

AVhat is Tg Just as Xj. represents the average time for ehp to recombine, so X represents the average time for ehp to be generated. It is intuitively obvious mat the mechanisms responsible for recombination and generation are quite different. We should, therefore, expect the recombination and generation lifetimes to be quite different as well. If in fact they are different what do they mean, how are they measured and how do they apply to device operation One must be very careful in the interpretation of lifetime measurements. 

The generation lifetime is measured with the pulsed MOS capacitor or the gate-controlled diode technique. It is important to understand that the lifetime measured by this technique can, and generally does, give very different values from the recombination lifetime measured by one of the techniques indicated above. 

Electrical Characterization of Semiconductor Materials and Devices 23 4.2.2 Generation Lifetime Pulsed MOS Capacitor... 

The pulsed MOS capacitor (MOS-C) method is the most common technique to measure the generation lifetime. To determine it is necessary for scr generation to be dominant For silicon devices this is generally true when the pulsed MOS-C measurement is done at room temperature. For Xg determination the MOS-C is pulsed from accumulation into deep depletion. As a result of thermal ehp generation the device relaxes to its inversion state. [21] To extract the generation lifetime the capacitance, C, is measured as a function of time, as shown in Fig. 10(a). From such a C-t plot one generates a Zerbst Plot. [89] A Zerbst plot is a plot of vs. (Cp/C-1). It is related to the device... 

The slope of a Zerbst Plot is given by determine the generation lifetime. We like to point out that the mtercept should not be interpreted as the surface generation velocity as is sometimes done. The intercept is a complicated function of surface generation, quasi-neutral bulk generation and it also depends on the assumptions used in the Zerbst analysis. [90] To extract a surface generation velocity from the intercept is very misleading. 

The question is Are the discrepancies between theory and experiment due to the limits in resolution of the experiments and data analysis, or do they have a more fundamental physical origin To clarify this, we have analyzed a computer-generated lifetime spectmm (for details, see Dlubek et al. [1998a, 1999a, b, 2003b]). The input... 

Generation lifetime 20-100 years Acid rain, heavy metal wet deposition, eutrophication... 

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