Drift region

In this mode, ions are formed continuously in the ion source (a), but the electrostatic accelerating potential is applied in pulses (b). Thus, a sample of ions is drawn into the drift region (c) with more ions formed in the source. As shown in Figure 26.1, the ions separate according to m/z values (d) and arrive at the detector (e), the ions of largest m/z arriving last. 

Upon acceleration through an electric potential of V volts, ions of unknown m/z value reach a velocity v = f2zeV/m]" ). The ions continue at this velocity (drift) until they reach the detector. Since the start (to) and end (r) times are known, as is the length d of the drift region, the velocity can be calculated, and hence the m/z value can be calculated. In practice, an accurate measure of the distance d is not needed because it can be found by using ions of known m/z value to calibrate the system. Accurate measurement of the ion drift time is crucial. 

In plasma chromatography, molecular ions of the heavy organic material to be analy2ed are produced in an ionizer and pass by means of a shutter electrode into a drift region. The velocity of drift through an inert gas at approximately 101 kPa (1 atm) under the influence of an appHed electric field depends on the molecular weight of the sample. The various sonic species are separated and collected every few milliseconds on an electrode. The technique has been employed for studying upper atmosphere ion molecule reactions and for chemical analysis (100). 

The mass separated, pulsed, and focused primary ions with the energy of 1 -25 keV, typically liquid metal ions such as Ga, Cs, and O", are used to bombard the sample surface, causing the secondary elemental or cluster ions to emit from the surface. The secondary ions are then electrostatically accelerated into a field-free drift region with a nominal kinetic energy of ... 

The operative mechanisms have been deduced by considering various peaks identified in the daughter spectrum employing the reflectron technique. One corresponds to a mass loss of 32 amu from a parent ion cluster that enters the drift region as H+(CH3OH) via ... 

Reflectron time-of-flight mass spectrometers can be used to generate fragment spectra. Before the introduction of delayed extraction, it was observed that the ions had a tendency to disassemble into fragments while they were traveling to the reflectron (Spengler et al., 1992). This behavior is called postsource decay (PSD). The ions collide with matrix ions in the gas phase when they are accelerated with several kilovolts. Their decay occurs in the first field-free drift region. 

Another interest in drift tube improvements is the poor performance of certain components such as the ion shutter, the device used to inject ions into the drift region... 

The second way to improve the mass resolution significantly is to use an electrostatic mirror (mass reflectron) placed in the drift region of ions (Fig. 1.27). 

The ions of the selected polarity will be accelerated down the drift region, left to right in Figure 10.1, while ions of the opposite polarity are repelled and pushed left. An important feature is the reverse flow air, a moderate velocity stream that resists the movement of the accelerating ions and sweeps the nonionized molecules and the ions of the opposite polarity into the exhaust stream. 

Donor ions have been somewhat neglected and one reason may be that the considerably smaller ionization energies reduce the importance of having full activation of implanted donor atoms research has therefore been concentrated on the acceptors. Another reason may also be that many of the expected SiC bipolar power-device applications require a thick low-doped n-type drift region grown by CVD in combination with a p-type emitter preferably manufactured by implantation. However, many transistor applications and also contacts to n-type material benefit from... 

Assuming also that the entire reverse voltage drop occurs over the drift region, the blocking voltage of the device can be expressed in terms of the maximum 1-D electric field under the junction E, as well as the doping N and thickness T of the drift region ... 

The number A in (5.4) is called the Lagrange multiplier. Solving this equation system with respect to T, N, and A results in the optimal epi parameters for the drift region ... 

Figure 5.2 Comparison of the practical minimum for a specific drift region on-resistance of 4H-SiC UMOSFET with thermal SiOj gate dielectric to the theoretical values calculated for 4EI-SiC and Si as a function of blocking voltage. Caughey-Thomas mobility parameters for Si used in this calculation are taken from [12].

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