Junctions in .NET Integrating Code 128 Code Set B in .NET Junctions

Junctions using barcode writer for none control to generate, create none image in none applications. iOS In devices desi none none gned for use at high reverse bias, care must be taken to avoid premature breakdown across the edge of the sample. This effect can be reduced by beveling the edge or by diffusing a guard ring to isolate the junction from the edge of the sample (Fig. 5-24).

The electric field is lower at the beveled edge of the sample in Fig. 5-24b than it is in the main body of the device. Similarly, the junction at the lightly doped p guard ring of Fig.

5-24c breaks down at higher voltage than the p+-n junction. Since the depletion region is wider in the p ring than in the p+ region, the average electric field is smaller at the ring for a given diode reverse voltage. In fabricating a p+-n or a p-n+ junction, it is common to terminate the lightly doped region with a heavily doped layer of the same type (Fig.

5-25a), to ease the problem of making ohmic contact to the device. The result is a p+-n-n+ structure with the p+-n layer serving as the active junction, or a p+-p-n+ device with an active p-n+ junction. The lightly doped center region determines the avalanche breakdown voltage.

If this region is short compared with the minority carrier diffusion length, the excess carrier injection for large forward currents can increase the conductivity of the region significantly. This type of conductivity modulation, which reduces the forward resistance R,. Figure 5-24 Bev eled edge and guard ring to prevent edge breakdown under reverse bias: (a) diode with beveled edge; (b) closeup view of edge, showing reduction of depletion region near the level; (c) guard ring.. *. Hill : ,.. {"" 1 j- w -. Figure 5-25 A p none for none +-n-n+ junction diode: (a) device configuration; (b) zero-bias condition; (c) reversebiased to punch-through.. (a) (b). 5 . can be very use ful for high-current devices. On the other hand, a short, lightly doped center region can also lead to punch-through under reversei>ias, as in Fig. 5-25c.

The mounting of a rectifier junction is critical to its ability to handle power. For diodes used in low-power circuits, glass or plastic encapsulation or a simple header mounting is adequate. However, high-current devices that must dissipate large amounts of heat require special mountings to transfer thermal energy away from the junction.

A typical Si power rectifier is mounted on a molybdenum or tungsten disk to match the thermal expansion properties of the Si. This disk is fastened to a large stud of copper or other thermally conductive material that can be bolted to a heat sink with appropriate cooling..

5.4.4 The Breakdown Diode As we discussed earlier in this section, the reverse-bias breakdown voltage of a junction can be varied by choice of junction doping concentrations. The breakdown mechanism is the Zener effect (tunneling) for abrupt junctions with extremely heavy doping; however, the more common breakdown is avalanche (impact ionization), typical of more lightly doped or graded junctions. By varying the doping we can fabricate diodes with specific breakdown voltages ranging from less than one volt to several hundred volts.

If the junction is well designed, the breakdown will be sharp and the current after breakdown will be essentially independent of voltage (Fig. 5-26a). When a diode is designed for a specific breakdown voltage, it is called a breakdown diode.

Such diodes are also called Zener diodes, despite the fact that the actual breakdown mechanism is usually the avalanche effect. This error in terminology is due to an early mistake in identifying the first observations of breakdown in p-n junctions..

Figure 5-26 A b none for none reakdown diode: (a) /-V characteristic; (b) application as a voltage regulator. -15 V . 17 V. -ir*-v.7 15 V T_. Junctions Breakdown diode s can be used as voltage regulators in circuits with varying inputs. The 15-V breakdown diode of Fig. 5-26 holds the circuit output voltage v0 constant at 15 V, while the input varies at voltages greater than 15 V.

For example, if vs is a rectified and filtered signal composed of a 17-V d-c component and a 1-V ripple variation above and below 17 V, the output v0 will remain constant at 15 V. More complicated voltage regulator circuits can be designed using breakdown diodes, depending on the type of signal being regulated and the nature of the output load. In a similar application, such a device can be used as a reference diode; since the breakdown voltage of a particular diode is known, the voltage across it during breakdown can be used as a reference in circuits that require a known value of voltage.

. We have conside none none red the properties of p-n junctions under equilibrium con- 5.5 ditions and with steady state current flow. Most of the basic concepts of junc- TRANSIENT AND tion devices can be obtained from these properties, except for the important A-C CONDITIONS behavior of junctions under transient or a-c conditions.

Since most solid state devices are used for switching or for processing a-c signals, we cannot claim to understand p-n junctions without knowing at least the basics of timedependent processes. Unfortunately, a complete analysis of these effects involves more mathematical manipulation than is appropriate for an introductory discussion. Basically, the problem involves solving the various current flow equations in two simultaneous variables, space and time.

We can, however, obtain the basic results for several special cases which represent typical time-dependent applications of junction devices. In this section we investigate the important influence of excess carriers in transient and a-c problems. The switching of a diode from its forward state to its reverse state is analyzed to illustrate a typical transient problem.

Finally, these concepts are applied to the case of small a-c signals to determine the equivalent capacitance of a p-n junction..
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