All the curves tend to merge to the same collector-emitter voltage V CE once breakdown has occurred. When the transistor is biased to operate in the forward active mode, I C begins to increase significantly before the actual breakdown voltage is reached. Typical l C versus V CE characteristics are depicted in Fig. In the CE configuration, the breakdown voltage mechanism also involves the transistor gain, as well as the breakdown phenomenon in the PN junction. The maximum rated voltage in a BJT is generally associated with avalanche breakdown in the reverse-biased base-collector junction. The maximum rated collector current l Cmax may be related to the maximum current that the wires connecting the semiconductor to the external terminals can handle, the collector current at which the current gain falls below a minimum specified value, or the current that leads to the maximum power dissipation when the transistor is biased in saturation. Typical current gain-collector current characteristics for the 2N3055 power BJT at various temperatures are shown in Fig. The current gain is generally smaller in power transistors, typically in the range of 20 to 100, and may be a strong function of collector current I C and temperature. Table 26.3 compares the parameters of a general-purpose small signal BJT to those of two power BJTs. The relatively wide basewidth implies a much smaller current gain β for power transistors in comparison to that of small switching transistors, and large area device implies a large junction capacitance and hence a lower cutoff frequency for a power transistor compared to a small switching transistor. Power transistors must also be large-area devices in order to handle large currents. A relatively large base width is required to avoid punch-through breakdown. A large base-collector voltage implies a relatively large space-charge width being induced in both the collector and base regions. The base region is also much wider than normally encountered in small devices. Another N region, with a higher doping concentration, reduces collector resistance and makes contact with the external collector terminal. The primary collector region has a low-doped impurity concentration so that a large base-collector voltage can be applied without initiating breakdown.
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