半导体物理学半导体 (73).pdf

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1、1Power Bipolar TransistorsVertical npn Power Transistor Structure The primary collector region has a low-doped impurity concentration so that a large basecollector voltage can be applied without initiating breakdown.Another n region,with a higher doping concentration,reduces collector resistance and

2、 makes contact with the external collector terminal.A relatively large base width is required to prevent punch-through breakdown.The collector terminal is at the“bottom”of the device:maximize the cross-sectional area through which current is flowing in the device Power transistors must also be large

3、-area devices in order to handle large currents.Interdigitated structure:Relatively small emitter widths are required to prevent the emitter current crowding effects.Vertical npn Power Transistor StructureAn interdigitated bipolar transistor structure showing the top view and cross-sectional viewPow

4、er Transistor Characteristics The relatively wide base width implies a much smaller current gain;Large area device implies a larger junction capacitance and hence lower cutoff frequency.The current gain is generally smaller in the power transistors,typically in the range of 20 to 100,and may be a st

5、rong function of collector current and temperature.Comparison of the characteristics and maximum ratings of small-signal and power BJTs The maximum rated collector current Ic,max may be related to the maximum current that the wires connecting the semiconductor to the external terminals can handle,th

6、e 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.All the curves tend to merge to the same collectoremitter voltage once breakdown has occurred.This voltage,VC-E,s

7、us,is the minimum voltage necessary to sustain the transistor in breakdown.Power Transistor CharacteristicsTypical dc beta curve(hFEv.s.IC)Typical ICv.s.collector-emitter V curve of a bipolar T,showing breakdown effects Second breakdown:operating at high voltage and high current,results in positive

8、feedback,until the semiconductor material melt.The maximum rated power PT for the transistor：The safe operating area(SOA);is bounded by IC,max,VCE,sus,PT,and the transistors second breakdown characteristic curve.Power Transistor CharacteristicsThe safe operating area(SOA)of a bipolar transistor plot

9、ted on(a)linear scales and(b)logarithmic scalesDarlington Pair Configuration Darlington pair:increase the effective current gain.The overall common-emitter current gain is then:The silicon dioxide that is shown completely penetrates through the p-type base region so that the base regions of the two transistors are isolated.An npn Darlington pair configurationAn integrated circuit implementation of the npn Darlington pair configuration