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B ipolar J unction T ransistors

B ipolar J unction T ransistors. ECE 2204. Three Terminal Device. Terminals Emitter The dominant carriers are emitted from the region (equivalent to the Source in a MOSFET) Base These now minority carriers travel through the base region

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B ipolar J unction T ransistors

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  1. Bipolar Junction Transistors ECE 2204

  2. Three Terminal Device • Terminals • Emitter • The dominant carriers are emitted from the region (equivalent to the Source in a MOSFET) • Base • These now minority carriers travel through the base region • Some recombine in the base, forcing a base current to flow • Collector • The remaining carriers from the emitter are collected from this region (equivalent to the Drain)

  3. Types of BJTs • n-p-n • Emitter is n+ type • Electrons flow from the emitter towards the collector • Base is p type • Some of the electrons from the emitter recombine with the holes in the base • Collector is n- type • p-n-p • Emitter is p+ type • Holes flow from the emitter towards the collector • Base is n type • Some of the holes from the emitter recombine with the electrons in the base • Collector is p- type

  4. Cross Section of npn Transistor

  5. Cross-Section of pnp BJT

  6. Circuit Symbols and Current Conventions npn pnp

  7. The one equation that will always be used with BJTs* * With the exception of reverse active. Then, the equation becomes

  8. Circuit Configurations

  9. I-V Characteristic: npn Transistor IC = b IB when VCE > VCEsat Measured in a Common Emitter Configuration Modified from https://awrcorp.com/download/faq/english/examples/images%5Cbjt_amp_oppnt_bjt_iv_curves_graph.gif

  10. Nonideal I-V Characteristic ICEO – leakage current between the collector and emitter when IB = 0, usually equal to the reverse saturation of the base-collection diode Effects from a change in the effective distance between emitter and collector VA – Early Voltage b is not a constant BVCEO – breakdown voltage of the transistor Modified from: http://cnx.org/content/m29636/latest/

  11. Current-Voltage Characteristics of a Common-Base Circuit In Forward Active Region: IC = aF IE, where aF < 1 Modified from Microelectronic Circuit Analysis and Design by D. Neamen

  12. Simplified I-V Characteristics

  13. Modes of Operation • Forward-Active • B-E junction is forward biased • B-C junction is reverse biased • Saturation • B-E and B-C junctions are forward biased • Cut-Off • B-E and B-C junctions are reverse biased • Inverse-Active (or Reverse-Active) • B-E junction is reverse biased • B-C junction is forward biased

  14. npn BJT in Forward-Active BE junction is forward biased BC junction is reverse biased

  15. Currents and Carriers in npn BJT iEn = iE – iEp iCn = iC – iCp where iCp ~ Is of the base-collector junction iEn > iCnbecause some electrons recombine with holes in the base iB replenishes the holes in the base

  16. Current Relationships in Forward Active Region

  17. DC Equivalent Circuit for npn in forward active npn pnp

  18. Simplified DC Equivalent Circuit IC = bF IB AND IE = (bF +1) IB npn pnp VBE = 0.7V VCE ≥ 50mV VEB = 0.7V VEC ≥ 50mV IB ≥ 0mA IB ≥ 0mA

  19. Saturation npn pnp IC ~ ISCIC≤bF IB VBE = 0.75V VCE = 50mV VEB = 0.75V VEC = 50mV

  20. Cut-Off IC = IB = IE = 0 VBE≤ 0.6V VEB≤0.6V

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