Bipolar Junction Transistors 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  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)

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

Cross Section of npn Transistor

Cross-Section of pnp BJT

Circuit Symbols and Current Conventions npnpnp

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

Circuit Configurations

I-V Characteristic: npn Transistor Measured in a Common Emitter Configuration Modified from I C =  I B when V CE > V CEsat

Nonideal I-V Characteristic I CEO – leakage current between the collector and emitter when I B = 0, usually equal to the reverse saturation of the base-collection diode Effects from a change in the effective distance between emitter and collector V A – Early Voltage  is not a constant BV CEO – breakdown voltage of the transistor Modified from:

Current-Voltage Characteristics of a Common-Base Circuit In Forward Active Region: I C =  F I E, where  F < 1 Modified from Microelectronic Circuit Analysis and Design by D. Neamen

Simplified I-V Characteristics

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

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

Currents and Carriers in npn BJT i En = i E – i Ep i Cn = i C – i Cp where i Cp ~ Is of the base-collector junction i En > i Cn because some electrons recombine with holes in the base i B replenishes the holes in the base

Current Relationships in Forward Active Region

DC Equivalent Circuit for npn in forward active npnpnp

Simplified DC Equivalent Circuit npnpnp V BE = 0.7V V CE ≥ 50mVV EB = 0.7V V EC ≥ 50mV I B ≥ 0mA I B ≥ 0mA I C =  F I B AND I E = (  F +1) I B

I C ~ I SC I C ≤  F I B V BE = 0.75V V CE = 50mVV EB = 0.75V V EC = 50mV Saturation npnpnp

Cut-Off I C = I B = I E = 0 V BE ≤ 0.6V V EB ≤ 0.6V