01-05-2012, 05:22 PM
Bipolar Transistor Operation
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pn-Junction Review
Recall that the boundary condition for the electron and hole minority
densities at the depletion region edges were npoeqVD/kT and pnoeqVD/kT
respectively, and that the minority density variation with distance is
linear for a neutral region which is short compared to the minority
diffusion length
Forward Active Operation - Current Components
Three current components in forward active operation, all of which can
be characterised from the appropriate minority gradient:
– “Linking current” due to electron transport from collector to emitter (1)
– “Back injection” due to hole injection from base to emitter (2)
– small component due to injection of holes from collector to base (3)
Reverse Active Region - Potentials
• When the base collector
junction is forward biased and
the base emitter junction is
reverse biased (implying VEC>
VBC), the device is in the reverse
active region of operation
• Basically the forward active
region with roles of emitter and
Transistor Action
• The term transistor action refers to the control of the large
collector-emitter (linking) current by the smaller base
(back injection) current in forward active operation, the
origin of “current gain” in a BJT
• Two features of the device are essential for transistor
action
– a narrow base, which forces all electrons injected from the emitter
to travel across the base neutral region to the collector
– a high emitter doping compared to the base doping, making base
(electron) injection the dominant term
collector reversed
Lecture Summary
• This lecture has introduced the fundamental current
components in the bipolar structure for each region of
operation
• Current components calculated explicitly from minority
carrier slopes using diffusion relationship from diode
• Transistor action identified – narrow base and highly
doped emitter allow control of large collector to emitter
linking current with small base to emitter back injection
component