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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

Bipolar Transistors

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Introduction

Bipolar transistors are one of the main ‘building-blocks’ in electronic systems
They are used in both analogue and digital circuits
They incorporate two pn junctions and are sometimes known as bipolar junction transistors or BJTs
Here will refer to them simply as bipolar transistors

An Overview of Bipolar Transistors

While control in a FET is due to an electric field, control in a bipolar transistor is generally considered to be due to an electric current
current into one terminaldetermines the currentbetween two others
as with a FET, abipolar transistorcan be used as a‘control device’

Bipolar Transistor Operation

We will consider npn transistors
pnp devices are similar but with different polarities of voltage and currents
when using npn transistors
collector is normally more positive than the emitter
VCE might be a few volts
device resembles two back-to-back diodes – but has very different characteristics
with the base open-circuit negligible current flows from the collector to the emitter

Summary of Bipolar Transistor Characteristics

Bipolar transistors have three terminals: collector, base and emitter
The base is the control input
Two polarities of device: npn and pnp
The collector current is controlled by the base voltage/current IC = hFEIB
Behaviour is characterised by the current gain or the transconductance

Bipolar Transistor Amplifiers

A simple transistor amplifier
RB is used to ‘bias’ thetransistor by injecting anappropriate base current
C is a coupling capacitorand is used to couple theAC signal while preventingexternal circuits fromaffecting the bias
this is an AC-coupled amplifier