ELEKTRONIKOS ĮTAISAI
1
2009
Isolation
Bipolar transistors
of elements
The vast majority of IC bipolar transistors are npn because of higher
current gain and better frequency response. npn transistors have these
properties because the main current flows due to electrons that have higher
mobility than holes.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
2
2009
Bipolar transistor
A major disadvantage of the simple integrated transistor is high collector series resistance
since the collector current is horizontally routed through the lightly doped collector.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
3
2009
Bipolar transistors
The resistance is less if the cross section for the transistor collector current
is greater. Using EDP we can increase this cross-section arranging Π type
contacts to emitter and collector region.
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stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
4
2009
Bipolar transistor
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
5
2009
Bipolar transistors
The resistance is also sufficiently reduced if the n+ buried layer is used. The buried layer
is formed making selective n+ diffusion before growing the epitaxial layer. The buried n+
layer is located below the collector. It provides a low resistance path from the
collector contact to the active portion of the transistor.
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VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
6
2009
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stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
7
2009
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VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
8
2009
Bipolar transistors
Processing complexity can be
reduced using the collectordiffusion-isolation (CDI) structure.
Using the CDI the p-type epitaxial
layer is taken as the base to
eliminate the base-diffusion step
employed in the EDP, thus
simplifying the processing steps.
What are disadvantages of the
CDI technology?
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
9
2009
Bipolar transistors
Sometimes pnp devices are required in ICs.
The lateral pnp and vertical or substrate pnp are the two kinds of such
transistors usually employed.
The lateral pnp transistor has a
considerably lower current gain than an
npn device. This is because the p-type
emitter cannot inject minority carriers into
the n-type base with the same efficiency as
the n+ emitter does into the p-type base of
an npn BJT. Furthermore, the larger base
area and the fact that some injected holes
migrate to the substrate cause the number
of holes reaching the collector to decrease.
A vertical pnp transistor can be used
only if its collector is at a fixed
negative voltage. Such a configuration
is called an emitter follower.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
10
2009
Bipolar transistors
Lateral pnp bipolar junction transistor
VGTU EF ESK
stanislovas.staras@el.vgtu.lt