MadSci Network: Physics |
Greetings:
References:
Ben G. Streetman, Solid State Electronic Devices, Prentice -
Hall, 1980.
S. M. Sze, Physics of Semiconductor Devices, John Wiley &
Sons , 1981.
Why are NPN bipolar junction transistors (BJT) more common than PNP
devices?
ANSWER
There are two basic reasons for the preference for NPN BJTs over PNP
devices:
1) The mobility of N type charge current carriers (negative
electrons)
in silicon (Si) is more than two times greater than the mobility of P
type
charge carriers (positive holes) in Si.
2)The mass processing of Si based BJTs and integrated circuits (ICs)
are most
economically manufactured using large N type silicon wafers.
DISCUSSION
Answer 1)
It is desirable to have BJTs (NPN and PNP) that switch at the highest
possible speeds (highest frequencies) using the lowest possible drive
voltages.
A measure of these parameters for a semiconductor material is it's
mobility.
The mobility = the average velocity of the charge carriers in the
material
measured in centimeters per second (cm/sec) divided by the electric
field
measured in volts per centimeter (V/cm). The greater the mobility the
better
the transistor's circuit parameters will be.
Mobility = (cm/sec)/ (V/cm) = (cm^2) / (V-sec)
Silicon has become the most common material used to fabricate
electronics
circuits and components. At room temperature (300 degrees Kelvin),
the
mobility's for silicon are:
P type Si = 1350 (cm^2 / V-sec)
N type Si = 480 (cm^2 / V-sec).
Thus P type Si material has more than twice the mobility of N type Si
material.
A Si based BJT is fabricated from a very thin base material sandwiched
between
two relatively thick emitter and collector materials. Thus an NPN
transistor
has a P type base with lower mobility sandwiched between an emitter
and a
collector of N type material with higher mobility. A PNP transistor
has an N
type base with higher mobility sandwiched between an emitter and a
collector
of lower mobility. Which type of BJT will have the best overall
mobility for
current flowing through the device? The NPN does, because it only has
a very
thin amount of low mobility material sandwiched between mostly high
mobility
material. The PNP is made mostly from low mobility P type material and
only a
very small amount of N type material.
Answer 2)
The economics and physics of manufacturing vast numbers of silicon
circuits
is much to complex to discuss here. They are discussed in Streetman's
book.
However; because N type silicon wafers have become the substrate of
choice
for manufacturing devices, only a thin layer of P type material
covered by
a second layer of N type material is required to form vast arrays of
NPN
devices. Making PNP devices on an N type substrate requires several
more
layers of materials on the N type wafer to isolate the PNP device from
the
wafer material. These additional steps increase the manufacturing cost
for
each wafer.
Best regards, Your Mad Scientist
Adrian Popa
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