MadSci Network: Physics
Query:

Re: Why NPN transistors are preferred over PNP ?

Date: Wed Apr 30 20:52:38 2003
Posted By: Adrian E. Popa, Director Emeritus
Area of science: Physics
ID: 1045404256.Ph
Message:



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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