The basic idea of a transistor was conceived back
in 1947, and this invention sparked a semiconductor revolution that paved the
way for integrating electronics.
The bulky packaging required for vacuum tubes
became many orders of magnitude smaller with the transistor. People started to
marvel at the small size of transistors and calculators that started to appear
in the late 50s. Computers that could barely fit into a closet in the early
50s are now pocket-sized and include even more computing power.
A bipolar junction transistor is basically two signal diodes connected
together in series within the same piece of semiconductor material such that
the two anodes (or cathodes) share a common semiconductor region. This region
is the thin middle layer of a transistor, which is a lightly doped P-type or N-type
semiconductor material. There are two junctions between the three layers, and
this is why the device is called a bipolar transistor. There are three terminals on
a bipolar transistor: base, which is the middle layer; collector, which is the
outer layer that is normally reverse biased with the base; and emitter, the
outer layer that is normally forward biased with respect to the base.
are current regulating devices that control the amount of current flowing
through them in proportion to the amount of biasing current applied to their
middle layer terminal (base).
The transistor can be used as a current
amplifier, voltage regulator, switch, and many other functions.
are two basic types of bipolar transistor construction, PNP and NPN, which
basically describes the physical arrangement of the P-type and N-type
semiconductor materials from which they are made. As the compositions of the
two types are opposite, it follows that biasing considerations for the NPN type
of transistor are opposite to those of the PNP type.
If two diodes are connected
back-to-back in a circuit, there would be no special characteristics when
biasing voltages are applied. As one diode is forward biased and the other is
reverse biased, the reverse biased diode would still have essentially no
current flowing through it. The reader might wonder why amplified current flows
through the reverse biased diode in a transistor while the other diode is
forward biased (the base-emitter diode).
There are basically two
reasons why this phenomenon appears in one but not the other. First, the base
region is made very thin, so the two outer layers can conduct current straight
through, as though the middle layer wasn't there. Majority carriers in the
collector region can shoot right through before they have a chance to recombine
with the opposite majority carrier in the middle layer.
The second reason is the fact
that the emitter region is very heavily doped while the collector and base
regions are lightly doped. This creates a situation like a valve controlling
water through a pipe that has high pressure on one end (the water supply). A
small pressure on the valve can control a large amount of water flowing through
the pipe. In the same way a small change in the base current can produce a much
larger change in the collector current. If the base current drops to zero or
the forward biased voltage is removed, the collector current would cease to
An NPN type of transistor is
considered here, although the same applies to the PNP type, except with
opposite voltages and currents.
The P material is sandwiched in between the two
layers of N material. As the base-emitter becomes forward biased in the manner
of a diode, a very large number of majority carriers (electrons) from the
emitter to be injected into the base, while at the same time, a very small
number of majority carriers (holes) from the base are injected into the
emitter. The carriers injected into the base are actually minority carriers in
the base, because of the opposite semiconductor type.
The emitter current is actually
equal to the sum of the collector and base currents for both NPN and PNP
transistors. It is surprising that only a small percentage (maybe about 1%) is
base current, even though it is forward biased with the emitter. Most of the current
flows through the collector region instead, even though the collector-base junction
is reverse biased. This is the phenomenon of a transistor's amplification, where
a small change in the base-emitter current can produce a much larger change in
the collector-emitter current. So very few of the electrons that enter the base
from the emitter region will come out through the base terminal. Another small percentage will recombine with
holes (majority carriers) in the base region, similar to diode characteristics.
The vast majority of the
emitter electrons will diffuse right through the thin base into the collector
through the base-collector depletion region.
This depletion region generates an
electric field that is proportional to the collector supply voltage. This
electric field normally blocks the flow of holes (majority carriers) that are
present in the base region. But now with an excess of electrons, they can
be accelerated directly through this field because it has an opposite effect on
electrons. Once electrons have passed through from the P-type base material
into the N-type collector material, they can freely flow as majority carriers
again. So when electrons enter in though the emitter, they exit the device
through the collector in far greater proportion to that of the base.
base-collector junction is lightly doped on both sides to increase
amplification and also to maintain a wider deletion region and a relatively
high reverse breakdown voltage. This allows a much higher collector supply
voltage, even up to hundreds of volts.
As the Bipolar
Transistor is a three terminal device, there are basically three possible ways
to connect it within an electronic circuit with one terminal being common to
both the input and output:
The Common Emitter configuration is used mostly as an amplifier or
switch and this is by far the most widely used configuration due to its
flexibility and high gain. This method is characterized by low input impedance,
high output impedance, 180 degree phase shift, controlled voltage and current
gain along with high power gain. The common emitter
amplifier configuration produces the highest current and power gain of all
three bipolar transistor configurations.
The Common-Base configuration
is characterized by low input impedance, high output impedance, high voltage
gain, and no current gain. In this configuration, the base connection is common to both the input signal and the
output signal with the input signal being applied between the base and the
emitter terminals. This method is very seldom used.
The Common-Collector configuration is characterized by high input
impedance, low output impedance, no voltage gain, and high current gain. The
connection to the collector is common to both the input signal and the output signal
through the power supply. This type is commonly used in Voltage Follower or Emitter
In the diagram on this page, the three terminals of each transistor type
are labeled as Emitter (E), the Base (B) and the Collector (C) respectively.
The construction and circuit symbols for both the PNP and NPN bipolar
transistor are given with the arrow in the circuit symbol always showing the
direction of "conventional current flow" between the terminals.