2. History
โข In 1951, William Schockley invented the fi rst junction transistor, a
semiconductor device that can amplify (enlarge) electronic signals
such as radio and television signals.
โข The transistor has led to many other semiconductor inventions,
including the integrated circuit (IC), a small device that contains
thousands of miniaturized transistors.
3. Outline
โข The Unbiased Transistor
โข The Biased Transistor
โข Transistor Currents
โข The CE Connection
โข The Base Curve
โข Collector Curves
4. Objectives
โข Describe the relationships among the base, emitter, and collector
currents of a bipolar junction transistor.
โข Draw a diagram of the CE circuit and label each terminal, voltage, and
resistance.
โข Draw a hypothetical base curve and a set of collector curves, labeling
both axes.
5. The Unbiased Transistor
โข The bottom region is called the emitter, the middle region is the
base, and the top region is the collector.
โข In an actual transistor, the base region is much thinner as compared
to the collector and emitter regions.
โข Recall that the majority carriers are free electrons in n-type material
and holes in p-type material.
โข An unbiased transistor is like two back-to-back diodes
6. Emitter and Collector Diodes
โข NPN has two junctions: one
between the emitter and the
base, and another between the
collector and the base.
โข a transistor is like two back-to-
back diodes.
โข The lower diode is called the
emitter-base diode
โข The upper diode is called the
collector-base diode
8. The Biased Transistor
Emitter Electrons
โข The minus signs represent free electrons.
โข The heavily doped emitter has the
following job:
โข to emit or inject its free electrons into the
base.
โข The lightly doped base also has a well-
defined purpose:
โข to pass emitter-injected electrons on to the
collector.
โข The collector is so named because it
collects or gathers most of the electrons
from the base.
9. Transistor Currents
How the Currents Compare
โข Because the emitter is the source of the electrons, it
has the largest current.
โข Since most of the emitter electrons flow to the
collector, the collector current is almost as large as
the emitter current.
โข The base current is very small by comparison, often
less than 1 percent of the collector current.
10. Relation of Currents
โข Kirchhoffโs current law gives us this important relationship
โข Since the base current is so small, the collector current approximately
equals the emitter current
โข and the base current is much smaller than the collector current:
11. Beta
โข The dc beta (symbolized รdc) of a transistor is defined as the ratio of
the dc collector current to the dc base current:
โข Also call a current gain because a small base current controls a much
larger collector current.
โข the current gain is typically 100 to 300.