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