Download Semiconductor Devices II

Semiconductor Devices II
Physics 355
Zener Diodes
With the application of sufficient reverse
voltage, a p-n junction will experience a
rapid avalanche breakdown and
conduct current in the reverse direction.
Valence electrons that break free under
the influence of the applied electric field
can be accelerated enough that they
can knock loose other electrons and the
subsequent collisions quickly become
an avalanche. When this process is
taking place, very small changes in
voltage can cause very large changes in
current.
The breakdown process depends upon the applied electric field, so by
changing the thickness of the layer to which the voltage is applied, zener
diodes can be formed which break down at voltages from about 4 volts to
several hundred volts.
Zener Diodes
When forward-biased, Zener diodes behave much the same as
standard rectifying diodes: they have a forward voltage drop
which follows the "diode equation" and is about 0.7 volts. In
reverse-bias mode, they do not conduct until the applied voltage
reaches or exceeds the so-called Zener voltage, at which point
the diode is able to conduct substantial current, and in doing so
will try to limit the voltage dropped across it to that Zener voltage
point. So long as the power dissipated by this reverse current
does not exceed the diode's thermal limits, the diode will not be
harmed.
Tunnel Diodes
Tunnel Diodes
Tunnel Diodes
Tunnel Diodes
Tunnel Diodes
Tunnel Diodes
Tunnel Diodes
NDC region
Tunnel Diodes: Applications
•
In the NDC region, the diode can be used
as either an oscillator, as in the case of the
Gunn diode, or as an amplifier. Low-noise,
tunnel-diode amplifiers represent an
important microwave application of tunnel
diodes. Tunnel-diode amplifiers with
frequencies up to 85 gigahertz have been
built in waveguides, coaxial lines, and
transmission lines. The low-noise
generation, gain ratios of up to 30 dB, high
reliability, and light weight make these
amplifiers ideal for use as the first stage of
amplification in communications and radar
receivers.
Transistors
Dr. John Bardeen, Dr. Walter Brattain, and Dr.
William Shockley discovered the transistor
effect and developed the first device in
December, 1947, while the three were
members of the technical staff at Bell
Laboratories in Murray Hill, NJ. They were
awarded the Nobel Prize in physics in 1956.
Transistors
•
•
•
•
Transistors are the main components of
microprocessors, which are essential to many of the
products we use every day such as televisions, cars,
radios, home appliances, and, of course, computers.
Transistors are miniature electronic switches. They are
the building blocks of the microprocessor which is the
brain of the computer.
At their most basic level, transistors may seem simple.
But their development actually required many years of
painstaking research. Before transistors, computers
relied on slow, inefficient vacuum tubes and mechanical
switches to process information.
In 1958, engineers (one of them Intel co-founder Robert
Noyce) managed to put two transistors onto a silicon
crystal and create the first integrated circuit, which led
to the microprocessor.
Transistors
The bipolar junction
transistor acts as a
current amplifier,
having many
applications for
amplification and
switching.
Unbiased Transistors
Transistors
Transistors
When a negative voltage is applied
to the base (point B), electrons in
the base region are pushed ('like'
charges repel, in this case both
negative) back creating insulation
boundaries. The current flow from
point E to point C stops. The
transistor's state has been changed
from a conductor to an insulator.
Transistors
We begin with the transistor acting as
an insulator. In order to have it
conduct, positive voltage must be
applied to the base (point B). As
opposite charges attract (in this case,
positive and negative), electrons are
'pulled' out of the insulating
boundaries and flow out of the base
region at point B. The barriers that
once restricted flow of electrons from
the emitter to the collector are
diminished. Electrons begin to flow in
at the emitter (point E), through the
base to the collector (point C). The
transistor's state has been changed
from an insulator to a conductor.
Transistors: Amplifier
Apply different values of the
base bias voltage and the
collector bias voltage to the
npn transistor and see
what happens.
With two ammeters (shown as red dots) and a voltmeter to
measure VCE we can determine the characteristics of the
transistor.
Transistors: Amplifier
The amount of base
current is determined
by the base bias
voltage.
IC

IB