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Introduction to Transistors
Instrument Systems
Spring 2007
Jose Rey
April 2007
Transistors
• Everywhere
• Similar diodes end-to-end (np-n and p-n-p junctions
• Sandwiched junction is very
thin
• Small current on the “base”
can control a large current
flowing from “collector” to
“emitter”
• Can be used as an amplifier
to make a weak signal
stronger
• Can replace mechanical
switches
Transistors
• First Transistor
• Bell Labs
• First transistor was
point contact Ge
bipolar junction
transistor
Bipolar Junction Transistors (BJT)
• C = Collector (Drain)
• B = Base (Gate)
• E = Emitter (Source)
• Shown on right: BJT
(Bipolar Junction
Transistor)
n-type
Contributor
(excess)
p-type
“holes”
(shortage)
n-type
Contributor
(excess)
Switch Application
From: http://natasha.eng.usf.edu/gilbert/courses/instrumentsystems/showtime/2005_presentations/npn_transistor.html
Field effect transistor (FET)
• Only two layers
• Electric Field Controls
Current Flow
• Voltage applied between the
Gate and Source terminals
modulates the current
between the Source and
Drain terminals
• Metal Oxide Semiconductor
Field Effect Transistor
(MOSFET) or junction field
effect transistor(JFET)
• Large amount of Chemical
Sensors are based on
MOSFETs
C = Collector (Drain)
B=
Base or
Gate
E = Emitter (Source)
Diodes Quick Review
• Diodes AND LED’s
Current flows from
Anode+ to Cathode• Place a LED in the
following circuit, how?
Transistors Quick Review
• In NPN Transistor
Collector AND Emitter are
n doped, Base is p doped
• When Transistor is
saturated, current flows at
maximum AND no longer
controlled by Base
End of presentation