SOLID STATE DEVICES and SYSTEMS – 4th Edition
Bipolar Junction Transistors(BJTs)
1) Bipolar Junction Transistors (BJTs)
a) Three layer semiconductor device
i) PNP
ii) NPN
b) Three terminal semiconductor device
i) May be three discrete terminals or the third terminal
may be the shell or case of the device
ii) Base – control element
(1) Similar in function to the gate on an SCR or Triac
(2) Determines whether or not current flows between
emitter and collector
(a) Current flows emitter to collector – PNP device
(b) Current flows collector to emitter – NPN device
iii) Emitter – one end of the primary current path
iv) Collector – other end of the primary current path
c) Schematic symbol
i) Direction of the emitter arrow identifies PNP or NPN
(1) Memory aid
(a) PNP = Pointing-iN-Place
(b) NPN = Not-Pointing-iN
(i) Most common
(ii) Works with negative ground circuits
d) Transistor packages
i) Conform to JEDEC standard (Joint Electron Device
Engineering Council)
(1) Defines the shape and configuration of devices
NOTES
(2) TO-3, TO-5, TO-92, and TO-220 are common
packages
e) Provides switching or amplifying functionality
2) Biasing Transistor Junctions
a) BJT transistors are current operated devices
i) Some small current must flow between the base and
emitter terminals before current can flow between
emitter and collector
b) Base-emitter junction must always be forward biased in
order for a BJT to conduct
i) .7 V(silicon)/.3V(germanium) junction potential (like a
diode)
c) Base-collector junction must always be reverse biased
d) The primary current path is between the emitter and
collector
i) Base layer is very thin and represents a higher resistance
path for current flow than the emitter-collector path
ii) Base current must be limited to prevent device damage
(1) Series–limiting resistor is one method commonly
used
3) Transistor Operating Characteristic Curves
a) Allow users to predict the operating characteristics of a
transistor in a circuit
b) Collector characteristic curves
i) Single
ii) Multiple
c) Current gain
i) Describes how many times greater is ICE than IBE
ii) Beta(β) is another term used to describe current gain
4) Transistors as DC Switches
a) Primary motivation for the development of the transistor
i) No mechanical moving
ii) Capable of very fast switching times
b) Two operating modes
i) Cutoff – very high CE resistance – “switch” is off
ii) Saturation – Very low CE resistance – “switch” is on
c) Transistor load line
i) Describes all possible operating conditions for a
transistor in a given circuit
(1) Drawn between the two extremes of operation –
cutoff and saturation
d) Biasing transistors
i) Base bias
(1) Base bias instability
(a) Ambient temperature change causes Q-point
shift
ii) Bias stabilization schemes
(1) Emitter-feedback bias stabilization
(2) Collector-feedback bias stabilization
(3) Combination bias stabilization
(4) Voltage divider with emitter bias stabilization
(5) Thermistor bias
(6) Thermistor-emitter feedback bias
5) Power Dissipation
a) Critical specification for transistors
b) If power rating is exceeded, thermal runaway will destroy
the device
6) Testing Transistors
a) Use an ohmmeter for basic testing
i) Treat as two diodes to test
(1) B-E = diode 1
(2) B-C = diode 2
(3) A minimum of 6 measurements are required to fully
evaluate transistor functionality
FWD Bias
REV Bias
B-E
Low Ω
High Ω
B-C
Low Ω
High Ω
C-E
High Ω
High Ω
7) Transistor Switching Applications