Solid State Electronic Devices[固态电子器件](-69).ppt

Solid State Electronics
Bipolar Junction Transistors
Ronan Farrell
Recommended Book:
Streetman, Chapter 7
Solid State Electronic Devices
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Solid State Electronics
Topics we’ll be covering
Qualitative explanation of the operation of the BJT
BJT equations and the Eber Molls model.
Physical Construction of BJT
Imperfections such as base narrowing, avalanche breakdown, thermal effects, emitter crowding
Small signal model and parasitic correspondence.
High Frequency Transistors
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Bipolar Junction Transistors
The BJT consists of a sandwich of either an N type material with P type on either side, or visa-versa.
The names used to identify the different type of bipolar junction transistor (BJT) is given by the structure of the device
PNP P-type N-type P-type
NPN N-type P-type N-type
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Historically, the left hand side region (the most positive side) is called the EMITTER (of holes).
The right hand side (the most negative) the COLLECTOR (of holes).
The centre is called the BASE.
Bipolar Junction Transistors
P
N
P
P
N
N
PNP
NPN
Collector
Emitter
Base
+V
-V
+V
-V
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Bipolar Junction Transistors
This construction for the transistor gives rise to two PN junctions within the device. In comparison with the FET, the current in a BJT device travels across the junctions.
The behaviour of the BJT is governed by the behaviour of charge carriers at a PN junction.
These are the same equations that we worked out in the PN junction section of this course.
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Bipolar Junction Transistors
In normal operation, one PN junction is forward biased and the other is reversed biased.
In a forward bias junction, the main mechanism for current flow is diffusion. Holes diffuse from the P-type material to the N-type material and recombine. There is also the reverse current of electrons diffusing from the N-type to the P-type material.
If one side is much more heavily doped than the other, then the majority of the current is carried by