
A transistor is a 3 terminal electronic device made of semiconductor material.

Transistors have many uses, including amplification, switching, voltage regulation, and the modulation of signals


Before transistors were invented, circuits used vacuum tubes:
Fragile, large in size, heavy, generate large quantities of heat, require a large amount of power

The first transistors were created at Bell Telephone Laboratories in 1947
William Shockley, John Bardeen, and Walter Brattain created the transistors in and effort to develop a technology that would overcome the problems of tubes


The first patents for the principle of a field effect transistor were registered in
1928 by Julius Lillenfield.
Shockley, Bardeen, and Brattain had referenced this material in their work
The word “transistor” is a combination of the terms “transconductance” and
“variable resistor”

Today an advanced microprossesor can have as many as 1.7 billion transistors.

• Silicon (Si) and Germanium (Ge) are the two most common single elements that are used to make Diodes. A compound that is commonly used is Gallium Arsenide
(GaAs), especially in the case of LEDs because of it’s large bandgap.
• Silicon and Germanium are both group 4 elements, meaning they have 4 valence electrons. Their structure allows them to grow in a shape called the diamond lattice.
• Gallium is a group 3 element while Arsenide is a group 5 element. When put together as a compound, GaAs creates a zincblend lattice structure.
• In both the diamond lattice and zincblend lattice, each atom shares its valence electrons with its four closest neighbors.
This sharing of electrons is what ultimately allows diodes to be build. When dopants from groups 3 or 5 (in most cases) are added to Si, Ge or GaAs it changes the properties of the material so we are able to make the P- and N-type materials that become the diode.
Si
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Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
The diagram above shows the 2D structure of the Si crystal. The light green lines represent the electronic bonds made when the valence electrons are shared.
Each Si atom shares one electron with each of its four closest neighbors so that its valence band will have a full
8 electrons.

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+4
+4
+4
+5
+4
+4
+4
+4
When extra valence electrons are introduced into a material such as silicon an n-type material is produced. The extra valence electrons are introduced by putting impurities or dopants into the silicon. The dopants used to create an n-type material are Group V elements. The most commonly used dopants from Group V are arsenic, antimony and phosphorus.
The 2D diagram to the left shows the extra electron that will be present when a Group
V dopant is introduced to a material such as silicon. This extra electron is very mobile.

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+4
+4
+4
+3
+4
+4
+4
+4
P-type material is produced when the dopant that is introduced is from Group III.
Group III elements have only 3 valence electrons and therefore there is an electron missing. This creates a hole (h+), or a positive charge that can move around in the material. Commonly used Group III dopants are aluminum, boron, and gallium.
The 2D diagram to the left shows the hole that will be present when a Group III dopant is introduced to a material such as silicon.
This hole is quite mobile in the same way the extra electron is mobile in a n-type material.

Current flows from P to N
No Current flows
Excessive heat can cause dopants in a semiconductor device to migrate in either direction over time, degrading diode
Ex: Dead battery in car from rectifier short
Ex: Recombination of holes and electrons cause rectifier open circuit and prevents car alternator form charging battery

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I
o
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
Early BJTs were fabricated using alloying - an complicated and unreliable process.

The structure contains two p-n diodes , one between the base and the emitter, and one between the base and the collector.

,
, and
.


BJTs are usually constructed vertically
› Controlling depth of the emitter’s n doping sets the base width
E B C p n n

The transistor is a versatile device usually configured to perform as a switch or as an amplifier. The bipolar junction transistor (BJT) is the most common type and has three leads:
3 Collector
3
Collector
Base
2
Base
2
1 Emitter 1 Emitter
PNP Transistor NPN Transistor
In a transistor, the flow of current from the collector to the emitter is controlled by the amount of current flowing into the base of the transistor. If no current flows into the base, no current will flow from the collector to the emitter (it acts like an open switch). If current flows into the base, then a proportional amount of current flows from the collector to the emitter (somewhat like a closed switch).
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Transistor Size (3/8”L X 5/32”W X 7/32”H)
No Date Codes. No Packaging.



›
› npn
 Base is energized to allow current flow pnp
 Base is connected to a lower potential to allow current flow
›
›
›
Current gain ( β )
Voltage drop from base to emitter when V
FB
Minimum voltage drop across the collector and emitter when transistor is saturated
BE
=V









Emitter is heavily doped compared to collector. So, emitter and collector are not interchangeable.
The base width is small compared to the minority carrier diffusion length. If the base is much larger, then this will behave like back-to-back diodes .
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(
) are probably the simplest form of transistor widely used in both analogue and digital applications they are characterised by a very high input resistance and small physical size, and they can be used to form circuits with a low power consumption they are widely used in very large-scale integration two basic forms: insulated gate FETs junction gate FETs

Many forms, but basic operation is the same a voltage on a control input produces an electric field that affects the current between two other terminals when considering amplifiers we looked at a circuit using a
‘control device’ a FET is a suitable control device



Analogous to BJT
Transistors
FET Transistors switch by voltage rather than by current
D
G
S




The “Field Effect”
The resulting field at the plate causes electrons to gather
As an electron bridge forms current is allowed to flow
Plate
Semiconductor

Three terminals:
Base (B): very thin and lightly doped central region (little recombination).
Emitter (E) and collector (C) are two outer regions sandwiching B.
Normal operation (linear or active region):
B-E junction forward biased; B-C junction reverse biased .
The emitter emits (injects) majority charge into base region and because the base very thin, most will ultimately reach the collector.
The emitter is highly doped while the collector is lightly doped .
The collector is usually at higher voltage than the emitter.

The emitter base junction of a transistor is forward biased whereas collector base junction is reversed biased.
In the absence of the emitter base junction no current would flow in the collector circuit because of the reversed biased and if it is present forward biased on it possess the emitter current to flow.
Emitter current almost entirely flows in the collector circuit,therefore the current in the collector circuit depends upon the emitter current.

Figure shows the energy levels in an
NPN transistor under no externally applying voltages.
In each of the N-type layers conduction can take place by the free movement of electrons in the conduction band .
In the P-type (filling) layer conduction can take place by the movement of the free holes in the valence band .
However, in the absence of any externally applied electric field, we find that depletion zones form at both PN-Junctions, so no charge wants to move from one layer to another.
NPN Bipolar Transistor

Apply a Collector-Base voltage
What happens when we apply a moderate voltage between the collector and base parts.
The polarity of the applied voltage is chosen to increase the force pulling the N-type electrons and P-type holes apart .
This widens the depletion zone between the collector and base and so no current will flow.
In effect we have reverse-biassed the Base-Collector diode junction.

Charge Flow
Apply an Emitter-Base voltage
What happens when we apply a relatively small Emitter-Base voltage whose polarity is designed to forward-bias the Emitter-Base junction .
This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary.
Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively-biassed
Collector region.
As a result the electrons which get into the
Base move swiftly towards the Collector and cross into the Collector region.
Hence a Emitter-Collector current magnitude is set by the chosen Emitter-Base voltage applied .
Hence an external current flowing in the circuit.

Charge Flow
Some of free electrons crossing the
Base encounter a hole and 'drop into it'.
As a result, the Base region loses one of its positive charges (holes).
The Base potential would become more negative (because of the removal of the holes) until it was negative enough to repel any more electrons from crossing the Emitter-Base junction.
The current flow would then stop.
Some electron fall into a hole

Charge Flow
To prevent this happening we use the applied E-B voltage to remove the captured electrons from the base and maintain the number of holes.
The effect, some of the electrons which enter the transistor via the
Emitter emerging again from the
Base rather than the Collector.
For most practical BJT only about 1% of the free electrons which try to cross Base region get caught in this way.
Hence a Base current, I
B
, which is typically around one hundred times smaller than the Emitter current, I
E
.
Some electron fall into a hole

Charge Flow
To prevent this happening we use the applied E-B voltage to remove the captured electrons from the base and maintain the number of holes.
The effect, some of the electrons which enter the transistor via the
Emitter emerging again from the
Base rather than the Collector.
For most practical BJT only about 1% of the free electrons which try to cross Base region get caught in this way.
Hence a Base current, I
B
, which is typically around one hundred times smaller than the Emitter current, I
E
.
Some electron fall into a hole

• Current flow in an npn transistor biased to operate in the active mode
• Forward bias of Emitter-Base Junction: current flows to emitter, electrons move towards base, holes to emitter
• Reverse bias of Base-Collector Junction: I
C independent of V
CB
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•Current in PNP mainly due to holes injected from emitter to base
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Combine the two diodes!
V
F p
 n h
V
R p
I forward
I
No transistor action reverse e
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E h e
Hole concentration is zero here, reverse biased

I
E
I
E
( 1
 
) I
E
R
V
F
V
R
The collector current I
C is almost equal to I
E
, and collector current is controlled by the E-B junction bias. The loss, i.e. corresponds to the recombination of holes in base.
 < 1
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
BE
FB
B
Transistor acts like an off switch

BE
FB
B
C
B
Transistor acts like a current amplifier

BE
FB
B
C,max
In this mode the transistor acts like an on switch
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i
C
-v
CB
●
characteristics for an npn transistor in the active mode.
• Collector is constant current source only controlled by emitter current i
C i
C
 
I
E
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Transistor as an amplifier. (b) The circuit of (a) with the signal source v be eliminated for dc (bias) analysis.
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I
E
= I
B
+ I
C and V
EB
+ V
BC
+ V
CE
= 0 V
CE
= 
As shown, the currents are positive quantities when the transistor is operated in forward active mode .
V
EC
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Emitter is heavily doped compared to collector. So, emitter and collector are not interchangeable.
The base width is small compared to the minority carrier diffusion length. If the base is much larger, then this will behave like back-to-back diodes .
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