Bipolar Junction Transistors

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2 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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Three
-
terminal, solid
-
state electronic device used for amplification and switching.

The transistor is an arrangement of semiconductor materials that share common
physical boundaries. Materials most commonly used are silicon, gallium
-
arsenide,
and germanium, into which impurities have been introduced by a process called
doping.

In n
-
type semiconductors the impurities or dopants result in an excess of
electrons, or negative charges; in p
-
type semiconductors the dopants lead to a
deficiency of electrons and therefore an excess of positive charge carriers or
"holes.

The PNP and NPN Junction Transistor


The Field
-
Effect Transistor


The Metal
-
Oxide Semiconductor Field
-
Effect Transistor (MOSFET)


Transistors

The First Transistor

The bipolar junction transistor was the first solid
-
state amplifier element and started
the solid
-
state electronics revolution. Bardeen, Brattain and Shockley at the Bell
Laboratories invented it in 1948 as part of a post
-
war effort to replace vacuum
tubes with solid
-
state devices (Nobel Prize in 1956).

A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are
joined back
-
to
-
back. This forms a sort of a sandwich where one kind of
semiconductor is placed in between two others. There are therefore two kinds of
Bipolar sandwich, the NPN and PNP varieties. The three layers of the sandwich are
conventionally called the Collector, Base, and Emitter.

Bipolar Junction Transistors


Bipolar Junction Transistors

The direction of the emitter arrow defines the type transistor. Biasing and power
supply polarity are positive for NPN and negative for PNP transistors. The transistor
is primarily used as an current amplifier. When a small current signal is applied to the
base terminal, it is amplified in the collector circuit.

Figure 1 shows the energy levels in an NPN transistor when we aren't externally
applying any 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
.

When we apply a moderate voltage between the Collector and Base and the polarity of
the applied voltage is chosen to increase the force pulling the N
-
type electrons and P
-
type holes apart. (i.e. we make the Collector positive with respect to the Base.) This
widens the depletion zone between the Collector and base and so
no current will
flow
. In effect we have
reverse
-
biased the Base
-
Collector diode junction
.

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
-
biased Collector region. As a result the electrons
which get into the Base move swiftly towards the Collector and cross into the Collector
region. Hence we see a Emitter
-
Collector current whose magnitude is set by the chosen
Emitter
-
Base voltage we have applied.

Each curve shows how the collector current, IC, varies with the Collector
-
Emitter
voltage, VCE, for a specific fixed value of the Base current, IB.

Characteristic curves from a working Bipolar Transistor


FET (Field Effect Transistor)

The field
-
effect transistor (FET) controls the current between two points but does so
differently than the bipolar transistor.


The FET operates by the effects of an electric
field on the flow of electrons through a single type of semiconductor material. There
are two basic types of FET.




The
J
-
FET (Junction Field Effect Transistor )

and the
MOS
-
FET (Metal
-
Oxide
-
Semiconductor FET)

are voltage controlled devices: that is a small change in input
voltage causes a large change in output current.


FET operation involves an electric field which controls the flow of a charge (current )
through the device. In contrast, a bipolar transistor employs a small input current to
control a large output current.


The source, drain, and gate terminal of the FET are analogous to the emitter,
collector,


and base of a bipolar transistor .


The terms n
-
channel and p
-

channel refer to the material which the drain and source
are connected.

In the junction FET (JFET), the gate material is made of the opposite polarity
semiconductor to the channel material (for a P
-
channel FET the gate is made of N
-
type semiconductor material).



Current is high if the junction is forward biased and is extremely small when
the junction is reverse biased
.



J
-
FET (Junction Field Effect Transistor)

JFET

In an n
-
channel device, the channel is made of n
-
type semiconductor, so the charges
free to move along the channel are negatively charged
-

they are electrons. In a p
-
channel device the free charges which move from end
-
to
-
end are positively (hence p)
charged
-

they are holes. In each case the source puts fresh charges into the channel
while the drain removes them at the other end.

JFET

Placing an insulating layer between the gate and the channel allows for a wider range
of control (gate) voltages and further decreases the gate current (and thus increases
the device input resistance)
MOSFET

Metal Oxide Semiconductor Field Effect
Transistor (MOSFET)

The insulator is typically made of an oxide (such as silicon dioxide, SiO
2
), This type of
device is called a metal
-
oxide
-
semiconductor FET (MOSFET).

The general structure is a lightly doped
p
-
type substrate, into which two regions, the
source and the drain, both of heavily doped
n
-
type semiconductor have been
embedded. The symbol
n
+ is used to denote this heavy doping.

MOSFET

The
p
-
type doped substrate is only very lightly doped, and so it has a very high
electrical resistance, and current cannot pass between the source and drain if there is
zero voltage on the gate.


Application of a positive potential to the gate electrode creates a strong electric field
across the
p
-
type material even for relatively small voltages, as the device thickness is
very small and the field strength is given by the potential difference divided by the
separation of the gate and body electrodes.


Since the gate electrode is positively charged, it will therefore repel the holes in the
p
-
type region. For high enough electrical fields, the resulting deformation of the energy
bands will cause the bands of the
p
-
type region to curve up so much that electrons will
begin to populate the conduction band.


The population of the
p
-
type substrate conduction bands in the region near to the oxide
layer creates a conducting channel between the source and drain electrodes,
permitting a current to pass through the device.


The population of the conduction band begins above a critical voltage, VT, below
which there is no conducting channel and no current flows. In this way the
MOSFET may be used as a switch.


Above the critical voltage, the gate voltage modulates the flow of current
between source and drain, and may be used for signal amplification.


The bias voltage on the gate terminal either attracts or repels the majority carriers of
the substrate across the PN junction with the channel.


This narrows (depletes) or
widens (enhances) the channel, respectively, as V
GS

changes polarity.



For N
-
channel MOSFETs, positive gate voltages with respect to the substrate and
the source (V
GS

> 0) repel holes from the channel into the substrate, thereby
widening the channel and decreasing channel resistance.


Conversely, V
GS

< 0
causes holes to be attracted from the substrate, narrowing the channel and
increasing the channel resistance.

If we apply a positive voltage to the gate we'll set up an electrostatic field between it
and the rest of the transistor. The positive gate voltage will push away the ‘holes’
inside the p
-
type substrate and attracts the moveable electrons in the n
-
type regions
under the source & drain electrodes. This produces a layer just under the gate's
insulator through which electrons can get into and move along from source to drain.

The positive gate voltage therefore ‘creates’ a channel in the top layer of material.

Increasing the value of the positive gate voltage pushes the p
-
type holes further
away and enlarges the thickness of the created channel. As a result we find that the
size of the channel we've made increases with the size of the gate voltage and
enhances or increases the amount of current which can go from source to drain