What is a Diode?
A diode is the simplest sort of
device. Broadly speaking, a semiconductor is a
material with a varying ability to conduct electrical
current. Most semiconductors are made of a poor
conductor that has had impurities (
material) added to it. The process of adding
impurities is called doping.
In the case of LEDs, the conductor material is
arsenide (AlGaAs). In
arsenide, all of the atoms bond perfectly to their nei
leaving no free electrons (negatively charged particles) to conduct electric current. In
doped material, additional atoms change the balance, either adding free electrons or
creating holes where electrons can go. Either of these alterations make th
e material more
A semiconductor with extra electrons is called N
type material, since it has extra negatively charged
particles. In N
type material, free electrons move
from a negatively charged area to a positively
or with extra holes is called P
material, since it effectively has extra positively
charged particles. Electrons can jump from hole to
hole, moving from a negatively charged area to a
positively charged area. As a result, the holes
themselves appear t
o move from a positively charged
area to a negatively charged area.
A diode consists of a section of N
bonded to a section of P
type material, with
electrodes on each end. This arrangement conducts
electricity in only one direction. When no
applied to the diode, electrons from the N
material fill holes from the P
type material along the
junction between the layers, forming a depletion zone.
In a depletion zone, the semiconductor material is
returned to its original insulating
all of the
holes are filled, so there are no free electrons or empty
spaces for electrons, and charge can't flow.
To get rid of the depletion zone, you have to get
electrons moving from the N
type area to the P
area and holes moving in the re
verse direction. To do
this, you connect the N
type side of the diode to the negative end of a circuit and the P
type side to the positive end. The free electrons in the N
type material are repelled by the
negative electrode and drawn to the positive elect
rode. The holes in the P
move the other way. When the voltage difference between the electrodes is high enough,
the electrons in the depletion zone are boosted out of their holes and begin moving freely
again. The depletion zone disappears, a
nd charge moves across the diode.
If you try to run current the other way, with the P
type side connected to the negative end
of the circuit and the N
type side connected to the positive end, current will not flow. The
negative electrons in the N
rial are attracted to the positive electrode. The
positive holes in the P
type material are attracted to the negative electrode. No current
flows across the junction because the holes and the electrons are each moving in the
wrong direction. The depletion
DC TRANSISTOR CIRCUITS
Most of us deal with electronic products on a routine
basis and have some experience with personal
computers. A basic component for the integrated
circuits found in these electronics and computers is the
terminal device known as the transistor.
Understanding the transistor is essential before an
engineer can start an electronic circuit design.
All about NPN and PNP transistors
A transistor is a semiconductor, meaning that
sometimes it conducts e
lectricity, and sometimes it doesn’t. Its internal
resistance varies, depending on the power that you apply to its base. NPN and
PNP transistors are bipolar semiconductors. They contain two slightly different
variants of silicon, and conduct using both pol
arities of carriers
The NPN type is a sandwich with P
type silicon in the middle, and the PNP
type is a sandwich with N
type silicon in the middle
All you need to remember
All bipolar transistors have three connections: Collector
, Base, and Emitter,
abbreviated as C, B, and E on the manufacturer’s data sheet, which will identify
the pins for you.
NPN transistors are activated by
voltage on the base relative to the
PNP transistors are activated by
on the base relative to the
In their passive state, both types block the flow of electricity between the collector and
emitter, just like an SPST relay in which the contacts are normally open. (Actually a
transistor allows a tiny bit of current k
nown as “leakage.”)
You can think of a bipolar transistor as if it contains a little button inside. When the button is pressed, it
allows a large current to fl
ow. To press the button, you inject a much smaller current into the base by
applying a small voltage to the base. In an NPN transistor,
the control voltage is positive. In a PNP transistor, the control voltage is negative.
NPN transistor basics
• To start
the flow of current from collector to emitter, apply a relatively positive voltage to the base.
• In the schematic symbol, the arrow points from base to emitter and shows the direction of positive current.
• The base must be at least 0.6 volts “more positi
ve” than the emitter, to start the flow.
• The collector must be “more positive” than the emitter.
PNP transistor basics
• To start the flow of current from emitter to collector, apply a relatively negative voltage to the base.
• In the schematic symbol, t
he arrow points from emitter to base and shows the direction of positive current.
• The base must be at least 0.6 volts “more negative” than the emitter, to start the flow.
• The emitter must be “more positive” than the collector.
ver apply a power supply directly across a transistor. You
WILL BURN IT OUT
with too much
Protect a transistor with a resistor, in the same way you would protect an LED.
Avoid reversing the connection of a transistor between positive and negative
Sometimes an NPN transistor is more convenient in a circuit; sometimes a PNP happens to fit
They both function as switches and amplifiers, the only difference being that you apply a relatively
positive voltage to the base of an NPN t
ransistor, and a relatively negative voltage to the base of a
PNP transistors are used relatively seldom, mainly because they were more difficult to manufacture
in the early days of semiconductors. People got into the habit of designing cir
cuits around NPN
Remember that bipolar transistors amplify current, not voltage. A small fluctuation of current
through the base enables a large change in current between emitter and collector.
Schematics sometimes show transistors with circle
s around them, and sometimes don’t. I’ll
circles to draw attention to them unless I forget.
You can think of a bipolar transistor as if it
contains a button that can connect the
collector and the emitter. In an NPN
transistor, a small positive pote
In a PNP transistor, a small negative
potential has the same effect of c
the emitter and collector. The arrows point
in the direction of “positive current flow.”
Schematics may show the emitter at the top and the collector at the bottom, or vice versa.
The base may
be on the left, or on the right, depending on what was most convenient for
the person drawing the schematic. Be careful to look carefully at the arrow in the
transistor to see which way up it is, and whether it is NPN or PNP. You can damage a
connecting it incorrectly.
Transistors come in various different sizes and configurations. In many of them, there is
no way to tell which wires connect to the emitter, the collector, or the base, and some
transistors have no part numbers on them. Before y
ou throw away the packaging that
came with a transistor, check to see whether it identifies the terminals.
If you forget which wire is which, some multimeters have a function that will identify
emitter, collector, and base for you.
There are two basic ty
pes of transistors: bipolar junction transis
tors (BJTs) and ﬁeld
effect transistors (FETs). Here, we consider only the BJTs, which were the ﬁrst of the
two and are still used today. Our objective is to present enough detail about the BJT to
enable us to apply the techniques developed in this chapt
er to analyze dc transistor
There are two types of BJTs: npn and pnp, with their circuit symbols as shown in Fig.
Each type has three terminals, designated as emitter (E), base (B), and collector (C).
The symbol for an NPN transistor always
has an arrow pointing from its base to its
emitter. Some people include a circle
around the transistor; others don’t bother.
The style of the arrow may vary, but the
meaning is always the same.
The symbol for a PNP transistor always has
an arrow pointing from its emitter to its
base. Some people include a circle around
the transistor; others don’t bother. The style
of the arrow may vary, but the meaning is
always the sa
How a BJT (Bipolar Junction
Its all in the doping
The way a transistor works can be described
based on the figure which shows the basic doping
of a junction transistor and figure below which
shows the method of operation of the
The operation of the transistor is ve
dependent on the degree of doping of
the various parts of the semiconductor
crystal. The N type emitter is very
heavily doped to provide many free
electrons as majority charge carriers.
The lightly doped P type base region is
extremely thin, and the N t
is very heavily doped to give it a low
resistivity apart from a layer of less
heavily doped material near to the base
region. This change in the resistivity of
the collector close to the base, ensures
that a large potential is present within
the collector material close to the base.
The importance of this will become apparent from the following description.
During normal operation, a potential is applied across the base/emitter junction so that the
base is approximately 0.6v more positive tha
n the emitter, this makes the base/emitter
junction forward biased.
A much higher potential is applied across the base/collector junction with a relatively
high positive voltage applied to the collector, so that the base/collector junction is heavily
rse biased. This makes the depletion layer between base and collector quite thick
once power is applied.
As mentioned above, the collector is made up of mainly low resistivity material with a
layer of high resistivity material next to the base/collector j
unction. This means that most
of the voltage between collector and base is developed across this high resistivity layer,
giving a high voltage gradient near the collector base junction.
When the base emitter junction is forward biased, a small current wil
l flow into the base.
Therefore holes are injected into the P type material. These holes attract electrons across
the forward biased base/emitter junction to combine with the holes. However, because the
emitter region is very heavily doped, many more elect
rons cross into the base region than
are able to combine with holes. This means there is a large concentration of electrons in
the base region and most of these electrons are swept straight through the very thin base,
and into the base/collector depletion
layer. Once here, they come under the influence of
the strong electric field across the base/collector junction. This field is so strong due to
the potential gradient in the collector material mentioned earlier, that the electrons are
swept across the depl
etion layer and into the collector material, and so towards the
Varying the current flowing into the base, affects the number of electrons attracted from
the emitter. In this way very small changes in base current cause very large chan
the current flowing from emitter to collector, so current amplification is taking place.
Circuit Analysis of Transistors
For the npn transistor, the currents and
voltages of the transistor are
speciﬁed as shown at right.
Applying KCL gives
where IE, IC , and IB are emitter, collector, and base currents,
Similarly, applying KVL to (b) gives
, and V
base, and base
collector voltages. The BJT can ope
rate in one of three modes: active,
cutoff, and saturation. When transistors operate in the active mode, typically V
where α is called the common
base current gain. α denotes the fraction of electrons
injected by the emitter that are collected by the collector. Also,
where β is known as the common
emitter current gain. The α and β are characteristic
properties of a given transistor and assume constant values for that transistor. Typically, α
takes values in the range of 0.98 to 0.999, while β takes values in the range 50 to 1000.
It is evident that
= (1 + β)I
equations show that, in the active mode, the BJT
can be modeled as a dependent current
current source. Thus, in circuit analysis, the dc
equivalent model in (b) may be used to replace the npn
transistor in (a). Since β is large, a small base cu
controls large currents in the output circuit.
Consequently, the bipolar transistor can serve as an
ampliﬁer, producing both current gain and voltage gain.
Such ampliﬁers can be used to furnish a considerable
amount of power to transducers such as
loudspeakers or control motors.
, and v
in the transistor circuit. Assume that the transistor operates in the
active mode and that β = 50. Hint: recall that Vbe = 0.7 V in active mode.
First find I
Next find I
= 50 × 165 µA = 8.25 mA
Now use KVL to find v
− 100IC + 6 = 0
= 6 − 100I
= 6 − 0.825 = 5.175 V
Note that v
= VCE in this case.