Task 1
The figure above shows the crystalline structure of silicon with all the atoms. Each atom has
valence electrons, which when shared, form four covalent bonds. This leaves no ‘free’
electron(s) for electrical conduction and so Silicon acts as an insulator at zero temperature.
Since no thermal energy is added at absolute zero temperatures, the conduction band is empty
while the valence band is filled with electrons. With increase in temperatures, thermal energy
is added to the semi-conductor and so this excites the electrons to move from the valence band
to the conduction band thus creating electron-hole pair(s) necessary for electrical conduction
when biased. Therefore, Silicon becomes a good conductor with increasing temperatures.
Task 2
(a)
Current - Voltage characteristics of a Diode
4
If
3
2
1
0
-5
-4
-3
-2
Vr
-1
-1
0
1
-2
-3
-4
At first, the diode current is ideally zero despite the increasing forward-bias voltage. This
happens till the cut-in voltage of the diode is attained (at approximately 0.7 [V]); beyond which
the diode behaves like an ohmic device i.e. the current flowing through it increases linearly
with the increasing bias voltage.
(b)
At first, the diode does not conduct despite the increasing reverse-biased voltage i.e. the diode
current is ideally zero. This happens till the reverse breakdown voltage is attained (at
approximately 4 [V]). Reverse breakdown voltage is the maximum voltage that can be applied
to the diode when connected in reverse-bias conditions. When attained, junction breakdown
occurs and the current increases instantaneously.
Task 3
Biasing
Application of external potential across the p-n junction results into biasing.
Reverse - biasing
In the case of reverse biasing, the negative terminal of this voltage is connected to the p-side
of the semiconductor while the positive side is connected to the n-type region. This causes a
huge number of delocalized electrons to move to the positive terminal. In so doing, the number
of positively charged uncovered ions left in the depletion region of the n-type material is
increased and so are the negative ones in the p-type material. Therefore, the depletion region
widens and consequently establishes a huge barrier to be overcome by the majority carriers.
Hence, the majority carrier flow is reduced to zero. Reverse saturation current flows in the
direction opposite to that of the flow of minority carriers as indicated in the diagram below.
The diode is reverse-biased when the potential difference is less than zero i.e.
𝑽𝑫 < 𝟎
Forward - biasing
In the case of forward biasing, the positive terminal of applied voltage is connected to the pside of the semiconductor while the negative side is connected to the n-type region. This results
into recombination of the holes in the p-substrate and electrons in the n-substrate with the
boundary ions. The depletion region thus narrows, and majority carriers heavily flow across
the junction.
Consequently, this causes the diode current to flow in the direction of the flow of majority
carriers as indicated in the diagram below.
The diode is forward-biased (conducts) when the potential difference is greater than zero i.e.
𝑉𝐷 > 0
And it is blocked when
𝑉𝐷 < 0
Increase in the bias voltage decreases the width of the potential barrier due to more rapid
recombination taking place.
Task 4
a)
Application
The application of Zener diode considered here is a shunt regulator. Below is a diagram of this
regulator.
Operation
Here, a Zener diode is connected in a reverse bias mode across the load. The Zener diode
ensures that the voltage across the load is always at a constant value despite the variations in
the input voltage. When the input voltage exceeds the breakdown voltage of the diode, the
diode starts to conduct and outputs a voltage equal to its breakdown voltage. This voltage is
always constant despite the variations in the input.
Other applications of the Zener diode include:
 Reference elements
 Voltage clippers
 Switching applications
 Surge suppressors
b)
An LDR operates on the principle that its resistance increases when it is dark and decreases
when light is stronger.
Consider the circuit of light automation using an LDR as shown below,
When sufficient light strikes on the LDR, its resistance becomes low. This causes the voltage
divider formed by the LDR and R2 to produce nearly 0 V at its output. The timer is reset thereby
keeping the light OFF.
During the dark, the resistance of the LDR increases. This pulls up the reset pin of the timer
hence setting the timer in astable mode. This switches ON the light.
c)
The diagram below illustrates the working principle of a photodiode.
When a photo of sufficient energy strikes the photo diode, it dislodges electrons and holes. The
electrons move towards the cathode while the holes move towards anode. This generates a
photocurrent and hence the diode conducts.
d)
The series resistor limits the current flowing through the diode hence preventing excessive
current that would otherwise blow out the light.
Task 5
Construction of FET
FETs have three terminals namely the drain, source and gate. The channel can be p-type or ntype. For a p-type channel, majority charge carriers flow from the source terminal to the drain.
Below is a diagram of the FET.
The gate terminal is reverse biased to a voltage source. This results into formation of a depletion
layer between the gate and the channel. An increase in the reverse voltage increases the width
of the depletion layer. This increases the overall resistance thereby impeding the flow of
current. This implies that FETs are current-controlled devices.
Construction of BJT
A BJT is a combination of two p-n junctions with 3 terminals namely the collector, emitter and
base. Depending on the nature of the p-n junction, a BJT can be classified as either PNP or
NPN. The diagram below shows the construction of a BJT.
For a PNP BJT, an n-type semiconductor is placed in between p-type semiconductors whereas
for an NPN BJT, a p-type substrate is sandwiched between 2 n-type semiconductors.
Differences between BJT & FET
FETs are operated in 3 modes i.e. active, cut-off and ohmic regions whereas BJTs are operated
in the active, saturation and cut-off modes.
FETs are current-controlled devices whereas BJTs are voltage-controlled devices.