Direct and Indirect band gap semiconductors

Planar LED
PN Junction Diode
 P
Diode

N -
- - - - - + + + + + +
- - - - - - ++++++
- - - - - - ++++++
- - - - - - ++++++
 P Formation of Depletion
Layer

N -
- - - - - + + + + + +
- - - - - - ++++++
- - - - - - ++++++
- - - - - - ++++++
PN Junction Diode
I-V Characteristics
of Diode
 Zener Diode
• The Zener diode is a special
kind of heavily doped PN
junction diode designed to
conduct in reverse the reverse
direction when a certain
specified voltage reached.
Symbol
• It is a silicon semiconductor
switch that permits the current
to flow in both directions,
either a forward or reverse
direction.

• The voltage drop of the diode

remains constant for the wide
range of the voltage.
 Construction
• The operation of a Zener diodes depends on the doping
level of the PN junction. The depletion region is very thin
and the electric field is very high even for a small
reverse bias voltage. And it allows the electron to move
from the valence band of P-type material to the
conduction band of the N-type material.
• The breakdown voltage of a diode can be accurately
control at the time of the doping level. The breakdown
voltage of a commonly available Zener diodes
• Diodes which are lightly doped and the breakdown
voltage is less than 5.6 V, the breakdown denoted by the
avalanche effect rather than the Zener effect.
 Zener

P N
Diode

e
e
-
-e - - +e +e +e
e
-
-
-
-
Conduction
Band
-
- - - ++ + -

- ve - - - ++ + Conduction
Band

- - - ++ + +
ve
Valence Band - - e- + + +
-e e- - +e +e +e
-
-
- Valence Band
-
- - - ++ + -
- h
+
 Zener

P N
Diode

e
-e - - + + +e
e
-
-
e
- -
Conduction
Band - - - ++ +
-

- - - ++ + Conduction
Band

- - - ++ +
Valence Band - - - ++ +
-e -e - + +e +e
e
-
-
Valence Band

- - - ++ +
-
- -
 Working
• When the PN junction diode is reverse bias, the depletion layer
becomes wider. And if the reverse breakdown voltage increase,
the depletion layer begins to wider.
• In this condition, the minority charge carriers get sufficient
kinetic energy due to a strong electric field after a certain
reverse breakdown voltage. And free electrons have sufficient
kinetic energy that collides with the stationary ions of the
depletion layer and releases more free electrons.
• These new free electrons will collide with other stationary ions
and produce more free electrons. This is a cumulative process.
And in a short time, it will produce more free electrons in the
depletion region and it becomes the entire diode conductive.
• This type of breakdown known as an avalanche breakdown, but
it is not quite sharp. The breakdown which is sharper than the
avalanche breakdown is known as the Zener breakdown.
• The voltage at which the breakdown occurs, that voltage known
as the Zener voltage. With the help of a proper doping level, the
Zener voltage can adjust at the time of manufacturing.
V-I Characteristic of Zener Diode
• When it connected in forward bias,
it reacts the same as normal PN
junction diode.
• When it connected in the reverse
bias and supply voltage is more
than Zener voltage, a very sharp
breakdown occurs as shown in
below VI characteristics.
• In reverse condition, the current is
very small for the starting. This
current is due to minority charges
carriers.
• When the breakdown voltage
increases than the Zener voltage,
the Zener breakdown will occur
and current will increase
immediately and the diode will
conduct without damage.
• The point at which current starts
flowing, that point is known as knee
point or Zener knee.
PHOTODETECTORS
Photoconductive Effect
➢ When light is incident on an intrinsic semiconductor, electrons are excited from the
valence band to the conduction band. Such electrons leave behind holes in the
valence band. Thus, free electrons and holes are generated in the material; but they
do not leave the material.
➢ Therefore, an increase of free charge carrier concentration occurs within the
semiconductor.
➢ This is known as internal photoelectric effect. An electron gets excited to the
conduction band from the valence band by a light photon provided the photon
energy, hv is greater than the band gap energy, Eg. That is, hv ≥ Eg. It means that
the frequency of the photon should satisfy the following condition.
➢ An increase in free charge carriers leads to an increase in the conductivity of the
semiconductor.
➢ The light-induced increase in the electrical conductivity called photoconductive
effect or simply photoconductivity.
➢ The application of an electric field to the semiconductor causes the drifting of
electrons and holes through the material and as a result, an electric current flows in
the circuit.
➢ Photodetectors are devices that absorb optical energy and convert it to electrical
energy. The operation of photoelectric detectors is based on the internal
photoelectric effect.
➢ There are three main types of photodetectors, namely, photodiodes, pin diodes and
avalanche photodiodes, which are widely used in optical communication systems.
Photodiode
➢ Photodiodes are essentially the same as the p-n junction diodes. During the
fabrication of the p-n diode, a depletion layer forms at the junction region by
immobile negatively charged acceptor atoms in the p type material and immobile
positively charged donor ions in the n type material.
➢ The electric field due to these ions stops the motion of majority carriers but
accelerates minority carriers across the junction. When a photon is incident on the
device, an electron-hole pairs are generated.
➢ In case of electron-hole pairs generated within the depletion region, the electric
field acting across the region causes the pair to separate as shown in Fig.
This charge separation can be utilized in two ways.
➢ If the diode is short-circuited externally, a current flows between p and n regions. It
is known as the photoconductive mode of operation. The diode is reverse biased
for photoconductive operation. On the other hand, if the diode is left on open-circuit,
an externally measurable voltage appears between p and n regions. This is known as
photovoltaic mode of operation. This mode of operation is used in solar cells.
➢ A semiconductor photodiode is a reverse biased p-n junction. The structure of a
photodiode is shown in Fig. When a reverse bias is applied across the junction the
depletion layer widens as mobile carriers are swept to their respective majority
sides. The motion of minority carriers causes the reverse leakage current of the
diode. Thus, even when no light radiation is present (zero light), a small leakage
current exists. This leakage current is called dark current. The amount of dark
current depends on the reverse bias voltage, the series resistance and the ambient
temperature.
➢ When the diode is illuminated by light, photons are absorbed mainly in the
depletion layer and also in the neutral regions. A photon of energy hv ≥ Eg incident
in or near the depletion layer of the diode will excite an electron from the valence
band to the conduction band. This process generates a hole in the valence band.
Thus an electron-hole pair is generated by the optical photon. These are known as
photocarriers.
➢ The electron-hole pairs generated in the depletion layer separate and drift in
opposite directions under the action of the electric field. Such a transport process
induces an electric current in the external circuit in excess of the already existing
dark current (reverse leakage current). The photocurrent created in the external
circuit is always in the reverse direction, i.e., from the n to the p region. Increasing
the level of illumination increases the reverse current flowing.
➢ The light incident in the neutral region, on either side of the depletion layer, also
produces electron-hole pairs. Electrons and holes generated within a diffusion
length of the depletion layer will move randomly and slowly diffuse into the
depletion region and are accelerated by the bias, thereby contributing to the
photocurrent. Thus, optical excitation leads to an increase in the reverse-biased
current.
➢ It is desirable that the depletion region be sufficiently wide so that a large fraction
of incident light can be absorbed. Therefore, the diode can be used as a
photodetector—using a reverse bias voltage—as the measured photocurrent is
proportional to the incident light intensity.
p-i-n Photodiode
➢ The structure of a p-i-n photodiode is shown in Fig. It is a device that consists of p
and n regions separated by a very lightly doped intrinsic region (i).
➢ The first and most important feature of p-i-n photodiode is that its depletion region
extends well into the intrinsic region, as it is lightly doped.
➢ Under sufficiently large reverse bias, the depletion region could extend through the
intrinsic region, whereby the entire intrinsic region could be made free of charge
carriers.
➢ The intrinsic layer in effect widens the depletion region and therefore increases area
available for capturing light.
➢ When photon is incident on the depleation layer elctron- hole pair is generated.
➢ These carriers are mainly generated in the depletion (depleted intrinsic) region where
most of the incident light is absorbed.
➢ The high electric field present in the depletion region causes the free carriers to
separate and be collected across the reverse biased junction. This gives rise to a
current flow in the external circuit.
advantage
➢ As the intrinsic layer is wide enough, most of the photons are absorbed and larger
photocurrent is produced. Therefore, p-i-n photodiode is more sensitive than pn-
photodiode.
Avalanche Photodiode
➢ An avalanche photodiode (APD) is more sophisticated than a p-i-n diode and
incorporates internal gain mechanism so that the photoelectric current is amplified
within the detector.
➢ . The structure of a typical APD is shown in Fig. This configuration is known as p+π
p n+ reach-through structure.
➢ The device is essentially a reverse-biased p-n junction. The n+ and p+ are heavily
doped semiconductors and have very low resistance. The p region is very lightly
doped and hence is nearly intrinsic.
➢ A photon that enters through the p+ region is absorbed in the intrinsic region and the
resulting electron-hole pair is separated by the electric field in the p region.

➢ The hole drifts towards the p+ and do not take part in the multiplication process.
➢ The electron drifts through the p region to the pn+ junction. There, the electric field
due to high reverse bias accelerates the electron. The electron acquires enough
kinetic energy to ionize neutral atoms in its path.
➢ The electrons thus produced get in turn accelerated and ionize atoms lying in their
paths. The effect is cumulative and builds up into an avalanche.
➢ As a result, one electron-hole pair will on an average produce M electron-hole pairs
in the process, where M is the multiplication factor.
➢ Thus there occurs a carrier multiplication and internal amplification. This internal
amplification process enhances the responsivity of the detector.
➢ Advantages of APD
● Internal current gain due to carrier multiplication
● High frequency response
SOLAR CELL
A solar cell is basically a p-n junction that can generate electrical power, when
illuminated.
Solar cells are usually large area devices typically illuminated with sunlight and are
intended to convert the solar energy into electrical energy.
➢ The schematic of a solar cell is shown in Fig.(a). It consists of a p-type chip on
which a thin layer of n-type material is grown.
➢ When the solar radiation is incident on the cell, electron-hole pairs are generated in
the n and p regions. The majority of them cannot recombine in the regions.
➢ They reach the depletion region at the junction where an electric field due to the
space charge separates them. Electrons in the p-region are drawn into the n-region
and holes in the n-region are drawn into the p-region.
➢ It results in accumulation of charge on the two sides of the junction and produces a
potential difference called photo emf.
➢ The power generated depends on the solar cell itself and the load connected to it.
➢ The I-V characteristic of a solar cell is shown in Fig. We identify the open-
circuit voltage, Voc, as the voltage across the illuminated cell at zero current.
The short-circuit current, Isc, is the current through the illuminated cell if the
voltage across the cell is zero.
➢ The short-circuit current is close to the photocurrent while the open-circuit
voltage is close to the turn-on voltage of the diode as measured on a current
scale similar to that of the photocurrent.
➢ The power equals the product of the diode voltage and current and at first
increases linearly with the diode voltage but then rapidly goes to zero
around the turn-on voltage of the diode.
➢ The maximum power is obtained at a voltage labeled as Vm with Im being
the current at that voltage. Solar cells can be connected in parallel or series
into solar panels, which can deliver power output of several kilowatts.
➢ Solar panels are used in numerous applications in remote locations and in
space. Solar cells of all kinds are used in different consumer products – from
watches and calculators to power supplies for laptop computers.
HALL EFFECT
➢ If a metal or a semiconductor carrying a current I is placed in a transverse magnetic
field B, a potential difference VH is produced in a direction normal to both the
magnetic field and current directions.
➢ This is known as Hall effect. This effect was discovered by E.H. Hall in 1879 and
showed that it is negatively charged particles that carry current in metals.
➢ The importance of Hall effect in the field of semiconductors is that it helps to
determine
(i) the type of semiconductor,
(ii) the sign of majority charge carriers,
(iii) the majority charge carrier concentration,
(iv) the mobility of majority charge carriers, and
(v) the mean drift velocity of majority charge carriers.
Applications
Hall effect is widely used in various fields for a variety of applications. In almost all cases a
Hall effect sensor is employed. A Hall effect sensor is a transducer that produces its output
voltage in response to changes in magnetic fields.
1. Determination of semiconductor type: The Hall coefficient is negative for a p-type
semiconductor and positive for a p-type semiconductor. Therefore, the sign of the Hall
coefficient can be used to determine whether a given semiconductor is n- or p-type.

2. Determination of carrier concentration: By measuring the Hall coefficient, the carrier

concentration in a semiconductor can be determined making use of the relations
n = 1/RHe or p = 1/RHe

3. Determination of carrier mobility: By measuring the Hall coefficient and conductivity

of the semiconductor, the carrier mobility can be determined using the relation μh = σ |RH|

4. Measurement of magnetic fields: Hall voltage is proportional to the magnetic field

intensity, for a given current through the sample. Therefore, one of the important
applications of Hall effect consists in measuring magnetic fields. Knowing the
parameters of the Hall probe, and applied current, we can determine the intensity of the
magnetic field. Hall probes can be used for static as well as high-frequency magnetic
fields. Hall probes measure variable magnetic fields up to a frequency of 1012 Hz.
Transistor
Formation of Depletion layer
BIASING THE TRANSISTOR
The two junctions of a transistor can be biased in four different ways.
(i) Both the junctions may be forward biased. It causes large currents to flow across
the junctions. The currents join together in the base and flow down the common
lead. Then the transistor is said to be operating in saturation region.
(ii) Both the junctions may be reverse biased. Very small currents flow through the
junctions. The transistor is said to be in cut-off region.
(iii) EB-junction may be reverse biased and CB-junction forward biased. The
transistor is said to operate in an inverted mode.
(iv) EB junction may be forward biased and the CB junction reverse biased. Such
biasing arrangement causes a large current to flow across the EB-junction as well
as CB-junction.
(v) Further, the collector current is controlled by the emitter current or base current.
With such biasing, the transistor is said to operate in active region or in normal
mode.
Transistor Configurations:
ACTION OF THE BIAS

S1 is closed
S2 is closed
S1 & S2 both are closed
Various types of currents flowing in the transistor