Semi_conductors

There are Different types of materials:

Conductors, semiconductors and insulators can be distinguished on the ground of their conductivity and other properties. Conductors like metals show conductivity at room temperature, but as the temperature increases, their conductivity gets reduced.

However, semiconductors act as the insulators at low temperature but as the temperature increases so their conducting properties also; however, insulators have no such effect of temperature variations as they do not have the conducting properties.

• Conductors are the substance that transmits heat or electricity through them.
• Semi_conductors:Such substance or materials that may act as a conductor, as well as insulators under different conditions.
• Insulators:are the substance that does not allow heat or electricity to pass through them.

We have to explain some definations :

• Holes is the lack of an electron at a position where one could exist in an atom.
• Valence Band is the highest range of electron energies in which electrons are normally present at absolute zero temperature.
• Band gap is the distance between the valence band of electrons and the conduction band.

The difference between the 3 types of metal is shown in the table .

Semiconductors: are materials which have a conductivity between conductors(generally metals) and nonconductors or insulators (such as most ceramics). Semiconductors can be pure elements, such as silicon or germanium, or compounds such as gallium arsenide or cadmium selenide.

It divide into 2 types:
1-intrinsic semiconductor:

An intrinsic semiconductor is an undoped semiconductor (Pure Semiconductor). This means that holes in the valence band are vacancies created by electrons that have been thermally excited to the conduction band,as by increasing the Temperature the bond between atom breaks down and electron leaves bond to be free electron leaving behind hole as in figure(3).So the conductivity of semiconductor increases.

2-extrinsic semiconductors:

Extrinsic semiconductors are semiconductors that are doped with specific impurities. The impurity modifies the electrical properties of the semiconductor and makes it more suitable for electronic devices such as diodes and transistors.
While adding impurities, a small amount of suitable impurity is added to pure material, increasing its conductivity by many times. But before we take about its type let’s take about doping:

• Dopping:

The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, since these foreign atoms incorporated into the crystal structure of the semiconductor provide free charge carriers (electrons or electron holes) in the semiconductor. In an extrinsic semiconductor it is these foreign dopant atoms in the crystal lattice that mainly provide the charge carriers which carry electric current.

Types of extrinsic semiconductor:

1) P-type:

An extrinsic semiconductor which has been doped with electron acceptor atoms is called a p-type semiconductor, because the majority of charge carriers in the crystal are electron holes (positive charge carriers).

2)N-type:

Is defined as a type of extrinsic semiconductor doped with a pentavalent impurity element which has five electrons in its valence shell. The pentavalent impurity or dopant elements are added in the N-type semiconductor to increase the number of electrons for conduction.

• A p–n junction:is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains an excess of electrons in the outer shells of the electrically neutral atoms there. This allows electrical current to pass through the junction only in one direction.

• In the pn junction, carrier particles move in 3 ways:

1. Random motion
2. Diffusion
3. Drift

• Due to the various types of carrier motion, pn junctions have 3 types of currents which their operation and usage depend on, each with their own laws:

1. Diffusion current
   
2. Drift current
3. External current

• Pn junctions are useful due to their bias towards electric current.
• Types of motion:

• Random motion:
  Both types of carriers are particles, so they have kinetic energy that allows them to move freely around the crystal lattice of their substance — crystal lattices are chemical configurations of materials which allow charge carriers to move freely between atoms. However, they move randomly till they collide with each other or with some atom, therefore their motion isn’t guided by some force and is totally random and is considered a type of brownian motion(which is the random motion of particles in a fluid due to their collision with each other). Bronwian motion is too complex to predict and therefore we don’t use equations to predict it, but we do account for it when analyzing pn junctions. Brownian motion of 5 particles among 800 others. Courtesy of Francisco Esquembre, Fu-Kwun and lookang,here.

• Diffusion motion:

  P-type semiconductors are rich in positively charged holes, and N-type semiconductors are rich in negatively charged electrons. Naturally, both these particle types exist in their own charge type material (P or N). However, when they are connected together a concentration gradient occurs, so particles in high concentration areas naturally diffuse or move to the areas with low concentration—a concentration gradient is when there is a difference between the concentration of the same particle at different parts. I.e different parts have different concentrations. The movement of holes to the N-type region and vice versa is called diffusion and The rate at which diffusion occurs is called diffusivity and it depends on the velocity of carriers and on the distance between scatterings. termed diffusivity and is measured in cm2s-1.(figure(5))

• Different types of current in PN-junction:

1. Diffusion current:

The diffusion current can be defined as the flow of charge carriers within a semiconductor travels from a higher concentration region to a lower concentration region. A higher concentration region is nothing but where the number of electrons present in the semiconductor. Similarly, a lower concentration region is where the less number of electrons present in the semiconductor. The process of diffusion mainly occurs when a semiconductor is doped non-uniformly.
In an N-type semiconductor, when it is doped non-uniformly then a higher concentration region can be formed at the left side whereas the lower concentration region can be formed at the right side. The electrons in the higher concentration region are more in the semiconductor so they will experience a repulsive force from each other.

2.Drift current:

Drift current can be defined as the charge carrier’s moves in a semiconductor because of the electric field. There are two kinds of charge carriers in a semiconductor like holes and electrons. Once the voltage is applied to a semiconductor, then electrons move toward the +Ve terminal of a battery whereas the holes travel toward the –Ve terminal of a battery.
Here, holes are positively charged carriers whereas the electrons are negatively charged carriers. Therefore, the electrons attract by the +Ve terminal of a battery whereas the holes attract by the -Ve terminal of a battery.

• Diodes:

A p-n junction diode is two-terminal or two-electrode semiconductor device, which allows the electric current in only one direction while blocks the electric current in opposite or reverse direction. If the diode is forward biased, it allows the electric current flow. On the other hand, if the diode is reverse biased, it blocks the electric current flow.

• Diodes are also known as rectifiers because they change alternating current (ac) into pulsating direct current (dc).
• Diodes are rated according to their type, voltage, and current capacity.
• Diodes have polarity, determined by an anode (positive lead) and cathode (negative lead). Most diodes allow current to flow only when positive voltage is applied to the anode. A variety of diode configurations are displayed in (figure-2-in this step).

Its symbol and operation modes:

When a diode allows current flow, it is forward-biased. When a diode is reverse-biased, it acts as an insulator and does not permit current to flow.

1. Zero Bias :

No external voltage potential is applied to the PN junction diode.
2. Reverse Bias:

The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN junction diode’s width. (figure-3-)

3. Forward Bias:

The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN junction diodes width.(figure-4-)

• No current flow : If the voltage is applied to the PN junction in the opposite sense no current flows. The reason for this is that the holes are attracted towards the negative potential that is applied to the P type region.Similarly the electrons are attracted towards the positive potential which is applied to the N type region. In other words the holes and electrons are attracted away from the junction itself and the depletion region increases in width. Accordingly no current flows across the PN junction.

• Current Flow : If the voltage is applied such that the P type area becomes positive and the N type becomes negative, holes are attracted towards the negative voltage and are assisted to jump across the depletion layer.Similarly electrons move towards the positive voltage and jump the depletion layer. Even though the holes and electrons are moving in opposite directions, they carry opposite charges and as a result they represent a current flow in the same direction.

Strange but true: The diode symbol's arrow points against the direction of electron flow.

Reason: Engineers conceived the symbol, and their schematics show current flowing from the positive (+) side of the voltage source to the negative (-). It's the same convention used for semiconductor symbols that include arrows—the arrow points in the permitted direction of "conventional" flow, and against the permitted direction of electron flow.(figure-5-in this step)

The types of diodes:

1. Zener diode.
2. PN junction diode.
3. Tunnel diode.
4. Varactor diode.
5. Schottky diode.
6. Photodiode PIN diode.
7. Laser diode Avalanche diode.
8. Light emitting diode.(figure-6-)

Applications of Diode:

Diodes are mainly used in rectifiers, clippers, clampers, snubber circuits and voltage regulators. Its few applications along with Circuit diagram is explained below:

• Use of Diode as Rectifier:

• Use of Diode as Clipper:

Clipping is a waveform shaping where the input signal is clipped or cut to produce an output which is a flattened version of the input signal. Diode-clipping circuits are used in voltage limiting applications as this circuit eliminates voltages below zero.(figure-8)

• Use of Diode for Clamping:

An electronic circuit that prevents the signal from exceeding a certain defined value is known as a clamping circuit. Diode’s clamping circuits are used as voltage multipliers and to remove distortions in the signal.(figure-9)

Advantages of Diode

The advantages of Diodes include:

• Diodes are compact in size and compatible.
• Designing electronic circuit is simple with certain diodes like Zener diodes.
• Diodes help in controlling the current flow.
• These diodes produce less unwanted noise.
• Certain diodes like Schottky diodes can operate at high frequencies.
• Light emitting diodes are highly efficient compared to other diodes and they can emit light of the expected colors.
• Operates at high switching speed.

Disadvantages of Diodes

The disadvantages of Diodes include:

• Power dissipation is more for Zener diodes and hence it is less efficient for heavy loads.
• Diodes are highly sensitive to temperature.
• Amplification is necessary in photodiode based circuits.
• Light emitting diodes are expensive compared to other diodes.
• Certain diodes like Schottky diodes have low maximum reverse voltageThey have high reverse current and impedance.

• Transistors:

is a semiconductor device that can both conduct and insulate. A transistor can act as a switch and an amplifier. It converts audio waves into electronic waves and resistors, controlling electronic current. Transistors have a very long life, smaller in size, can operate on lower voltage supplies for greater safety, and required no filament current. The first transistor was fabricated with germanium. A transistor performs the same function as a vacuum tube triode but using semiconductor junctions instead of heated electrodes in a vacuum chamber. It is the fundamental building block of modern electronic devices and found everywhere in modern electronic systems.

• Transistor Basics:

A transistor is a three-terminal device. Namely,

1. Base: This is responsible for activating the transistor.
2. Collector: This is the positive lead.
3. Emitter: This is the negative lead.

The basic idea behind a transistor is that it lets you control the flow of current through one channel by varying the intensity of a much smaller current that’s flowing through a second channel.

• Types of Transistors:

• Types of transistor:

1. Bipolar Junction Transistor:

has three terminals connected to three doped semiconductor regions. It comes with two types, P-N-P and N-P-N.

• P-N-P transistor:

consisting of a layer of N-doped semiconductor between two layers of P-doped material. The base current entering the collector is amplified at its output.

That is when PNP transistor is ON when its base is pulled low relative to the emitter. The arrows of the PNP transistor symbol the direction of current flow when the device is in forwarding active mode.(figure-2-)

• N-P-N transistor:

consisting a layer of P-doped semiconductor between two layers of N-doped material. By amplifying current the base we get the high collector and emitter current.

That is when NPN transistor is ON when its base is pulled low relative to the emitter. When the transistor is in ON state, the current flow is in between the collector and emitter of the transistor. Based on minority carriers in the P-type region the electrons moving from emitter to collector. It allows the greater current and faster operation; because of this reason, most bipolar transistors used today are NPN.(figure-3-)

2-Field Effect Transistor (FET):

The field-effect transistor is a unipolar transistor, N-channel FET or P-channel FET are used for conduction. The three terminals of FET are the source, gate, and drain. The basic n-channel and p-channel FET’s are shown above. For an n-channel FET, the device is constructed from n-type material. Between the source and drain, then-type material acts as a resistor.
This transistor controls the positive and negative carriers concerning holes or electrons. FET channel is formed by moving of positive and negative charge carriers. The channel of FET which is made of silicon.

There are many types of FET’s, MOSFET, JFET, etc. The applications of FET’s are in a low noise amplifier, buffer amplifier, and an analog switch.(figure-4-)

3- IGBT Transistor :

An Insulated Gate Bipolar Transistor (IGBT) is a three-terminal power semiconductor device typically used as an electronic switch. IGBT's are types of transistors that are capable of switching electric power in many modern appliances such as electric cars, trains, variable speed refrigerators, air-conditioners and even stereo systems with switching amplifiers.(figure-5-)

4-MOSFET Transistor :

A Metal-Oxide-Semiconductor Field-Effect Transistor (MOFET) is used in integrated circuits to control the conductivity of a channel. MOSFETs are highly dependent on negative and positive charges. They have many purposes, including limiting a device's power levels, storing data, and being used as a switch for a wide variety of electronic devices.(figure-6-)

• Modes of biasing: (figure-7-)

1. Current biasing: As shown in Fig.1, two resistors RC and RB are used to set the base bias. These resistors establish the initial operating region of the transistor with fixed current bias. The transistor forward biases with a positive base bias voltage through RB. The forward base-emitter voltage drop is 0.7 volts. Therefore the current through RB is IB = (Vcc – VBE ) / IB

2. Feedback biasing: Fig.2 shows the transistor biasing by the use of a feedback resistor. The base bias is obtained from the collector voltage. The collector feedback ensures that the transistor is always biased in the active region. When the collector current increases, the voltage at the collector drops. This reduces the base drive which in turn reduces the collector current. This feedback configuration is ideal for transistor amplifier designs.

3. Double Feedback Biasing: Fig.3 shows how the biasing is achieved using double feedback resistors. By using two resistors RB1 and RB2 increase the stability concerning the variations in Beta by increasing the current flow through the base bias resistors. In this configuration, the current in RB1 is equal to 10 % of the collector current. 4. Voltage Dividing Biasing: Fig.4 shows the Voltage divider biasing in which two resistors RB1 and RB2 are connected to the base of the transistor forming a voltage divider network. The transistor gets biases by the voltage drop across RB2. This kind of biasing configuration is used widely in amplifier circuits.

5. Double Base Biasing: Fig.5 shows double feedback for stabilization. It uses both Emitter and Collector base feedback to improve the stabilization by controlling the collector current. Resistor values should be selected to set the voltage drop across the Emitter resistor 10% of the supply voltage and the current through RB1, 10% of the collector current.

• Advantages of Transistor:

1. Smaller mechanical sensitivity.
2. Lower cost and smaller in size, especially in small-signal circuits.
3. Low operating voltages for greater safety, lower costs, and tighter clearances.
4. Extremely long life.
5. No power consumption by a cathode heater.
6. Fast switching.

In the next articles, we will talk about applications of (Diodes&Transistors).