Transistor Operation

1.) Transistor construction
NPN transistor.

The transistor consists of two PN junctions to form three regions. And, these three regions of the transistors are called:

- Base, very thin and lightly doped, thus less mobile charge carriers.

- Emitter, highly doped, thus more mobile charge carriers.

- Collectors, largest region, and is less doped than the emitter.

PNP Transistor.

2.) Transistor Biasing

The setting up of dc voltage properly onto a transistor is called biasing.

For proper and correct biasing, please refer to the picture below,

- The emitter-base (or base-emitter) must be forward-biased.

- The collector-base must be reversed-biased.

Free electrons in the emitter region move to the base because the emitter-base is forward-biased. Because the base is deliberately made very thin and is very lightly doped. Most of these free electrons do not combine.

These free electrons diffused into the collector-base depletion where they are swept across the junction into the collector and are attracted by the higher positive potential (voltage) of collector supply.

Almost all these free electrons that enter the base, will pass the collector.

3.) Bipolar Transistor

For transistor, the charge carriers are:
- Electrons
- Holes

Since the conduction of a transistor is by electrons, as well as holes. a transistor is often called the bipolar transistor (BJT).

The two junctions of a bipolar transistor are known as:
- BE junction
- BC junction

Electrostatic Discharge (ESD)

1.) Electrostatic
It is the study of electrical charges at rest.

A charged object is created by the separation of charges:

- An atom is electrically neutral; it has the same number of protons (positive charges) as it does electrons (negative charges).

- Objects are charged by adding or removing electrons.

- A positive charge occurs when there are fewer electrons than protons; its classical definition is the charge accumulated by a glass rod rubbed with silk or wool.

- A negative charge occurs when there are more electrons than protons; its classical definition is the charge accumulated by a hard, rubber rod rubbed with fur.

2.) How is static charge generated?
Static charges are produced whenever there is friction between 2 materials or between human and material, for example touching or rubbing material, touching a plastic bag, clothing, foam coffee cup, or vinyl traveler. Demonstrate generation of static charges using a comb, small pieces of paper, a mylar  tape or balloon.

3.) What is Electrostatic Discharge?
ESD is a sudden discharge of the stored up of static charge. This sudden discharge can cause rupture on material inside devices. ESD sensitive devices therefore can be damaged when in contact with skin.

4.) What are ESD sensitive components?
Class 1 (0kV to 1kV)
- MOS IC
- VLS IC
- Linear IC, voltage regulators
- MOS capacitor
- FET, SCR
- Microwave, VHF transistors
- Thin film resistors

Class 2 (1kV to 4kV)
- MOS IC (CMOS)
- High speed bipolar logic IC
- Monolithic ceramic
- Schottky deiode

Class 3 (4kV to 15 kV)
- Small signal diodes
- Transistor
- Low speed bipolar TTL, DTL
- Quartz piezo-electric crystals

5.) ESD Safety Devices
The following safety precautions are used to prevent damages by ESD:

Handle static sensitive devices and PCB assemblies only at an anti-static workstation.

Discharge human body by wrist or heel straps.

Keep away static generators such as common plastic, tapes, and static generating tools away from work stations.

Use ionizers to reduce charge.

Store and transport sensitive devices in ESD protective packaging, (e.g. anti-static/conductive magazines, conductive bags/tote boxes) which are labeled clearly with a 'CAUTION" message.

Use treated plastics.

6.) Ionisers

An ioniser is a device that is used to clean the air of dust particles.

The ionisers uses a minute amount of electricity to generate billions and billions of negative ions (charged air molecules). These negative ions shoot down dirt particles that are circulating in the air by electrostatic action.

7.) How Static electricity is generated?

Examples:
- Pulling a tape from a reel.
- Clothing rubbing against a chair.
- Walking across a carpeted floow.
- Separating.rubbing 2 foils.
- Combing your hair.

Zener Diode Applications

1.) Zener Diode Application

It is commonly used as voltage regulator in DC power supplies.

Two types of regulation are:
- Line regulation
- Load regulation

2.) Line regulation

The zener diode regulates a varying input power supply as shown below.

Given: Izk = 0.25mA to Izm = 100mA.

Vin = Vr + Vz

For minimum current, voltage across 220 Ω:

Vr = IzkR = (0.25mA)(220Ω)

= 55mV

Therefore:

Vin = Vr + Vz

= 55mV + 10V

= 10.055V

For maximum current, voltage across 220Ω:

Vr = IzmR = (100mA)(200W)

= 22V

Therefore:

Vin = 22V + 10V

=32V

The current limiting resistor is included to prevent the zener from conducting too much and overheated.

Notice that the diode is reverse biased. The output voltage (across the zener doiode) is maintained constant in this manner.

3.) Load Regulation

A zener regulator with variable load resistor. A zener diode maintains constant voltage across RL as long as the zener current is greater than Izk and less than Izm.

At no load, IL = 0 & the total current, IT will flow through the zener diode.

When RL is connected, IT = Iz + IL.

When Iz reaches its minimum, Izk, load current is at its maximum.

Zener Diodes Principles

1.) Zener Diode

The zener diode is a special type of diode - for it is designed to operate at reversed biased condition.

2.) Characteristic of the Zener Diode

Zener diode is specially designed to operate at its reversed breakdown voltage region. This voltage is known as the Zener voltage, Vz.

At this region, a change in zener diode current will cause only a small change in the zener voltage. The zener voltage is reasonably stable.

3.) Zener Breakdown

Two types of reverse breakdown are avalanche and zener breakdown.

Avalanch breakdown occurs at a sufficiently high reverse voltage while Zener breakdown occurs at low reverse voltages.

Zener breakdown occurs at low reverse voltages.

4.) Breakdown Characteristic

As reverse voltage is increased, zener begins to breakdown at Izk.

A minimum value of reverse current, Izk, must be maintained to keep the diode in breakdown.

Maximum current, Izm, must not be exceeded otherwise the diode will be damaged.

Rectifier Calculations

1.) Calculations

-Full-wave Rectifier:

 

The average value of DC output voltage at the rectifier output

For full-wave rectifier: Vavg = 2Vp / π

For silicon diode: Vout = Vmax - 0.7V
= Vp

-Bridge Rectifier:

 

The average value of DC output voltage at the rectifier output

For bridge rectifier: Vavg = 2Vp/π

For silicon diodes (2 diodes) = Vout = Vmax - 0.7V - 0.7V

=V max - 1.4V 

=Vp

2.) Diode Rating
Diodes are generally reated by its voltage , current or power.

3.) Peak Inverse Voltage
It is defined as the maximum reverse bias voltage that can be applied to a doide without the diode breaking down.

The PIV across each reverse-biased diode = Vp + 0.7V

4.) Maximum forward Current Rating

It is referred to the maimum current that is allowed to flow through the diode.

5.) Ripple Voltage 

It is the AC variation of the DC output voltage after filtering.

Rectifier Principles

1.) Definition of Rectifiers
Rectification, is the term used to describe the conversion of voltages or current.
Rectifier, diodes used in a circuit to convert AC to DC voltage or current.

2.) Types of Rectifier Circuits
- Full-wave Rectifier
- Full-wave Bridge Rectifier

3.) Operation of Full-wave Rectifier
The circuit shown below is a full-wave rectifier circuit.

The full-wave rectifier requires a center-tapped transformer so that current can be made to flow in the load resistor on both halves of the ac wave.

In a full-wave rectifier circuit, a diode rectifier is placed in series with each half of the center-tapped transformer secondary winding and the load. Effectively this circuit has 2 half-wave rectifiers working into the same load.

During the first half cycle, when point A of the transformer is positive:
- It makes the diode D1 anode positive so that it can conduct current.
- Current will flow from A through diode D1 to point C through the load resistor to point D and then to ground at the transformer center-tapped terminal.
- A positive voltage will be developed across the load resistor.

During the next half cycle when point B of the transformer is positive:
- It makes the diode D2 anode positive so that it will conduct.
- Current will flow from B through the diode D2 to point C, though the load resistor to point D and then earth, the center-tapped of the transformer.
- This current flow will also develop another positive voltage across the load resistor.

Since both pulses of the current flowing through the load are in the same direction, a pulsating dc voltage now appears across the load. The full-wave rectifier has changed both halves of the ac input voltage to a pulsating dc output voltage. The sequence half cycles of the input ac voltage will be rectified in the same way. The output voltage develop across the load resistor will be shown in the figure below:

The disadvantage of the full-wave rectifier is that it requires a center-tapped transformer. It becomes costly and heavier in weight.

4.) Operation of Full-wave Bridge Rectifier
The circuit shown in figure below is a full-wave bridge rectifier that requires a non-center-tapped transformer and 4 diodes. At any one time 2 diode will be in operation.

 

During the positive half cycle, when point A is positive.

- The diodes D2 and D4 anodes will be positive, while the remaining 2 diodes will be negative.

- Current will flow from point A via D2 to point C, through the load resistor to point D, through diode D4 to point B, through the trransformer and back to point A.

- This current flow will developed a positive voltage across the load resistor.

During the next half cycle, point B is positive,

- The diode D1 and Diode D3 anode will be positive, while the remaining 2 diode will be negative.

- Current will flow via diode D3 to point C, through the load resistor to point D, through diode D1 and back to point B.

- This current will also develop a positive voltage across the load resistor.

The full-wave bridge rectifier has the advantage of using a non center-tapped transformer.As a result the rectifier circuit become lighter in weight. Since diode are inexpensive, the full wave bridge rectifier is commonly used in modern solid-state electronic equipment. In many cases, a special package of 4 diodes are available to aboid the extra wiring required in this circuit.

5.) RC Filter Circuit
Figure below shows an RC filter circuit.
- Consisting of 2 capacitors C1 and C2 and resistor R.
- The capacitor C1 is called reservoir capacitor.
- The capacitor C2 is called smoothing capacitor.
- The resistor R is called Dropper resistor.

 

6.) RC Filter Circuit Description
When point X in figure above received the positive peak voltage Vp at the rectifier output, correspond to point T1 in figure below, the capacitor C1 will be charged up to its peak value.

When the rectifier reduces its output voltage from its peak value to zero, the capacitor C1 start to discharge its energy. The time taken for the rectifier output voltage to reach its peak value is faster than the time taken for the capacitor C1 to discharge.

The reservoir capacitor still remain at certain amount of energy (voltage) when the rectified voltage reaches zero, correspond to point T2 as shown below.

The next positive puls will charge up capacitor C1 to its peak voltage again. The process of charging and discharging across C1 will continue in the subsequent positive pulses.

Since C1 does not discharge to zero, the load resistor will continue to be supplied with electrical energy from the capacitor C1. As a result the voltage develop across the resistor will contain an AC ripple as shown below.

The resistor R will reduce the ripple further as shown below.

The capacitor C2 will now received these ripple voltage. The charging and discharging action of C2 will further reduce the ripple to a very small percentage. The voltage across the load restor will be almost a pure DC voltage as shown below.

7.) Resistor Inductor (RL) Filter Circuit

 

It is similar in its circuit and its operations to that of the rc filter except that an iron cored choked is used instead of resistor.

The choke give a better filtering because of its high Inductive Reactance. It also prove high resistance to AC but low resistance to DC. Hence, this is good for filtering AC ripple.

Diode Applications

1.) Rectifier
A rectifier is a device that changes AC to DC.

It is a process by which alternating current is changed to direct current. Since diodes conduct in only one direction, they serve as rectifiers.

2.) Detector

A diode detector in an AM receiver recovers the original information from the AM transmitter.

3.) Half wave rectifier diagram

 

During the positive half cycle of the AC supply, the diode is forward biased and it conducts. This signal is thus present at the load.

During the negative half cycle of the AC supply, the diode is reversed biased and it does not conduct.

 

Thus throughout the negative half cycle there is no signal present at the load. Therefore the signal output of load appears to be shown as follows:

 

Average value, is the value you would measure on a DC voltmeter.

 

Vp is the peak value of the voltage.

4.) Peak Inverse Voltage (PIV)
Occurs at the peak of negative cycle alternation of an AC input when the diode is reverse biased.

It is define as the maximum reverse bias voltage that can be applied to a diode without the diode breaking down. PIV equals the peak value of input (reverse) voltage before going through the diode.

The diode must be capable of withstanding this amount of repetitive reverse voltage.

5.) Summary
The table below shows the maximum ratings for a certain series of rectifier diodes.

These are the absolute maximum values under which the diode can be operated without damage to the device.

The greatest reliability, the diode should always be operated well under these maximums.

Generally the maximum ratings are specified at 25 degree Celsius and must be adjusted downward for higher temperatures.

Diode Principles

1.) Definition of PN Junction Diode
A diode is made by joining P-type and N-type materials.
The junction of a diode is the region where the P-type material ends and the N-type material begin.

2.) Depletion Region
At the instant when junction is formed, diffusion takes place. That is some holes will move from the P-type material intro the N-type material.

Likewise some electron will move from the N-type material into the P-type materials.

These holes and free electrons, which moves across the junction, recombine and produces a depletion region at the junction.

The depletion region is where there is no mobile majority charge carrier.

It contains positively and negatively charge atoms on either sides of the junction.

3.) Barrier Potential Difference (or Barrier Voltage)
The opposite charge that build up on each side of the junction create a barrier voltage or potential barrier which resist any further free electrons and holes from cross the junction.

The barrier voltage is about 0.3V for a Germanium junction and 0.7V for a Silicon junction.

4.) Reverse Bias
When P-type material is connected to the negative terminal of the supply and the N-type material is connected to the positive terminal of the supply, the diode is said to be reverse biased.

The external voltage supply causes the majority carriers:
- Holes from P-type material
- Electrons from N-type material

To move away from the junction: As a result. the depletion region at the junction becomes larger and wider.

5.) Diode Reverse and Forward Bias Characteristics Curve

 

 Where the diode is reversed-bias, a very small current (due to the minority carrier) called the reverse current or leakage current will flow only.

This means that, the reverse biased resistance of the diode is very high.

When the reverse bias voltage is high enough, it will cause damage or breakdown at the PN junction. this reverse voltage is called the breakdown voltage.

For any diode, the Peak Inverse Voltage (PIV) is the maximum safe reverse voltage of a diode specified by the manufacturer.

6.) Forward Bias
When the P-type material is connected to the positive terminal of the supply and the N-type material is connected to the negative terminal of the supply, the diode is said to be forward biased.
The potential of the external supply forces:
- Holes from P-type material
- Electrons from N-type material to cross the junction

A large current called the forward current flow through the diode.

7.) Diode forward Bias Characteristic Curve
When the amount of forward bias voltage equals the barrier voltage, there is no longer any depletion region and the junction can conduct current.

This barrier potential difference, VF is 0.3V for Germanium diode and 0.7V for Silicon diode.

8.) Characteristic of a diode

 

The anode is the P-type material of the diode. The cathode is the N-type material of the diode.

A diode is a non-linear device. When it is forward biased, current can flow through it easily since it acts a very low resistance.

When the diode is forward biased, it acts as a closed switch in series with a small forward voltage. (0.7V for Silicon)

 

When it is reverse biased, current does not flow through it since it now acts as a very high resistance.

When the diode is reverse biased, it acts as an open switch.

Semiconductor Principles

1.) Definition of Semiconductors
Semiconductors are neither good conductors nor good insulators, but belong to a class of material between the two categories.

The semiconductor materials are:
- Germanium (Ge)
- Silicon (Si)

2.) Intrinsic Semiconductor
Intrinsic semiconductors are pure Germanium or Silicon crystal.
- A stable needs 8 electron on the outermost shell.
- For Germanium or silicon atoms, each atom has 4 valence electrons on its outermost shell (valence shell).
- Hence by forming covalent bond with other atoms of the same material, it results in a stable crystalline structure.

3.) Covalent
It is the sharing of electrons with neighboring atoms in order to fill its outer shell with 8 electrons.

 

4.) Holes

Each time, an electron breaks away from  covalent bond, a hole is created.

- A hole represents the absence of an electron in a covalent bond.

- The hole acts like a positive chare because it will attract and capture any free electron that happens to come near it.

- A hole (positive charge) can attract adjacent bound electron move to fill up the hole, the hole at the original position disappears.

- But a new hole is created at that adjacent bound electron when it moves out of its shell.

- In this way, a hole can move from one atom to another atom.

5.) Doping

It is the process of adding controlled amount of impurities to pure semiconductor to improve conductivity.

6.) Extrinsic Semiconductor

It is the process of adding controlled amount of impurities to pure semiconductor to improve conductivity,

There are two types of extrinsic semiconductor, namely:

- N-type semiconductor/material

- P-type semiconductor/material

7.) N-type Semiconductor

By doping the pure crystal with pentavalent (donors) impurities, that is, a material with 5 valance electron, e get N-type semiconductor.

Examples of pentavalent impurities are:

- Antimony (Sb)

- Arsenic (As)

- Phosphorus (P)

Only free electrons are produced by doping with pentavalent impurities.

As a result, for N-type semiconductor, there are more free electrons than holes.

Hence, the majority carriers are the free electrons and the minority carriers are the holes.

Illustration:

 

8.) P-type Semiconductor

By doping the pure crystal with trivalent (acceptors) imurities, that is, material having 3 valence electrons, we get P-type semiconductor.

Examples of trivalent impurities are:

- Aluminium (Al)

- Boron (B)

- Gallium (Ga)

-  Indium (In)

Only the holes are produced by doping with trivalent impurities.

As a result, for P-type semiconductor, there are more holes than free electrons.

Hence, the majority carriers are holes.

The minority carriers are the free electrons.

Illustration: