P-N junction diode work

P-N junction diode:
Semiconductor materials was to join a P-type material with an N-type to form a simple but impotant semiconductor device called a diode.
the term diode denotes a two-electrode device. A semiconductor diode is simply a P-N junction with connecting leads or terminals on the two sides of the P-N junction. P-N junction  is grown out off crystal. A diode is a unidirectional devices permitting the easy flow of current in one direction but restraining the flow in opposite direction.

A major application of diodes is in rectification i.e., in conversion of ac into dc.semiconductor diode is becoming more and more popular these days due to its smaller size,cheapness,robustness and higher operating efficiency.

 gray color is denoted a cathode terminal.
The P-type material of the diode is the positive electrode and is called the anode whereas the N-type material is the negative electrode and is called the cathode. The arrow symbol used for the anode represents the direction of flow of conventional current. the electrode current flow in a direction opposite to the arrow.

The symbol for semiconductor diode is shown in figure Diode have polarity and, therefore,must be connected in circuits correct direction. small diodes are marked with a band to indicate the cathode end. larger diodes use the diode symbol to indicate their polarity. Diodes are available in many package configuration,depending on their ratings.Small diodes,used for low current application in circuits up to 1A of average current,are generally packaged in a simple glass or  plastic tube.Diode for handling larger currents are packaged in metal plates to dissipate heat.

THERMISTORS

THERMISTORS:

The thermistors is a combination of a thermal and resistor. A thermistor is a resistor with definite thermal characteristics.
most thermistors have a negative temperature coefficient,but positive temperature devices are also available. the reason of negative temperature coefficient of resistance of a semiconductor is that when its temperature increased, the concentration of charge carriers increases resulting in a decrease in resistance.

Thermistors are essentially semiconductor devices, that behave as resistors with high negative temperature coefficient. Thermistor has a very nonlinear resistance-temperature relation is

                                  β/T
                   R=Roα e

symbol of thermistor

 where,
 R is a resistance of a thermistor.
T is a temperature.
α and β are constants depending upon the material and manufacturing technique employed.

positive temperature coefficient (PTC) Thermistors.
such thermistor are resistors with a high positive temperature coefficient of resistance. They differ from NTC  thermistors is the following aspects.
1. the temperature coefficient of a PTC thermistor is positive  only between a certain temperature  range,outside this range the temperature coefficient is either zero or negative.
2.the absolute value of the temperature coefficient of PTC thermistor is usually much higher than of NTC thermistors.

 PTC thermistors are employed as current limiters, temperature  sensors, and protectors again ts overheating in equipment such as electric motors.They are also employed as level indicators,time delay devices,thermostats,compensation resistors etc.
PTC thermistors are prepared from BaTiO3  and  SrTiO3 in a similar way as  NTC thermistors.If prepared in the absence of Oxygen, these semiconductors have a weak negative temperature coefficient.A strong positive temperature coefficient is obtained by firing the sample in an oxygen-rich atmosphere. this is  caused by the penetration of oxygen along pores and crystal boundaries during cooling after the firing process.
The oxygen atoms, absorbed on the crystal surface attract electrons from a thin Zone of the semiconducting crystals.In this way  electrical potential barriers are formed consisting of a negative surface charge with,on both sides,thin layers having a positive space charge resulting from the now uncompensated foreign ions.These 
                                                                                                                                                  barriers to because an extra resistance of the thermistor. At very high temperatures,i.e.,above 160 degree c to 200 degree c the electrons captured at the boundaries  are gradually liberated causing the potential barriers to decrease in strength.This means  that the PTC loses its properties and may eventually act as an NTC if the temperature becomes too thigh.Therefore the applications of PTC thermistors are restricted by a certain temperature .

Zener diode

Zener diode:

Zener diode,also sometime called the breakdown diode,is a P-N junction diode specially designed for operation in the breakdown region in reverse bias condition. the diode may use either zener breakdown mechanism or avalanche breakdown mechanism.
the breakdown diode may be silicon or germanium one but silicon is preferred over germanium because of higher operating tempature and current capability.

symbol of a zener diode

Zener diode are connected into circuits with their cathodes as the positive terminal because they operate in the reverse-bias mode.small zener diodes are marked the same way as conventional diodes with a cathode band.The larger diodes are marked with the zener diode.

The symbol of zener diode is given figure this is similar to that of a normal diode except that the line representing the cathode is bent at both ends i.e the bar is turned into  Z-shape; stand for zener.

construction of a zener diode:

Zener diode is like an ordinary P-N junction diode except that it properly doped so as to have a sharp breakdown voltage. There are several methods used in the manufacture of zener diodes, such as diffused structure ,diffused and passivated structure and alloy diffused structure.
 Diffused structure zener diode consists of two N and P substrates diffused together and has metallic layers deposited on both sides to connect anode and cathode terminals as shown in figure. the drawback of such a construction is that the edge of the junction are exposed to contamination.
In passivated structure of a zener diode,the edges of the junction are covered by layer of sio2, as shown in fig 3.
similarly,all junctions are protected in alloy diffused structures as shown in figure 4.
 In general, the alloy diffused structure gives better zener characteristics at lower zener voltages.on the other hand, diffused and passivated zener diode is used at higher zener voltages 

what is a Active and passive components?

Active components: 

Active components or devices used in electronic circuits are numerous but all of these active components or devices can be broadly classified into two categories-tube type and semiconductor type devices. Tube devices can be further divided into two categories -vacuum tubes(such as vacuum diode,vacuum triode,vacuum tetrode and vacuum pentode)
and gas tubes.
semiconductor devices include junction diode,bipolar junction transistor(BJT),field effect transistor(FET),uni junction

example of a active component:picture of  a BJT

transistor(UJT),silicon controlled rectifier(SCR)tunnel diode.

Passive components:

passive components include resistors,inductor and capacitors,these components are split into two categories:those which dissipate energy and those which store it. in the latter category is the inductor,which stores energy by virtue of a current passing through it, and the capacitor,which stores energy by virtue of a voltage exiting across it. The third components, the resistor ,is the only one of the virtue of a voltage exitsing across it.

example of a passive components:picture of a capacitor

Zener Diode application

Zener diode:

Zener diode,also sometime called the breakdown diode,is a P-N junction diode specially designed for operation in the breakdown region in reverse bias condition. the diode may use either zener breakdown mechanism or avalanche breakdown mechanism.

the breakdown diode may be silicon or germanium one but silicon is preferred over germanium because of higher operating tempature and current capability.

Zener Diode have numerous applications in transistor circuitry. some of their common applications are explained below. 

       1. zener Diode shunt Regulator(or voltage regulation).

 voltage regulation is a measure of a circuit's ability to maintain a constant output voltage even when either input voltage or load current varies.

electronic symbol of zener diode

Zener diode can be use a voltage regulator to provide a constant voltage from a source whose voltage may very  appreciably. 

2.Meter protection:

 Zener diodes can be used as meter protection. a zener diode protect a meter movement or circuit against damage from an accidental application of excessive voltage. in this arrangement zener diode is connected in parallel with meter.

3.Switching Operation:

Zener diode can produce a sudden change from low current to high current,so it is useful for switching operations. the use of zener diode in switching operation has made possible an extremely fast performance in computer applications.

OR GATE,AND GATE and NOT GATE.

OR GATE:
The OR gate performs logical addition, more commonly known as the OR function. an OR gate has two or more inputs signals with only one output signal.
In OR gate, output voltage is very high if any or all of the input voltages are high. Diodes may be used to build and OR gate.
TRUTH TABLE of OR gate

INPUT          OUTPUT
a       b               c
0       0               0
0       1               1
1       0               1
1       1               1

where 1 is represents  a high signal and 0 is represents a low signal
a and b is inputs of gate and c is a output.
 1->high signal(given voltage/current)
 0->low signal(not given any signal voltage/current)

AND GATE:

The AND gate performs logical multiplication,more commonly known as AND function.
 the AND gate has two or more inputs and a single output. The AND gate provides high output only when all inputs are high.
TRUTH TABLE of AND gate
INPUT                 OUTPUT

a       b                    c
0       0                   0
0       1                   0
1       0                   0
1       1                   1

where 1 is represents  a high signal and 0 is represents a low signal
a and b is inputs of gate and c is a output.
 1->high signal(given voltage/current)
 0->low signal(not given any signal voltage/current)

NOT GATE OR INVERTER:

The inverter (NOT circuit)performs a basic logic function called inversion or complementation. the purpose  of the inverter is to change one logic level to the opposite level. in terms of bits, it changes a 1 to a 0 and vice versa.

  

This gate has only one input and one output. NOT gate is called the inverter because output state is always opposite to the input state,so when the input is low signal, output signal is high and vice versa. it is called NOT gate because the output state is always opposite to that of input state.

1----->0(using NOT gate)

0----->1(using NOT gate)

fig: AND GATE,NAND GATE,OR GATE,NOR GATE and XOR GATE,XNOR GATE. .

figures show that:

 row 1: AND gate then use NOT gate it's make NAND gate.

 row 2: OR gate then use NOT gate it's make NOR gate.

 row 3: NOR gate then use NOT gate it's make XNOR gate.

TRUTH-TABLE

TRUTH-TABLE:

A TRUTH-TABLE as suggested is a tabular or graphic technique for listing all possible combinations of input variables, arguments,or whatever they may be called,in a vertical order,listing each input combination one row at a input combination one row at a time .when every possible combination is recorded,each  combination can be studied to determine whether the "output"or "combined interaction" of that combination should be ASSERTED or NOT-ASSERTED.

the TRUTH-TABLE came into existence during the early year of logic study when logician used them to validate their thinking. it is somewhat unfortunate that is TRUE/FALSE connotation has been carried over into digital hardware design . in digital design, the TRUTH-TABLE is used primarily to specify an INPUT/OUTPUT relation for a digital network,not to determine the truth value of an argument.

example of a truth table:

---

INPUTS          OUTPUTS
 a      b                  c
 0      0                  0
 0      1                  1 
 1      0                  1
 1      1                  1

---

this is a example of a OR gate truth table where a and b is a input and c is a output