Make your own free website on Tripod.com

www.udaycse.tripod.com

 

                          

 

 

 

 

 

 

 

 

TRANSISTOR

                 CHARACTERISTICS  

     

 

 

INTRODUCTION TO BIPOLAR JUNCTION TRANSISTOR:    

 

      A Bipolar junction transistor (BJT) is a three terminal semi conductor device in which the operation depends on the interaction of both majority and minority carriers.

      Hence it is called as Bi polar junction transistor.

      It has wide applications in computers, satellites and other modern communication systems.

 

CONSTRUCTION OF BJT:    

 

 

      The BJT consists of a silicon or (germanium) crystal in which a thin layer of N-type silicon is sandwiched between two layers of P-type silicon.

      This transistor is referred to as P N P.

      The three portions of BJT are Emitter, base, and collector represented as E, B, and C respectively.

 

 

TYPES OF CONFIGURATION:    

 

      When a transistor is to be connected in a circuit, one terminal is used as an input terminal, the other terminal is used as an output terminal and the third terminal is common to both input and output terminal.

      Depending on the input, output and common terminal, a transistor can be connected in three configurations.

 

  They are,

 

      Common base configuration (CB)

      Common emitter configuration (CE)

      Common collector configuration (CC)

 

 

 

 

 

 

 

CB configuration:    

 

 

      This is also called grounded base configuration.

      In this configuration, emitter is the input terminal, collector is the output terminal and    base is the common terminal.

 

CE configuration:    

 

 

      This is called as grounded emitter configuration.

      In this configuration, base is the input terminal, collector is the output terminal, and emitter is the common terminal.

 

CC configuration:    

 

 

      This is called as grounded collector configuration.

      In this configuration base is the input terminal, emitter is the output terminal and collector is the common terminal.

 

 Current amplification factor:   

 

 

      In a transistor amplifier with a. c. input signal, the ratio of change in output current to the change in input current is known as the current amplification factor.

 

BREAKDOWN IN TRANSISTORS:    

 

 

      Break down is defined as the sudden change in reverse current.

      There are two types of breakdowns.

They are,

 

      Avalanche multiplication or Avalanche Break down.

      Reach - through or punch - through

 

      It is possible to raise the punch-through voltage by increasing the doping concentration in the base, but this automatically reduces the emitter efficiency.

      Punch-through takes place at a fixed voltage between collector and base and is not dependent on circuit configuration, where as Avalanche multiplication takes place at different voltages depending upon the circuit configuration.

      Therefore, the voltage limit of a particular transistor is determined by either of the two types of Break down, which ever occurs at lower voltage.

 

 

INTRODUCTION TO FIELD EFFECT TRANSISTOR:   

 

 

      The FET is a device in which the flow of current through the conducting region is controlled by an electric field.

      As current conduction is carried only by majority carriers, the FET is said to be an uni polar device.

      Based on the construction, the FET can be classified into two types as junction

 

      FET (JFET) and metal oxide semiconductor FET (MOSFET) or insulated gate

      FET (IGFET) or metal oxide silicon transistor (MOST). 

 

      Depending upon the majority carriers, JFET has been classified into two types, namely,

 

1.         N-channel JFET with electrons as the majority carriers

2.         P-channel JFET with holes as the majority carriers.

 

 

  CONSTRUCTION OF N-CHANNEL JFET:   

 

      It consists of a N-type silicon bar which is made of silicon.

      Ohmic contacts (terminals), made at the two ends of the bar, are called source and drain.

 

 Source (S):   

 

      This terminal is connected to the negative pole of the Battery.

      Electrons which are the majority charge carriers in the N-type bar enter the bar through this terminal.

 

 Drain (D):   

 

      This terminal is connected to the positive pole of the battery.

      The majority carriers leave the bar through this terminal.

 

Gate (G):    

 

      Heavily doped P-type silicon is diffused on both sides of the N-type silicon bar by which PN junctions are formed.

      These layers are joined together and called Gate (G).

 

 

 APPLICATIONS OF JFET:   

 

 

      FET is used as a buffer in measuring instruments, receivers since it has high input impedance and low output impedance.

      FET’s are used in RF amplifiers in FM tuners and communication equipment for the low noise level.

      Since the device is voltage controlled, it is used as a voltage variable resistor in operational amplifiers and tone controls.

      Since the input capacitance is low, FET s are used in cascade amplifiers in measuring and testing equipments.

      FET s are used in mixer circuits in FM and TV receivers, and communication equipment because inter modulation distortion is low.

      It is used in oscillator circuits because frequency drift is low.

      As the coupling capacitor is small, FET s are used in low frequency amplifiers in hearing aids and inductive transducers.

      FET s is used in digital circuits in computers, LSD and memory circuits because of its small size.

 

 

  METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR   

 

MOSFET is the common term for the Insulated Gate Field Effect Transistor (IGFET).

There are two basic forms of MOSFET:

 

1.         Enhancement MOSFET

2.         Depletion MOSFET

 

      Principle: By applying a transverse electric field across an insulator, deposited on the semi conducting material, the thickness and hence the resistance of a conducting channel of a semi conducting material can be controlled.

 

      In a depletion MOSFET, the controlling electric field reduces the number of majority carriers available for conduction, where as in the enhancement MOSFET, application of electric field causes an increase in the majority carrier density in the conducting region of the transistor.

 

  Applications:   

 

 

MOSFET s are widely used in digital VLSI circuits than JFET s because of the following advantages: 

 

      The input resistance of MOSFET is very high.

      The drain resistance of MOSFET is low.

      The transverse electric field induced across an insulating layer deposited on the semiconductor material controls the conductivity of the channel.

      MOSFET is very susceptible to overload voltage and needs special handling during       installation. It gets damaged easily if it is properly handled.

      MOSFET has zero offset voltage. As it is a symmetrical device, the source and drain can be interchanged. This is very useful in analog signal switching.

 

 

UJT (UNI JUNCTION TRANSISTOR):    

 

 

      UJT is a three terminal semi conductor Switching device. As it has only one PN junction and three leads, it is called as uni junction transistor.

      The basic structure of UJT is it consists of a lightly doped N-type silicon bar with a heavily doped P-type material alloyed to its one side closer to base (B2) for producing single PN junction.

      A unique characteristic of UJT is, when it is triggered, the emitter current increases regeneratively until it is limited by emitter power supply.

      Due to this negative resistance property, a UJT can be employed in a variety of applications, viz. saw tooth wave generator, pulse generator, switching, timing, and phase control circuits.

 

 PNPN DIODE (SHOCKELY DIODE):   

 

      It is a semiconductor device having three or more junctions.

      The family of thyristors consists of PNPN diode (Shockely diode), SCR, LASCR, TRIAC,DIAC, and UJT etc.

      Shockely diode is a Four Layer PNPN silicon device with two terminals.

      This diode acts as a switch during forward bias condition.           

 

 

  SILICON CONTROLLED RECTIFIER (SCR):  

 

      It is a four layer three terminal device in which the end P-layer acts as anode, the end     N-layer acts as cathode and P-layer nearer to the cathode acts as gate.

      As leakage current in silicon is very small compared to germanium, SCR s are made of silicon and not germanium.

      SCR acts as a switch when it is forward biased.

      SCR is used in relay control, motor control, phase control, heater control, battery charges, inverters, regulated power supplies and as static switches.

      SCR is basically a switch; it can be used in linear applications like rectification.

      SCR s are much superior in performance than ordinary diode rectifiers.

      The main advantage of SCR full wave rectifier over ordinary full wave rectifier is that      any voltage can be made available at the output by simply changing the firing angle of        the SCR s.

 

Holding current:    

 

      Holding current is the minimum value of current to hold the device in ON-state.

       For turning the device OFF, the anode current should be lowered below holding current by increasing the external circuit resistance.

 

 

 

 Gate current:   

 

      Gate current is the current applied to the gate of the device for control purposes.

      The minimum gate current is the minimum value of current required at the gate for    triggering the device.

      More the gate current, easier is the triggering of the device and device and vice-versa.

 

 LASCR (LIGHT ACTIVATED SCR):   

 

      The LASCR is triggered by irradiating with light.

      The incident light generates electron- hole pairs in the device thus increasing the number of charge carriers.

      This leads to the instantaneous flow of current within the device and the device turns ON.

 

 TRIAC (TRIODE A C SWITCH):   

 

      TRIAC is a three terminal semi conductor Switching device which can control alternating current in a load.

      TRIAC is nothing but two SCRs connected in parallel but in the reverse direction.

      TRIAC also acts as a switch for both directions.

      TRIAC is used for illumination control, temperature control, liquid level control, motor   speed control and as static switch to turn a. c. power ON and OFF.

      Its main limitation in comparison to SCR is its low power handling capacity.

 

DIAC (DIODE A. C. SWITCH):    

 

      DIAC is three layers, two terminal semi conductor devices.

      It acts as a Bi -directional avalanche diode. It has two junctions J1 and J2.

      It exhibits negative resistance characteristics, i.e. the current flowing in the device starts increasing and the voltage across it starts decreasing.

      The DIAC is not a control device.

      It is used as triggering device in TRIAC phase control circuits used for light dimming,     motor speed control and heater control.

 

                                                                 

                                                       

 

 

 

 

 

 

 

pink_flower_divider_md_wht.gif

EDC Material

Home | Things To Remember | AHCET | Sports | Hardware | EDC Material | EDC Part-2 | C&DS MATERIAL | Contact Information

On this page we will provide some tips that can improve the performance on your 1st Year Subjects.

EDC MATERIAL

UNIT~1

  • INTRODUCTION:
  • An electron device is that in which Electrons flow through a vacuum or gas or semiconductor.
  • The operation of an electronic device depends upon the motion of electrons under the influence of electric and magnetic fields. The behavior of electrons under the influence of these fields is termed as electron ballistics.

Field intensity

  • The electric field intensity is defined as force on a unit positive charge at any point in an electric field is called electric field intensity 'E' at that point.
  • The unit of electric field intensity is volts per meter (V/m).

Motion of electron in electric field:

In case of Two Dimensional motion of electron in electric field, the electron moves in the region between the two plates.

Motion of electron in magnetic field:

  • The magnetic force acting on a charged particle in a uniform magnetic field can be expressed as Fm = q B v This type of force makes the electron to move in a circular path with uniform speed.
  • This force is same as the centripetal force which always tries to push the electron towards the center.

Pitch of the helix:

The pitch of the helix is defined as the distance travelled by an electron along the direction of the magnetic field in one revolution.

Electrostatic deflection in cathode ray tube:

  • Electro static deflection sensitivity of a pair of deflection plates of a CRO is defined as the amount of deflection of electron spot produced when a voltage of one volt d. c. is applied between the corresponding deflection plates.
  • The amount of deflection can be calculated by knowing the dimensions of the tube components deflecting voltage.
  • D = (V d* l L ) / (Va*2d)
  • D is the amount of deflection
  • L = distance from center of the plate to the screen. V a = Accelerating potential.

Magnetic deflection in cathode ray tube:

  • The force exerted on the electron beam by the magnetic field bends the electron beam in a direction perpendicular to both the field and the direction of electron movement.
  • The magnetic field continues to deflect the electron beam at right angles to its movement and hence the path taken by the electron beam within the magnetic field is a part of the circle whose radius R is given by
  • , R = m v/q B
  • B is magnetic flux density
  • V is velocity of the electron beam
  • The magneto static deflection sensitivity of a CRO is defined as the ratio of the deflection to the unit magnetic field intensity.

Defects of deflection:

When an electron beam is deflected from the axial direction the spot on the fluorescent screen tends to distort and enlarge. This phenomenon is called as deflection defocusing.

Cathode Ray Oscilloscope(CRO):

  • The CRO is a versatile electronic testing and measuring instrument that allows the amplitude of the Signal which may be voltage, current, power etc.
  • To be displayed primarily as a function of time.
  • It is used for examining the waveforms, from d. c or very low frequency to very high frequencies.
  • The CRT is the heart of the oscilloscope.
  • CRT's are useful in displaying numeric, alphanumeric and graphic displays with high resolution.

Applications of CRO:

  • The modulation index of Amplitude Modulation (AM) waves can be measured using a CRO.
  • The voltage-current characteristics of PN junction diode and transistor, and characteristics of a transformer core can be displayed on CRO
  • Some of the applications are:
  1. Measurement of voltage.
  2. Measurement of current.
  3. Measurement of frequency. Using signal wave form. Using lissajous figure.
  4. Measurement of phase difference.
  5. CRO is also used to measure the distortion using lissajous figure.

PREPARED BY UDAY CSE

 

chrome_shimmer_md_wht.gif

Semiconductor

Diodes and Rectifiers

Introduction:

The PN junction diode is one of the semiconductor devices with two semiconductor materials in physical contact, one with excess of holes (P-type) and other with excess of electrons (N-type).

  • The most important characteristic of a PN junction is its ability to allow the flow of current in only one direction.
  • In the opposite direction it offers very high resistance.

Classification of semiconductors:

Semiconductors are classified as

  • Intrinsic semiconductor(pure)
  • Extrinsic semiconductor (impure).

Intrinsic semiconductor:

  • A pure semiconductor is called as intrinsic semiconductor.
  • In an intrinsic semiconductor, even at room temperature, electron hole pairs are created.

Extrinsic semiconductor:

  • Due to poor conduction at room temperature, the intrinsic semiconductor as such, is not useful in the electronic devices.
  • Hence, the current conduction of an intrinsic semiconductor should be increased.
  • This can be achieved by adding a small amount of impurity to the intrinsic semiconductor, so that it becomes an impure or extrinsic semiconductor.
  • The process of adding impurity is called as doping

 

 

P-type semiconductor:

  • A small amount of trivalent impurities such as aluminium or boron is added to the pure semiconductor to get the P-type semiconductor.

N-type semiconductor:

  • A small amount of pentavalent impurities such as arsenic, antimony or phosphorus is added to the pure semiconductor (germanium or silicon crystal) to get N-type semiconductor.

 

 

 

 

Breakdown in PN junction diodes:

  • Breakdown occurs due to sudden change in reverse current.
  • Once breakdown occurs, the diode no longer blocks the current and diode current can be controlled only by the resistance of the external circuit.
  • Breakdowns are of two types.
  • They are
  1. Avalanche breakdown.
  2. Zener breakdown.

Applications of PN junction diode:

  • Rectifiers in d. c. power supplies.
  • Switch in digital logic circuits used in computers.
  • Clamping network used as d. c. restorer in T V receivers and voltage multipliers.
  • Clipping circuits used as wave shaping circuits used in computers, radars , radio and T V receivers.
  • Demodulation (detector) circuits.

Rectifiers:

  • Rectifier is defined as an circuit used for converting a. c. voltage into unidirectional voltage.
  • Rectifiers are classified based on period of conduction as Half wave rectifier and Full wave rectifier.

Half wave rectifier: It converts a. c. voltage into a pulsating d. c. voltage using only one half of the applied voltage.

Full wave rectifier: It converts a. c. voltage into a pulsating d. c. voltage using both half cycles of the applied a. c. voltage.

Ripple factor: The ratio of rms value of a. c. component to the d. c. component in the output is known as ripple factor.

Efficiency: The ratio of d. c. output power to a. c. input power is known as rectifier efficiency.

 

Peak inverse voltage: It is defined as the maximum reverse voltage that a diode can withstand without destroying the junction.

 

 

Transformer utility factor:

  • In the design of any power supply, the rating of the transformer should be determined.
  • This can be done with the knowledge of the d. c. power delivered to
  • The load and the type of rectifying circuit used.

Form factor: The ratio of r. m. s. value to the average value is called as form factor.

Peak factor: The ratio of peak value to the r. m. s. value is called as peak factor.

 

Filters: Filter is used to minimise the undesirable a. c. that is ripple leaving only the d. c. component to appear at the output.

Some important filters are

  • Inductor filter.
  • Capacitor filter.
  • LC or L-section filter.
  • CLC or Pi-type filter.
  • ZENER DIODE: When the reverse voltage reaches breakdown voltage in normal PN-junction diode, the current flows through the junction and the power dissipated at the junction will be high.
  • This results in the damage of the device.
  • So, the diodes are designed with adequate power dissipation capabilities to operate in the breakdown region.
  • One such diode is called as Zener diode.
  • Zener diode is heavily doped than ordinary diode.

Application: Zener diode is used as a voltage regulator.

  • VARACTOR DIODE: The varactor, is also called a varicap, tuning or variable voltage capacitor diode, is also called junction diode with a small impurity dose at its junction, which has its useful property that its junction or transition capacitance is easily varied electronically.
  • At zero volt, the varactor depletion region W is small and the capacitance is large at approximately 600 p F.

USES: The varactor diodes are used in FM radio and receivers, AFC circuits, self-adjusting bridge circuits and adjustable band pass filters.

TUNNEL DIODE: The tunnel diode or Esaki diode is a thin-junction diode which exhibits negative resistance under low forward bias conditions.

Applications:

  1. The tunnel diode is used as an ultra high speed switch with switching speed of the order of ns or ps.
  2. As logic memory storage devices.
  3. As microwave oscillator.
  4. In relaxation oscillator circuit.
  5. As an amplifier.

Advantages:

  1. Low noise.
  2. Ease of operation.
  3. High speed.
  4. Low power.

 

PHOTO DIODE: Silicon photo diode is a light sensitive device, also called photo detector, which converts light signals into electrical signals.

USES:

  • Photo diodes are used as light detectors, demodulators and encoders.
  • These are also used in optical communication systems, high speed counting and switching circuits.
  • Photo diodes are also used in computer card punching and tapes, light operated switches, sound track films and electronic control circuits.

 

HALL EFFECT: When a transverse magnetic field B is applied to a specimen carrying current I, an electric field E is induced in the direction perpendicular to both I and B.

  • This phenomenon is called as Hall effect.

 

 

 

 

phead.gif

pn_1.gif

PN-JUNCTION
pn_4.gif

pn_2.gif

AL HABEEB COLLEGE OF ENGINEERING AND TECHNOLOGY