Varactor Diode : It is always used in reverse biased.
- Operate the principle of transition capacitance:
- CT μ V-n
- Popularly material used in GaAs.
- Used in low noise amplification (Parametric Amplifier.)
- Tuning stages of a radio receiver used in microwave frequency multiplication.
- It is used in direct generation of FM.
LED (Light Emitting Diode)
- LED can emit light when it is forward biased.
- Principle is electro-luminescence.
- Fabricate using direct band gap material.
- In the invisible spectrum of light, LED emits infrared light.
- GaASP Orange,Yellow
- With a forward current of 20 mA, LED gives out the maximum intensity of light.
- When reverse bias did not emit any light and works as a normal diode.
- Cut in voltage 1.3 V, Power dissipation mW, response time m sec.
- The color of light given by LED depends on the wave length of radiation:
- In optocouplers, remote control transmitter
- As a display device
Point contact Diode
- It is metal semiconductor junction diode.
- It is made up of N-type semiconductor and tungsten metal in the form of Cat’s Whisker.
- The smallest value of junction capacitance is obtained with point contact diode.
- Both P and N region are highly doped (1: 105)
- Always operated under reverse bias and fabricated with only silicon.
- Its breakdown voltage is generally less than 6V.
- Zener has large electric field intensity
- When it is forward biased, it works as a normal diode.
- it is popularly known a constant voltage device.
- Major application is as a voltage regulator circuit.
- Break down voltage reduces with increase in temperature therefore zener voltage have negative temperature coefficient.
- It is lightly doped than Zener diode.
- It is due it electron multiplication.
- Break down voltages is generally greater than 6V.
- Break down voltage increase with temperature so it is positive temperature coefficient device.
Schottky Barrier Diode
- It is a metal-semiconductor junction diode.
- Current flows because of majority carries only that’s why it is unipolar device
- Can be fabricated with Ge or Si, but Ge is more popularly used because of higher mobility of charge carriers.
- This diode is used along with a BJT to reduce the switching time of BJT
- It is used in microwave detection and mixing
- These diodes have a very small reverse recovery time and almost nil storage capacitance.
LCD (Liquid Crystal Display)
- Operating principle is dynamic scattering of light.
- Power dissipation is of the order ofmW.
- Response time is of the order of msec.
- Operating life is 50,000 + hrs.
- It is used as display device.
- Material used is liquid crystal material.
LDR (Light Dependent Resistor)
- It is also called as photo resistor or light activated resistor (LAR)
- Principle of operation is photo resistive effect.
- Range of resistance is 5W to 0.75 MW.
- Dark resistance of LDR is 0.75 MW.
- It is used in optocouplers.
- Materials used for fabrication are CDs, Se.
- LDR characteristic is as in the figure below.
- Principle of operation is Photo conductive effect.
- Photo sensitive material used are CdS, Se, ZnS.
- It is also called as light operated switch.
- Ge- photo diode responds to visible light while Si-photo diode responds to infrared light.
- Photo sensitive coating is provided at junction only.
- Compared to normal diode photo diode has larger depletion in width obtained from lower level of doping.
- It is always operated under reverse biased condition.
- Compared to normal diode it is 10 times faster, 100 times higher sensitive but power handling capacity is low.
- Magnitude of photo current increase with increase in intensity of the light falling at junction.
- Current in photo diode is given by
Where, Is → Short circuit current of photo diode,
Io → Reverse saturation current
V → Voltage applied,
VT → Thermal voltage
- Photo current flows from n to p.
- Photo current is a minority carrier current.
- Photo diode current is almost independent of applied voltage.
- Photo current is ‘diffusion current’.
- It is used in remote control sensor, in designing of optocouplers and to read audio track recorded on motion picture film.
- When photo diode is forward biased it behaves as a normal diode and effect of light on current is zero.
- Principle of operation is photo conductive effect.
- Coating of photo conductive material is done at collector base junction.
- It is basically a light operated switch.
- n-p-n photo transistor is faster than p-n-p photo transistor. It is always operated under active region.
- Its principle of operating is photo voltaic effect.
- Terminal of voltage of solar cell can’t exceed the barrier potential of diode that’s why an array solar cells is used to achieve higher voltage.
- We can measure terminal voltage of solar cell using voltmeter.
- Popularly used solar cells are Se cells, Ni-Cd cells, PbS cell.
- Ni- Cd cells are rechargeable cells used in satellites.
- It is used in automatic traffic signal lightening.
- It is generally operated under open circuit condition.
- It can be operated in forward biased condition and has cut in voltage equal to zero.
- These are optically coupled but electrically isolated.
- Optocouplers are faster than conventional devices.
- It is widely used in industrial application where very good dc isolation better than transformers is required.
- It is fastest switch.
- Its response time is of the order of p sec.
- It is a p+ n+ diode having doping level of 1: 103.
- Popularly used materials is GaAs.
- It works on the principle of tunneling effect.
- It is negative resistance device.
- It has very narrow depletion layer 100 A to 200 A.
- It is used as linear device as well as negative resistance device.
- Best material used in GaAs which is having highest swing.
- It is used as normal diode, in designing of microwave oscillators, as a relaxation oscillator, in designing of pulse and switching circuits, and as parametric amplifier.
- It is p+ -i-n+ diode
- If I is replaced by p-type then called as ppn diode and if I is replaced by n- type then called as pin diode.
- In PIN diode light doped intrinsic SC sandwiched between highly doped p and n.
- It has low response time because of high resistivity of region.
- It is two- terminal, three-layer, single junction device.
- It always operated under reverse biased con dition.
- When whole I-region is covered by depletion layer than it is called as swept out condition.
- In PIN diode if whole I-region is not swept out then signal loss will occur.
- It is used in handling microwave power, as microwave mixer, as a duplexer, in designing of transmit received switch in designing of anti transmit-receive switch.
- It stands for light amplification by stimulated emission of radiation.
- It is source of coherent light.
- It is fabricated with direct band gap material having larger carrier life time.
- Emission in laser is both spontaneous and stimulated.
- Population inversion occurs in laser.
- Lasers are highly directional.
The basic principles of lasers are
- Spontaneous emission
- Pumping and population inversion
- Stimulated emission of EMT radiation
FABRICATION OF INTEGRATED CIRCUITS
Integrated circuits compose the major portion of the field of microelectronics and may consist of film monolithic or hybrid circuits. A monolithic IC consists of active and passive components formed by diffusion into a single silicon chip, with interconnection provided by an aluminium metallization process.
The monolithic fabrication process consists of water preparation, epitaxial growth, diffused isolation, base and emitter diffusions, pre-ohmic etch, metalization, circuit probing, dicing mounting and packaging, wire bonding, encapsulation and final testing.
- Each diffusion process involves the processes such as items dioxide layer, photo resist mask, ultraviolet exposure, etching, scrubbing and diffusion.
- Practical IC resistance values obtainable range from 25W to 50 kW, depending upon the sheet resistivity measured in ohms per square P-type diffused resistors have values from 50W to 250 W per square and pinch resistors have a value of 5000 W per square.
- Because the tolerances of resistance values are at best + 30%, ICs are designed to utilize resistance rations, which may be controlled to within 3%.
- Capacitors may be obtained by utilising the P–n junctions in transistor type structures or the MOS capacitive effects employing the silicon – dioxide layer. Practical values range from 3 to 30 pF because of the excessive area used for larger values.
- By suitable interconnecting CMOS/MOS ICs, an inverter circuit with very low quiescent dissipation can be made available, and by further interconnections of many inverters many of the logic gates found in digital systems may be implemented
IC fabrications steps
1. Water cleaning
5. Diffusion/epitaxial growth
7. Ion Impanation
Thyristor (Silicon Controlled Resistor)
- It is a four layer, three junction p-n-p-n semiconductor switching device.
- It is employed in high power controlled devices.
- Like the diode, it is a unidirectional device that blocks the current flow from cathode to anode.
- Unlike the diode, a thyristor also blocks the current flow from anode to cathode until it is triggered into conduction by a proper gate signal between gate and cathode terminals
SCR I-V Characteristic
Reverse Blocking Mode
Cathode is made positive with respect to anode. Two outer junction J1 an J3 are reverse biased and J2 is
Forward Blocking Mode
When anode is positive with respect to cathode, with gate circuit open, outer junction are forward biased but inner junction J2 is reverse biased.
In this mode a small current, called forward leakage current flows. SCR offers high impedance.
- SCR treated as an open switch even in this mode.
Forward Conduction Band
- When anode to cathode forward voltage is increased with gate circuit open, breakdown occurs at voltage VBO, and current flows from anode to cathode and SCR represents forward conduction mode.
- Thus SCR can be made ON by other methods which are follows-
(i) A positive triggering
(ii) dv/dt triggering
(iii) Temperature Triggering
(iv) Light triggering
Some Important Points about SCR
- Fabricated with only si, as Ge is not practical
- Can be unidirectional or bidirectional.
- Thyristors are faster than BJT.
- Bistable device
- In SCR gate is made with p-type SC.
- It is a controlled rectifier and can be used in poly phase rectifiers.
- SCR is generally specified in terms of break over voltages (VBO) – 50 V to 1800 V.
- SCR is not negative resistance device.
- SCR can be used to speed control of DC motor.
Some Important Definitions
Turn ON time: It is time required to switch ON the thysistor
- TON increases with temperature
- TON increases with anode current
Turn OFF time: It is time required to switch off SCR.
- TOFF increases with temperature
- TOFF increases with anode current.
SCR can be switched off by using
(i) By disconnecting the power supply
(ii) By giving anode negative w.r.t. cathode.
(iii) By reducing anode supply voltage to that the anode current falls below holding current (IH), then SCR is in OFF state.
If IA > IH, SCR is no ON state.
IA < IH, SCR is in OFF state
(i) SCR can handle power up to 50 mW.
(ii) Break over voltage is in the range of 50 V – 1800 V
(iii) Switching time n-sec.
(iv) It can handle current up to 2000 A.
Latching Current: it may be defined as the minimum value of anode current which it must attain during turnon process to maintain conduction when gate signal is removed. Typical value is 1 mA.
Holding current: It may be defined as the minimum value of anode current below which it must fall for turning of the thyristor.
For turning off a thyristor requeires.
(i) Anode current is reduced below holding current
(ii) Anode current is allowed to reverse
(iii) Anode voltage is reversed
- For normal SCR , turn ON time is less than turn off time.
- Turn ON time of SCR can be reduced by using a rectangular pulse of high amplitude and narrow width.
- Turn ON time of an SCR in series with RL circuit can be reduced by decreasing L.
- Turn ON time increases with temperature
(i) dv/dt protection is achieved through the use of RC across SCR.
(ii) di/dt protection is achieved throughC the use of L in series with SCR.
DIAC (Bi-directional Thyristor Diode) Symbol and I-V Characteristic
Radiation Resistance of Dipole:
Half wave dipole: Rrad =73W
Quater wave dipole: Rrad = 36.5W
- It is used at microwave frequencies.
- It improves directivity of waveguide and reduces diffraction.
- Directivity D =
A→ Area of horn mouth
• Gain Gp =
- It is used in point to point communication.
- Highly directional broadband non-resonant antenna.
- Requires large space for installation
- Input impedance and radiation pattern do not change rapidly over considerable frequence range.
- Produces circularly polarized ro elliptically wave.
- Used at VHF, UHF frequencies.
- Wide Bandwidth and highly directive.
- Two half wave dipole at 90º to each other
- Used in FM transmission and TV Broadcasting.
Log Periodic Antenna;
- It is frequency independent antenna:
- Impedance and pattern remain constant as function of frequency.
- Super directive antenna.
- Unidirectional pattern.
- Used as receiving antenna in television systems.
- Consist following elements:
- Driven element l= 1/2
- Reflection l> 1/2
- Directors l< 1/2
Power gain of Gp = k=0.65
Directivity D = 9.87
Bandwidth between first nulls (BWFN)
Beam width between half power points
(HPBW) = 701/d
Array of two isotropic point sources :
d → Phase angle between I2 and I1
d → Angle
f → Angle
Case-I: Broad side Array → Maximum radiation is normal to the axist of array for broadside
f=0 d = 1/2
For maximum radiation f = 90º or 270º
f = 180o d = 1/2
For maximum radiation f = 0º, 180º