Electric Circuit and Electron Device - SPECIAL SEMICONDUCTOR DEVICES (Unit V)–Lecture Notes

Anna University

Electric Circuit and Electron Device

Subject Code : EC2151

Subject Name : Electric Circuit and Electron Device

Year : 1st yr

Semester : 2nd Sem

Department : CSE & ECE

UNIT-V

SPECIAL SEMICONDUCTOR DEVICES

Tunnel Diodes

  • When the impurity concentration is of the order of one part to 103 parts then tunnel diode is formed.
  • This diode has negative resistance region.
  • Due to which it is used as an oscillator.
  • This diode is uses the tunneling phenomenon.

Tunneling

The process that an electron from n-side of a pn diode directly penetrates through the junction into the p-side of diode is called tunneling. It is a quantum –mechanical behaviors.

Operation

  • When a tunnel diode is under unbiased condition then there will not transfer of electrons from n-side to p-side hence the net current will be zero.
  • When the diode is reverse biased under this condition the electrons from n-side are attracted by the positive plate and hence move away from the junction.
  • As a result the energy level in the n-side decreases when compared to the unbiased state.
  • Now, there will be some empty state in valence band of p-side quite opposite to the empty conduction band.
  • Hence tunneling takes place from p to n-side.
  • As reverse bias is increased this current increase.

Applications

Ø Tunnel diode is used as Ultra-high speed switch.

Ø Used in relaxation oscillator.

Ø Used as an amplifier.

Ø Used as logic memory storage device.

Ø Used as microwave oscillator.

Advantages

  • High speed operation
  • Ease of operation
  • Low noise
  • Low cost
  • Low power

Disadvantages

  • It is two terminal device, there is no isolation between the input and output circuit.
  • Voltage range over which it can be operated is 1 V or less.

PIN Diode

  • It has highly improved switching time in comparison with a PN diode.
  • PIN diodes are used in microwave switches.
  • In PIN diode high resistivity intrinsic layer is sandwiched between the P and N regions. This results in improved switching time.
  • Quite often instead of I-region we actually use either a high resistivity P-region is called π region and the high resistivity N-region is called γ region.
  • The I-region has typically resistivity of 10 Ωm.

Applications of PIN Diode

Ø Used as pulse and phase shifter.

Ø Used as SPST and MPST switches.

Ø Used in amplitude modulation.

Ø Used as photo detectors in fiber optic systems.

Ø Used as T-R switch.

Ø Used as attenuator and duplexer.

Varactor diode

  • Varactor diode is a specially manufactured reverse biased PN junction diode with a suitable impurity concentration profile.
  • It is also called as varicap or voltacap.
  • It is used as a variable reactance capacitance.

Characteristics of Varactor diode

  • The diode conducts normally in the forward direction.
  • At relatively low voltage the reverse current saturates and then remains constant.
  • It is rising rapidly at avalanche point.
  • At the saturation point the maximum junction capacitance is obtained and a point just above avalanche the minimum junction capacitance is obtained.
  • Therefore there are two conditions which are limiting the reverse voltage swing and the capacitance variation.

Applications

Ø Used as a tuning device in receivers.

Ø It is used in High frequency.

Ø It is used in adjustable band-pass filter

Ø It is used in FM modulation.

Ø It is used in automatic frequency control devices.

Ø It is used in parametric amplifier.

SCR

  • SCR consist of four semiconductor layers forming a PNPN structure.
  • It has three PN junctions namely J1, J2, J3.
  • There are three terminals called anode (A), cathode (K) and the gate (G).
  • The anode terminal is taken out from P1 layer, and the gate (G) terminal from the P2 layer. It conducts the current in forward direction only.

Operation of SCR

  • SCR is forward bias with a small voltage, it is in ‘OFF’ and no current flows through the SCR.
  • The applied forward voltage is increased, a certain critical voltage called forward break over voltage (VBO).It reaches at the junction J2 breakdown. At this case the SCR switched ‘ON’ position.
  • If the SCR is reverse bias, the junction J1 and J3 are reverse bias and junction J2 is forward bias.
  • It has found that most of the voltage will drop across junction J1 only.
  • When the applied reverse voltage is small, the SCR is OFF, and there is no current flow through the device.

SCR characteristics

  • It is the relationship between the anode –cathode voltage and anode current at different gate current.
  • Two types of V-I characteristics

Ø Forward Characteristics

Ø Reverse Characteristics

Forward Characteristics

  • It is the current drawn between anode-cathode voltage (VAK) and anode current (IA) at different gate current.
  • Adjust the gate current to zero value by keeping the switch open.
  • Increase the applied voltage across the SCR in small suitable steps at each step.
  • Note the anode current & plot the graph.

Reverse Characteristics

  • The reverse characteristic is obtained by reversing the connections of the d.c. supplies VAA and VGG .
  • Adjust the gate current to any suitable value.
  • Increase the reverse applied voltage in suitable steps.
  • Note the anode current for each steps.
  • Now we plot a graph with anode current and anode cathode voltage.

Turning ON (Triggering) SCR

The SCR can be turned ON, from OFF position by anyone of the following methods.

Ø Gate triggering

Ø Forward break over voltage

Ø Light triggering

Ø Rate-effect

Latching

Once the SCR is turned ON, it starts to conduct and remains in conduction state even when the gate signal is removed. This ability of the SCR to remain conducting, even when the gate signal is removed, is known as latching.

Turning OFF

One of the following methods is applied to turn OFF the SCR.

  • Reversing polarity of anode-to-cathode voltage called as Gate turn OFF switch (GTO).
  • The second method is anode current interruption. Changing anode current by means of momentarily series or parallel switching arrangement.
  • Third method is forced commutation. In this, the current through SCR is reduced below the holding current.

Applications of SCR

Ø Power control device

Ø Relay control

Ø Regulated power supplies

Ø Static switches

Ø Motor control

Ø Battery charges

Ø Heater controls

Ø Phase controls

Ø For speed control of DC shunt motor

Advantages of SCR

  • SCR controls large current in the load by means of a small gate current.
  • SCR size is very compact.
  • Switching speed is high.

UniJunction Transistor (UJT)

  • UniJunction transistor is a three terminal semiconductor device consisting of only one PN junction.
  • It differs from ordinary PN diode in the sense that it has three terminals namely Emitter, Base1 and Base 2.
  • The behavior of UJT differs from other transistors like BJT and FET in the sense that it has no ability to amplify.
  • However, it has ability to control large ac power with a small signal.
  • It also exhibits a negative resistance characteristic which allows it to be used as an oscillator.

Applications of UJT

Ø Non sinusoidal oscillator

Ø Timing circuits

Ø Saw tooth generators

Ø Triggering device for SCR and TRIAC

Ø Switching circuits

Ø Voltage regulated supply

Diac (Diode A.C. switch)

  • A DIAC is two terminal semiconductor device and three layer bidirectional device, which can be switched form of it’s OFF to ON state for either negative or positive polarity of applied voltage.
  • The two leads are connected to p-region of silicon separated by an n-region. It consists of two 4-layer diodes connected in parallel in opposite direction.
  • The diodes are P1N1P2N2 and P2N1P1N3.
  • It has two main terminals namely Main terminal 1 and Main terminal 2.

Applications of DIAC

Ø Temperature control

Ø Triggering of TRIAC

Ø Light dimming circuits

Ø Motor speed control

Triac (Triode A.C. switch)

  • TRIAC is a three terminal semiconductor switching device which can conduct in either forward or reverse direction.
  • The TRIAC is the combination of two SCR’s connected in parallel but in opposite direction.
  • The anode of one SCR is connected to the cathode of another SCR.
  • The gates are connected together.
  • It consists of two four layer switches in parallel and the switches are P1N1P2N2 and P2N1P1N4.
  • The TRIAC has two main terminals namely main terminal1 and main terminal2 and one Gate terminal.

Applications of TRIAC

Ø Heater control

Ø Phase control

Ø Light dimming control

Ø Static switch to turn a.c. power ON and OFF.

Ø Speed control of motor.

Light Activated SCR (LASCR)

  • LASCR is similar to that of a SCR except the light triggering.
  • It has a window and lens which focuses light on the gate junction area.
  • It can be triggered ON by a light input on the gate area, but does not turn OFF, when light source is removed.
  • The LASCR acts like a latch.
  • To reduce the holding current, it can be turned OFF.
  • Depending on its size a LASCR is capable of handling larger amount of current.
  • It can be handled by a photo transistor or a photo diode.

Applications of LASCR

Ø Optical light controls

Ø Phase control

Ø In relays

Ø Motor control

LASER DIODE

· The term Laser comes from the acronym for light amplification for stimulated emission of radiation.

· The Laser medium can be a gas, liquid, amorphous solid or semiconductor.

· Two commonly used Laser structure

Ø PN homojunction laser

Ø Double hetrostructure laser

Laser Action

  • The light traveling through a semiconductor, then a single photon is able to generate an identical second photon.
  • This photon multiplication is the key physical mechanism of lasing.
  • The carrier inversion is the first requirement of lasing.
  • It is achieved at the PN junction by providing the conduction bandwidth electrons from the N-doped side and the Valence band with the holes from the P-doped side.
  • The photon energy is given by the band gap, which depends on the semiconductor material. The optical feedback and the confinement of photon in an optical resonator are the second basic requirement of lasing.

Photodiode

  • It is a light sensitivity device used to convert light signal into electrical signal.
  • It is also called Photo detector.
  • The light energy fall on the junction through lens, when, the PN photodiode junction is reverse bias.
  • The hole-electrons pairs are created.
  • The movement of the hole-electron pairs in a properly connected circuit results in current flows.
  • The current is proportional to the intensity of light and the frequency of the light falling on the junction of the photo diode.
  • It is used in demodulator, encodes and light detectors systems.

Phototransistor

  • The photo transistor is a light detector.
  • It combines a photodiode and phototransistor.
  • The phototransistor cannot be directly used in control applications. Because, it produces a very low current.
  • Before applying to control circuit the current should be amplified.
  • A lens focuses the energy on the base-collector junction.
  • It has three terminal, but only two leads are generally used (emitter and collector).
  • The base current is supplied by the current created by the light falling on the Base-collector photodiode junction.
  • In phototransistors, the current is dependent mainly on the intensity of light entering into the lens and the voltage applied to the external circuit.

Photoconductive sensors

  • Photoconductive sensor is also called as Light Depending Resistor (LDR).
  • It is made of thin layer of semiconductor material (cadmium sulfide).
  • There is no light falls on the sensor the resistance is very high and the current is low.
  • Hence, the voltage drop across R is high. It is used in control circuits to control the current.

Photovoltaic sensors

  • It is a light-sensitive semiconductor device, and it produces a voltage, when the voltage increases and the intensity of light falling on the semiconductor junction of this photovoltaic cell increases.
  • It consists of a piece of semiconductor material (silicon or germanium).
  • The photovoltaic cells are produced more power, as in solar cells. These are called photovoltaic devices.
  • It is used in light meters.

LIGHT EMITTING DIODE (LED)

  • An LED is a semiconductor p-n junction diode which converts electrical energy to light energy under forward biasing.
  • It emits light in both visible and IR region.
  • The amount of light output is directly proportional to the forward current.
  • LED structure can be divided into two categories.

Ø Surface - emitting LED

Ø Edge - emitting LED

  • Surface emitting LED’s emit light perpendicular to the PN junction plane.
  • Edge-emitting LED emits light parallel to the PN in the plane.

Principle and Working

  • Injection luminescence is the principle used in LED’s.
  • When LED is forward biased, the majority charge carriers moves from p to n and similarly from n to p region and becomes excess minority carriers.
  • These excess minority carriers diffuse through the junction and recombines with the majority carriers in n and p region respectively to produce light.
  • The light thus produced is emitted from the p-n junction of the diode.

Advantages of LED

  • They are smaller in size.
  • Its cost is very low.
  • It has long life time.
  • It operates LED’s are available in different colours at low cost.
  • even at very low voltage.
  • Response time of LED is very fast in the order of 10 9 seconds.
  • Its intensity can be controlled easily.
  • It can be operated at a wide range of temperature (0-70˚) C.

Applications of LED

  • Used for numeric display in pocket calculators.
  • Used for applying input power to lasers.
  • Used for entering information into optical computer memories
  • Used for solid video displays.
  • Used in image sensing circuits.

Liquid Crystal Display (LCD)

  • Liquid crystal display is not a semiconductor device as LED.
  • LCD’s display the light, it doesn’t radiate light energy.
  • Therefore, LCD’s require an external (or) internal source of light so that it can either transmit (or) reflect the incident light.
  • LCD is a passive type display device used to display alpha numeric character and is seven segment display, watches calculators etc., in which the digits are displayed by the transmission (or) deflection of the incident light, with very low power consumption.
  • Molecules in ordinary liquids have random orientation but in a liquid crystal they are oriented in a definite crystal pattern.
  • Types of LCD’s
    • Dynamic Scattering Displays.
    • Twisted nematic display (or) Field effect display

Advantages of LCD

  • Low power is required
  • Good contrast
  • Low cost

Disadvantages of LCD

  • Speed of operation is slow
  • LCD occupy a large area
  • LCD life span is quite small, when used on d.c. Therefore, they are used with a.c. suppliers.

Applications of LCD

Ø Used as numerical counters for counting production items.

Ø Analog quantities can also be displayed as a number on a suitable device. (e.g.) Digital multimeter.

Ø Used for solid state video displays.

Ø Used for image sensing circuits.

Ø Used for numerical display in pocket calculators.

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