Electron Devices - A Broader Perspective (Compiled by Prof.N.Shanmugasundaram)

Electron DevicesA broader perspective

Prof. N.ShanmugasundaramProfessor, ECE Department

Vidyaa Vikas College of Engg & Tech.

Tiruchengode, Namakkal Dt.

Apr 7, 2023 NSS / VVCET 2

Overview of Presentation

• Semiconductor Theory

• PN Junction Diode

• BJT

• FET (JFET & MOSFET)

• Special Function Devices

Semiconductor Basics


• Stable atom has 8 electrons in outermost orbit

• Insulator has 8 electrons

• Semiconductor has exactly 4 electrons (eg. Si)

• Metal has less than 4 electrons

Energy Levels in Semiconductor


The more distant the electron from the nucleus, the higher the energy state.

Energy level of electrons at Outermost orbit is VALENCE BAND.

Energy needed in Electrons for conduction is CONDUCTION BAND.

Intrinsic Semiconductor


• No Impurities (pure semiconductor)

• No Charge carriers at 0º K (−273.15°

Celsius)

• Only few carriers at room temperature

• N = P

• Not suitable for Electron Devices

Semiconductor Theory


Extrinsic Semiconductor

P Type:

• Trivalent Impurity (eg. Boron)

• Holes – Charge carriers

N Type:

• Pentavalent Impurity (eg. P)

• Electrons – Charge carriers



1. Comparison of Metals, Semiconductors & Insulators

2. Forbidden Energy gap, VB, CB

3. Comparison of Intrinsic & Extrinsic semiconductor

4. Comparison of P-type and N-type Semiconductors




PN Junction Diode

PN Junction Diode


PN Junction Diode


PN Junction Diode


1. Forward & Reverse Biasing

2. Depletion region

3. Reverse saturation current (Is)

4. Equation for Forward current (If)

5. Barrier potential / Knee voltage (Si & Ge)

6. Breakdown voltage (PIV)

7. Static and Dynamic resistance

8. Parameters (If, PIV, Pd, Ot)

9. Applications

BJT – Bipolar Junction Transistor


Model of First Transistor

• BJT is a 3-Terminal device.

• BJT is a CURRENT CONTROLLED device.

• Terminals are EMITTER, BASE, COLLECTOR.

• Main application of BJT: SWITCH and AMPLIFIER.

• Can be operated in 3 regions: CUT-OFF, ACTIVE and SATURATION.

• 3 BJT configurations are CB, CE and CC.

• Commonly used configuration: CE

• Biasing methods: Base resistor, Collector Feedback, Potential divider.

Two Types of BJT: NPN & PNP

IE = IC + IB

Common Emitter Configuration

Common Base Configuration

Common Collector Configuration


BIASING

Biasing is a process through which collector current IC is kept constant

withstanding the variations in β, Temperature.

Three types of Biasing are:

1. Fixed Bias (Simple, but not stable)

2. Collector Feedback Bias

3. Voltage Divider Bias (Stable and Best)

Fixed Biasing (Simple)

Collector Resistor (Voltage Feedback) Biasing

Voltage Divider Biasing (Stable)


IC = Collector current

IB = Base current

IE = Emitter current

Simulation of transistor as an amplifier

Transistor Configuration Comparison

AMPLIFIER TYPE COMMON BASE COMMON EMITTER

COMMON COLLECTOR

INPUT/OUTPUT PHASE

RELATIONSHIP0° 180° 0°

VOLTAGE GAIN HIGH MEDIUM LOW

CURRENT GAINLOW = a IC / IE

MEDIUM = b IC / IB

HIGH γ = IE / IB

POWER GAIN LOW HIGH MEDIUM

INPUT RESISTANCE

LOW MEDIUM HIGH

OUTPUT RESISTANCE

HIGH MEDIUM LOW

APPLICATION NOT USED AMPLIFICATIONIMPEDANCE MATCHING

Transistor Terminal Identification

Transistor Testing

1. Curve Tracer - Provides a graph of the characteristic curves.

2. DMM - Some DMM’s will measure DC or HFE.

3. Ohmmeter

BJT


1. Application

2. Biasing

3. Regions of Operation

4. Classes of Operation

5. Frequency Response

6. h-Parameters

7. Important relationships of a transistor

FETApr 7, 2023 NSS / VVCET 37

FIELD EFFECT TRANSISTOR (FET)


• FET is a UNI-POLAR transistor.

• FET is a VOLTAGE CONTROLLED device.

• 3-terminals of FET: SOURCE, GATE, DRAIN.

• Main application of FET: SWITCH and AMPLIFIER.

• Can be operated in 3 regions: CUT-OFF, ACTIVE and SATURATION.

• Biasing methods: Base resistor, Collector Feedback, Potential divider.

• Advantage of FET over BJT:

FET requires virtually no input (bias signal) current and gives an extremely high input resistance.

High noise immunity and thermal stability





Field Effect Transistor (FET) Bipolar Junction Transistor (BJT)

1 LOW VOLTAGE GAIN HIGH VOLTAGE GAIN

2 HIGH CURRENT GAIN LOW CURRENT GAIN

3 VERY INPUT IMPEDANCE LOW INPUT IMPEDANCE

4 HIGH OUTPUT IMPEDANCE LOW OUTPUT IMPEDANCE

5 LOW NOISE GENERATION MEDIUM NOISE GENERATION

6 FAST SWITCHING TIME MEDIUM SWITCHING TIME

7 EASILY DAMAGED BY STATIC ROBUST

8SOME REQUIRE AN INPUT TO TURN IT "OFF"

REQUIRES ZERO INPUT TO TURN IT "OFF"

9 VOLTAGE CONTROLLED DEVICE CURRENT CONTROLLED DEVICE

10 MORE EXPENSIVE THAN BIPOLAR CHEAP

11 DIFFICULT TO BIAS EASY TO BIAS














MOSFET




1. Features

2. Characteristics (Transfer, Drain)

3. Biasing

4. Regions of Operation

5. Frequency Response

6. Important Parameter - gm

7. Important relationships of a transistor


Other Special Purpose Devices

FIGURE - Zener diode symbol.

1. ZENER DIODE

Zener Diode:- is a silicon pn junction device that differ from rectifier diodes because

it is designed for operation in the reverse- breakdown region.

- if Zener diode is forward-biased, it operates the same as a rectifier diode.

Function:- to provide a stable reference voltage for use in power supplies, voltmeter & other instruments, voltage regulators.

FIGURE - General diode V-I characteristic.

Zener breakdown:- occurs in a Zener diode at low reverse voltages. - Zener diode is heavily doped to reduce the breakdown voltage. - This causes a very thin depletion region.

FIGURE - Tunnel diode symbols.

2. TUNNEL DIODE

A tunnel diode or Esaki diode is a

type of semiconductor diode which is

capable of very fast operation, well

into the microwave frequency region,

by using quantum mechanical effects.

Forward bias operation

Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow p–n junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states on the p-side of the pn junction.

As voltage increases further these states become more misaligned and the current drops – this is called negative resistance because current decreases with increasing voltage.

As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the p–n junction, and no longer by tunneling through the p–n junction barrier.

Thus, the most important operating region for a tunnel diode is the negative resistance region.

FIGURE - Tunnel diode characteristic curve.

FIGURE - Parallel resonant circuit.

FIGURE - Basic tunnel diode oscillator.

3. VARACTOR DIODE

The reverse-biased varactor diode acts as a variable capacitor.

FIGURE - The reverse-biased varactor diode acts as a variable capacitor.

FIGURE - Varactor diode capacitance varies with reverse voltage.

FIGURE 6 - A Resonant band-pass filter using a varactor diode for adjusting the resonant frequency over a specified range.

FIGURE - Symbol for an LED. When forward-biased, it emits light.

4. LED

FIGURE - Electroluminescence in a forward-biased LED.

FIGURE - Basic operation of an LED.

FIGURE - Examples of typical spectral output curves for LEDs.

FIGURE - Typical LEDs.

FIGURE - The 7-segment LED display.

5. LASER DIODE

A Laser diode, also known as an injection

laser or diode laser, is a semiconductor device

that produces coherent radiation (in which the

waves are all at the same frequency and

phase) in the visible or infrared (IR) spectrum

when current passes through it.

Laser diodes are used in •optical fiber systems, •compact disc (CD) players, •laser printers, •remote-control devices, •and intrusion detection systems.

Figure: Structure of DH LASER Diode

http://upload.wikimedia.org/wikipedia/commons/9/93/Simple_dh_laser_diode.svg

FIGURE - Basic laser diode construction and operation.

FIGURE - Photodiode.

6. PHOTODIODE

FIGURE - Typical photodiode characteristics.

FIGURE - Operation of a photodiode.

FIGURE - PIN diode.

7. PIN DIODE

A PiN diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor regions.

The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts.

The wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (the normal function of a diode),

but it makes the PIN diode suitable for

•attenuators, •fast switches, •photo detectors, and •high voltage power electronics applications.

FIGURE - PIN diode characteristics.

FIGURE - Diode symbols.

8. SILICON CONTROLLED RECTIFIER

Two Transistor model of SCR

The switching action of gate takes place only when

(i) SCR is forward biased i.e. anode is positive with respect to cathode.

(ii) Suitable positive voltage is applied between the gate and the cathode.

Once the SCR has been switched on, it has no control on the amount of current flowing through it.

The current through the SCR is entirely controlled by the external impedance connected in the circuit and the applied voltage. The forward current through the SCR can be reduced by reducing the applied voltage or by increasing the circuit impedance.

A minimum forward current must be maintained to keep the SCR in conducting state. This is called the holding current rating of SCR. If the current through the SCR is reduced below the level of holding current, the device returns to off-state or blocking state.

Note : The gate can only trigger or switch-on the SCR, it cannot switch off.

Firing Angle

The angle (in the input AC) at which the gate is triggered is known as 'firing angle'.

Holding Current

It is the minimum anode current (with gate being open) required to keep the SCR in ON condition.

Break Over voltage

It is the minimum forward voltage with gate being open, at which an SCR starts conducting heavily (i.e., the SCR is turned ON) .

Terminology

A unijunction transistor (UJT) is an electronic semiconductor device that has only one junction.

The UJT has three terminals: an emitter (E) and two bases (B1 and B2).

The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends.

The emitter is of p-type and it is heavily doped.

9. UNIPOLAR JUNCTION TRANSISTOR

Intrinsic Standoff Ratio

Unijunction transistor: (a) emitter characteristic curve, (b) model for VP .

Application of UJT – RELAXATION OSCILLATOR

REVIEW:

• A unijunction transistor consists of two bases (B1, B2) attached to a resistive bar of silicon, and an emitter in the center.

• The E-B1 junction has negative resistance properties; it can switch between high and low resistance.

• The intrinsic standoff ratio is η= RB1 /(RB1 + RB2), for a unijunction transistor. The trigger voltage is determined by η.

• Unijunction transistors and programmable unijunction transistors are applied to oscillators, timing circuits, and Thyristor triggering.

Documents

Electron Devices - A Broader Perspective (Compiled by Prof.N.Shanmugasundaram)