lesson 5 - university of nairobi

Lesson 5 Electronics:

• Semiconductors

• Doping

• p-n Junction Diode

• Half Wave and Full Wave Rectification

• Introduction to Transistors- Types and Connections

Semiconductors

• If there are many free electrons to carry current, the semiconductor acts more like a conductor.

• If there are few electrons, the semiconductor acts like an insulator.

• Silicon is the most commonly used semiconductor.

• Atoms of silicon have 14 electrons.

• Ten of the electrons are bound tightly inside the atom.

• Four electrons are near the outside of the atom and only loosely bound.

• The relative ease at which electric current flows through a material is known as conductivity.

• Conductors (like copper) have very high conductivity.

• Insulators (like rubber) have very low conductivity.

• The conductivity of a semiconductor depends on its conditions.

• For example, at low temperatures and low voltages a semiconductor acts like an insulator.

• When the temperature and/or the voltage is increased, the conductivity increases and the material acts more like a conductor.

Semiconductors

Semiconductors

• Pure semiconductors – thermal vibration results in some bonds being broken generating free electrons

which move about – these leave behind holes which accept electrons from adjacent atoms and

therefore also move about – electrons are negative charge carriers – holes are positive charge carriers – At room temperatures there are few charge carriers – pure semiconductors are poor conductors – this is intrinsic conduction

• Doping – the addition of small amounts of impurities drastically affects its properties – an excess of electrons produce an n-type semiconductor – an excess of holes produce a p-type semiconductor – both n-type and p-type materials have much greater conductivity than pure

semiconductors – this is extrinsic conduction

Semiconductors

• The dominant charge carriers in a doped semiconductor (e.g. electrons in n-type material) are called majority charge carriers. Other type are minority charge carriers

• The overall doped material is electrically neutral

p-n Junction Diode

• Potential Barrier – the barrier opposes the flow of

majority charge carriers and only a small number have enough energy to surmount it • this generates a small diffusion

current

– the barrier encourages the flow of minority carriers and any that come close to it will be swept over • this generates a small drift current

– for an isolated junction these two currents must balance each other and the net current is zero

p-n Junction Diode

• The diffusion of positive charge in one direction and negative charge in the other produces a charge imbalance – this results in a potential barrier across the junction

• When p-type and n-type materials are joined this forms a p-n junction – majority charge carriers on each side diffuse across the junction where they

combine with (and remove) charge carriers of the opposite polarity

– hence around the junction there are few free charge carriers and we have a depletion layer (also called a space-charge layer)

p-n Junction Diode • Forward bias

– if the p-type side is made positive with respect to the n-type side the height of the barrier is reduced

– more majority charge carriers have sufficient energy to surmount it

– the diffusion current therefore increases while the drift current remains the same

– there is thus a net current flow across the junction which increases with the applied voltage

1.5V

p-n Junction Diode

• Reverse bias – if the p-type side is made negative with respect to the

n-type side the height of the barrier is increased – the number of majority charge carriers that have

sufficient energy to surmount it rapidly decreases – the diffusion current therefore vanishes while the drift

current remains the same – thus the only current is a small leakage current caused by

the (approximately constant) drift current – the leakage current is usually negligible (a few nA)

1.5V

Forward I–V Diode Characteristics

The load line plots all possible combinations of diode current (ID) and voltage (VD) for a

given circuit. The maximum ID equals E/R, and the maximum VD equals E.

The point where the load line and the characteristic curve intersect is the Q-point, which

identifies ID and VD for a particular diode in a given circuit.

p-n Junction Diode

• Forward and Reverse Currents – p-n junction current is given approximately by

– where I is the current, e is the electronic charge, V is the applied voltage, k is Boltzmann’s constant, T is the absolute temperature and (Greek letter eta) is a constant in the range 1 to 2 determined by the junction material

– for most purposes we can assume = 1

1exp

ηkT

eVII s The Shockley Equation

Different types of Diodes

Different types of Diodes

Different types of Diodes

Diode Applications - The Half-Wave Rectifier

A Typical Battery Charging Circuit

Diode Applications - The Half-Wave Rectifier

Diode Applications - The Full-Wave Rectifier

The full-wave rectifier

Voltage across each half of the transformer secondary

Full-wave load voltage

Diode Applications - The Bridge Rectifier

Transistors Transistors

BJT Transistors

FET Transistors

JFET Transistors

MOSFET Transistors

npn pnp

n channel p channel Depletion MOSFET

Enhancement MOSFET

n channel p channel n channel

(NMOS)

p channel (PMOS)

3

2

1

3

2

1

NMOS +PMOS=CMOS

BJT Transistors

The BJT is a nonlinear, 3-Terminal device based on the junction diode. A representative structure sandwiches one semiconductor type between layers of the opposite type. We first examine the npn BJT

A pnp BJT and its schematic symbol.

A npn BJT and its schematic symbol.

BJT Transistors The npn Transistor

BJT Transistors

BJT Transistors

the constant β is called the common-emitter current gain

BJT Transistors

BJT Transistors

Input Characteristics

Circuit for measuring BJT characteristics.

BJT Transistor

Output Characteristics

BJT Transistor

(a) Conceptual circuit for measuring the iC –vCE characteristics of the BJT. (b) The iC –vCE characteristics of a practical BJT.

Output Characteristics

BJT Applications

The Common-Emitter Amplifier

The Field-Effect Transistor (FET)

• The field-effect transistor, or FET, is also a 3-terminal device, but it is constructed, and functions, somewhat differently than the BJT.

• There are several types. We begin with the junction FET (JFET), specifically, the n-channel JFET

• The p-n junction is a typical diode. Holes move from p-type into n-type .

• Electrons move from n-type into p-type . • Region near the p-n junction is left without any

available carriers -depletion region • Carriers are still present in the n-type channel . • Current could flow between drain and source

FET Transistors

FET Transistors

N-channel JFET P-channel JFET

FET Transistors

n-channel JFET

Typical output characteristics, n-channel JFET.

Transfer characteristics, n-channel JFET.

MOSFET Transistors

N-channel MOSFET P-channel MOSFET

MOSFET Transistors

Depletion mode N-channel MOSFET Enhancement mode N-channel MOSFET

MOSFET Transistors

P-channel MOSFET N-channel MOSFET

MOSFET Transistors Static characteristics of a Depletion and Enhancement mode N-channel MOSFET

MOSFET Transistors Enhancement only N-channel MOSFET (NMOS)

NMOS can never operate with a negative gate voltage

MOSFET Transistors Enhancement only N-channel MOSFET (NMOS) Enhancement only P-channel MOSFET (PMOS)

MOSFET Transistors Enhancement only N-channel MOSFET (NMOS) symbol, drain and transfer characteristics

CMOS

Q1PMOSFET

Q2NMOSFET

+10v

01

NMOS

PMOS