Electronics

Semiconductors

Semiconductors have electrical conducting properties but they are not as bad as insulators and nowhere as good as conductors.

The electrical conducting properties of semiconductors may be greatly increased by:

• Raising the temperature

• The addition of small amounts of suitable impurities .

Examples are silicon, germanium and gallium arsenide.

An extrinsic semiconductor is a semiconductor that has been doped, that is, into which a doping agent has been introduced, giving it different electrical properties than the intrinsic (pure) semiconductor.

They are the foundation of modern electronics, including radio, computers, telephones, and many other devices.

Their conductivity may easily be modified by introducing impurities into their crystal lattice.

Doping is the addition of impurities. Doping a semiconductor such as silicon with a small amount of impurity atoms, such as antimony or boron, greatly increases the number of free electrons or holes within the semiconductor.

Donor impurities are called n-type (excess of negative charge carriers), Common donor impurities are Phosphorous, arsenic and antimony. They all have 5 valence electrons (outer shell electrons).

Those doped with acceptor impurities are known as p-type(positive charge carriers). Common acceptor impurities are Boron and Gallium. They all have 3 valence electrons (outer shell electrons).

Junctions forming where n-type and p-type semiconductors join together are called p-n junctions.

A semiconductor diode is a device made from a single p-n junction.

The junction region itself has no charge carriers and is known as the depletion region. The junction (depletion) region has a physical thickness that varies with the applied voltage.

When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a depletion layer which is approximately 0.5 to 0.7v for silicon diodes and approximately 0.3 of a volt for germanium diodes.

When a junction diode is Forward Biased the thickness of the depletion region reduces and the diode acts like a short circuit allowing full current to flow.

When a junction diode is Reverse Biased the thickness of the depletion region increases and the diode acts like an open circuit blocking any current flow, (only a very small leakage current).

Features of the semi-conductor diode

• The diode acts as a one way electrical valve , they allow current to flow in one direction only.

• The diode converts a.c. to d.c. This process is known as rectification. A Single diode will give half wave rectification.

Bridge Rectifiers are widely used for supplying power to various electronic basic components. The purpose of the Bridge Rectifier is to convert the AC power into DC power. It is the most resourceful rectifier circuit from others. The power conversion in this device is very efficient.

A capacitor is a device for storing electric charge.

Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting substance, or dielectric. You can easily make a capacitor from two pieces of aluminum foil and a piece of paper.

A capacitor's storage potential, or Capacitance, is measured in units called farads. It is the ratio of charge stored on the capacitor to the potential difference across it.

A 1-farad capacitor can store one coulomb of charge at 1 volt.

Equation → Q = CV WHERE

• Q is the charge on the capacitor

• C is the capacitance of the capacitor

• V is the Voltage across the capacitor

Capacitors in Series Circuits

Since the potential difference between points A and B is independent of path, the battery voltage V must equal the sum of the voltages across each capacitor.

Equation → V_battery = V₁ + V₂ + V₃

Equivalent C_e for capacitors in series: 1/C_e = 1/C₁ + 1/C₂ + 1/C₃

Capacitors in Parallel Circuits

Equivalent C_e for capacitors in a parallel circuit: C_e = C₁ + C₂ + C₃

When a capacitor is connected to a battery current flows. Therefore energy transfers are taking place in the circuit. Energy is transferred from the source of e.m.f to the capacitor. We can say the capacitor stores energy.

Energy stored in capacitor → E= ½ QV= ½ CV²

Applications of Capacitors

• Sometimes, capacitors are used to store charge for high-speed use. That's what a flash does. Big lasers use this technique as well to get very bright, instantaneous flashes.

• Capacitors can also be used for Smoothing a rectified AC (eliminate ripples). If a line carrying DC voltage has ripples or spikes in it, a big capacitor can even out the voltage by absorbing the peaks and filling in the troughs /valleys.

A capacitor can block DC voltage. If you hook a small capacitor to a battery, then no current will flow between the poles of the battery once the capacitor charges.

The plate on the capacitor that attaches to the negative terminal of the battery accepts electrons that the battery is producing.

The plate on the capacitor that attaches to the positive terminal of the battery loses electrons to the battery.

Here you have a battery, a light bulb and a capacitor. When you connect the battery, the light bulb will light up as current flows from the battery to the capacitor to charge it up.

The bulb will get progressively dimmer and finally go out once the capacitor reaches its capacity. If you then remove the battery and replace it with a wire, current will flow from one plate of the capacitor to the other. The bulb will light initially and then dim as the capacitor discharges, until it is completely out.

Capacitors and AC Voltage

However, any alternating current (AC) signal continues unimpeded. That's because the capacitor will charge and discharge as the alternating current fluctuates, making it appear that the alternating current is flowing. Therefore a lamp connected in series with an AC supply and a capacitor can light.

Logic Gates and Circuits

A transistor serves as a switch in electronic circuits in the telecommunication industry.

• It can also be used to amplify the current.

• Transistors are made from extrinsic semiconductors.

• Logic gates are circuits in which transistors act as high speed switches.

• Logic gates may consist of multiple transistors.

• Microprocessors have billions of transistors.

An example of a transistor is the npn. This is a transistor that has been made of n-type and p-type semiconductor materials.

Integrated Circuits

Integrated circuits (ICs) are used in microelectronics. These circuits consist of transistors, diodes, resistors and small capacitors.

• The various components are all connected together on a ‘chip’ of silicon.

• It is by the use of integrated circuits that artificial intelligence has developed and hence a machine can be used to do many tasks.

• The ICs can be used to produce different logic gates depending on how it is connected.

Logic gates are circuits in which transistors act as high speed switches. They are used in pocket calculators, digital watches, computers , robots etc.

• Their outputs and inputs involve only two levels of voltage. High is near the supply voltage and low is near 0 V.

• High is referred to as logic level 1 and low as logic level 0.

• The behavior of a logic gate can be summarized using a truth table

Types of Logic Circuits

1. NOT Gate → It produces a high output if the input is low and vice versa.

2. AND Gate → The output is high only when both inputs are high.

3. OR Gate → The output of an OR gate is 0 only when both inputs are 0.

4. NAND Gate → It’s output is 0 only when both inputs are high.

5. NOR Gate → Its output is high only when both inputs are low.

Application of Logic Circuits

Transistors and logic circuits are the fundamental components for all modern electronic devices.

Electronics and Society

• In the home devices such as washing machines, burglar alarms, telephones, microwave ovens etc. all contain electronic components.

• In industry microprocessor controlled equipment is being increasingly used.

• A microprocessor (MPU or microP) is a miniature version of the CPU of a digital computer.

• In many cases production lines and even whole factories e.g. sugar refineries and oil refineries are almost entirely automated.

• Communication satellites enable the transmission of information from one part of the world to another.

• Using computers we are now able to communicate extensively via the internet.