Lecture # 1 Introduction to Electronics (Semiconductors)

Introduction to Electronics (Semiconductors)

1. Engr. Athar Baig Lecturer a [email protected] 1 Lecture 1 & 2 Introduction to Electronics & Semiconductors Department of Electronics Engineering University of Chakwal Electronic Devices & Circuits

2. Introduction (Applications) We are living in an electronic era where machine robots are capable to do human work with more ease and high efficiency. Capsules and tablets contain wireless sensors that collect information from the body to diagnose. Transparent smartphones will exist in the coming days, we can see through them and they may lead to the use of windows or mirrors in our home to be used as PC screens and TV monitors. Sensors are placed on the plants to detect the shortage of water and alert the farmers . Electronic devices are made up of  active & passive elements  and smaller IC memories. The ICs, diodes, and transistor are made of semiconductor materials and their working is dependent on current flow through them . 2 Department of Electronics Engineering

3. Introduction “Electronics”, as the name implies relating to electrons. The word electronics arrived from electron mechanics (Behavior of the electron when it is subjected to externally applied fields ). The  definition of electronics technically says “ Electronics is an engineering branch that concerns with the flow of current through semiconductor, gas or any form of matter”. The world is growing at a fast rate and it is relevant for the technology enthusiast to upgrade with latest changes happening in the society. Moreover, it is difficult to spend few hours without electronics gadgets and they had become an important part of our everyday routine. 3 Department of Electronics Engineering

4. Applications 4 Department of Electronics Engineering

5. History of Electronics Vacuum Tubes The invention of Vacuum tube –   Invented by John Ambrose Fleming in 1897 brought in the age of electronics . The basic working principle of a vacuum tube is a phenomenon called thermionic emission . It works like this: you heat up a metal, and the thermal energy knocks some electrons loose . Fleming’s device consisted of two electrodes, a cathode and an anode, placed on either end of an encapsulated glass tube . When the cathode is heated, it gives off electrons via thermionic emission. Then, by applying a positive voltage to the anode (also called the plate), these electrons are attracted to the plate and can flow across the gap. By removing the air from the tube to create a vacuum, the electrons have a clear path from the cathode to the anode, and a current is created. 5 Department of Electronics Engineering

6. History of Electronics A simplified diagram of a vacuum tube diode. When the cathode is heated, and a positive voltage is applied to the anode, electrons can flow from the cathode to the anode. Note: A separate power source (not shown) is required to heat the cathode . This type of vacuum tube, consisting of only two electrodes, is called a diode . Diodes are commonly used for rectification , that is, converting from an alternating current (AC) to a direct current (DC). 6 Department of Electronics Engineering

7. History of Electronics In 1907, American inventor Lee de Forest added a third electrode to the mix, creating the first triode tube. This third electrode, called the control grid, enabled the vacuum tube to be used not just as a rectifier, but as an amplifier of electrical signals. Further enhancements of vacuum tubes placed an additional grid (called the screen grid) and yet another (called the suppressor grid) even closer to the anode, creating a type of vacuum tube called a  tetrode  and a  pentode , respectively. These extra grids solve some stability problems and address other limitations with the triode design, but the function remains largely the same. 7 Department of Electronics Engineering

8. History of Electronics Transistors In 1947, the trio of physicists William Shockley, Walter Brattain and John Bardeen created the world’s first transistor , and marked the beginning of the end for the vacuum tube. The transistor could replicate all the functions of tubes, like switching and amplification, but was made out of semiconductor materials . Transistors are much more durable (vacuum tubes, like light bulbs, will eventually need to be replaced), much smaller (imagine fitting 2 billion tubes inside an iPhone), and require much less voltage than tubes in order to function (for one thing, transistors don’t have a filament that needs heating ). They had better reliability and probably most importantly, their characteristics were far more linear and stable . 8 Department of Electronics Engineering

9. History of Electronics V acuum tubes are still used in high power RF transmitters, as they can generate more power than modern semiconductor equivalents. For this reason, you’ll find vacuum tubes in particle accelerators, MRI scanners, and even microwave ovens . The real development started with the invention of the transistor in 1948 in Bell Laboratories. Large Bulky Vacuum diodes are replaced with junction transistor. Transistors are initially made with germanium material, later on, silicon BJT (Bipolar Junction Transistor) are grown up. Most of the devices developed today are made up of silicon only due to its low cost. 9 Department of Electronics Engineering

10. History of Electronics IC (Integrated Circuit)  – Jack Kilby To reduce the size and cost of the entire circuit Jack Kilby introduced a new concept. This idea entirely changed the world. The complete interconnected circuit is placed on a single chip commonly called VLSI (Very Large Scale Integrated). Computer processors used today are made up of billions of transistors integrated on a single IC. 10 Department of Electronics Engineering

11. History of Electronics The miniaturization that has occurred in recent years leaves us to wonder about its limits. Complete systems now appear on wafers thousands of times smaller than the single element of earlier networks. Today, the Intel® i7 extreme edition processor has 731 million transistors in a package that is only slightly larger than a 1.67 sq. inches. In 1965, Dr. Gordon E. Moore presented a paper predicting that the transistor count in a single IC chip would double every two years.   11 Department of Electronics Engineering

12. All ICs used in Electronics are fabricated with Semiconductors. OR All Electronics depends upon the semiconductors. 12 Department of Electronics Engineering

13. Materials Before moving towards semiconductors let’s see how many major types of materials exist. 13 Materials Insulators Semi Conductors Conductors Department of Electronics Engineering

14. Conductors M aterials allows charges to flow easily. High Conductivity – low resistivity. No energy band gap, Positive temp. coefficient. Valence electron is loosely bound with nucleus. Examples are Copper, Silver, Gold, Aluminum etc. and Copper is very cheap among all . Vastly used in Electrical Engineering (Generation, Transmission and distribution). Some special conductors turn into super conductors at 0K. 14 Department of Electronics Engineering

15. Insulators Materials do not allow to flow of charges. low conductivity – high resistivity. Huge energy band gap (5eV), Negative temp. coefficient Valence electron is tightly bound with nucleus as it has stable orbit (8 electrons). Examples are mica, glass, wood, rubber etc. Vastly used in Electrical Engineering (Transmission and distribution side). Resistance increases when cooled down. 15 Department of Electronics Engineering

16. Semiconductor Materials (Ge , Si, & GaAs) The construction of every discrete (individual) solid-state (hard crystal structure) electronic device or integrated circuit begins with a semiconductor material of the highest quality. Semiconductors are a special class of elements having a conductivity between that of a good conductor and that of an insulator, having negative temp. coefficient, Energy band gap of 1eV, behaves as insulator at absolute zero temp (0K, - 273 °C ). They have four valence electrons. Backbone of Electronics. 16 Department of Electronics Engineering

17. Semiconductor Materials (Ge , Si, AND GaAs) 17 Semiconductor Materials Single Crystal Compound Crystal repetitive crystal structure (Si, Ge) constructed of two or more semiconductor materials of different atomic structures (GaAs, CdS , GaN , GaAsP ) The three semiconductors used most frequently in the construction of electronic devices are Ge, Si, and GaAs Department of Electronics Engineering

18. Semiconductor Materials (Ge , Si, AND GaAs) In the early few decades (1940s -1950s), Ge was used for the fabrication of diodes and t ransistors because easy to find available in fairly large quantities easy to refine (to obtain very high levels of purity) But suffered from low levels of reliability due primarily to its sensitivity to changes in temperature High reverse saturation current 18 Department of Electronics Engineering

19. Semiconductor Materials (Ge , Si, AND GaAs) At the time, scientists were aware that another material, silicon, had improved temperature sensitivities with low reverse saturation current But The refining process for manufacturing silicon of very high levels of purity was still in the development stages. Finally, however, in 1954 the first silicon transistor was introduced, and silicon quickly became the semiconductor material of choice . Not only is silicon less temperature sensitive, but it is one of the most abundant materials on earth, removing any concerns about availability. 19 Department of Electronics Engineering

20. Semiconductor Materials (Ge , Si, AND GaAs) As time moved on, however, the field of electronics became increasingly sensitive to issues of speed and communication systems were operating at higher levels of performance . A semiconductor material capable of meeting these new needs had to be found. The result was the development of the first GaAs transistor in the early 1970s. 20 GaAs Si Speeds of operation up to five times that of Si Speeds of operation is less than GaAs More expensive (Difficult to manufacture) Cheaper to manufacture Little design support Highly efficient design strategies Department of Electronics Engineering

21. Semiconductor Materials (Ge , Si, AND GaAs) However , in time the demand for increased speed resulted in more funding for GaAs research, to the point that today it is often used as the base material for new high-speed, very large scale integrated (VLSI) circuit designs. 21 Department of Electronics Engineering

22. Semiconductor Materials (Ge , Si, AND GaAs) This brief review of the history of semiconductor materials is not meant to imply that GaAs will soon be the only material appropriate for solid-state construction. Germanium devices are still being manufactured, although for a limited range of applications. Even though it is a temperature-sensitive semiconductor, it does have characteristics that find application in a limited number of areas. Given its availability and low manufacturing costs, it will continue to find its place in product catalogs. 22 Department of Electronics Engineering

23. Ge Germanium is in limited production due to its temperature sensitivity and high reverse saturation current. It is still commercially available but is limited to some high-speed applications (due to a relatively high mobility factor) and applications that use its sensitivity to light and heat such as photodetectors and security systems. Si Without question the semiconductor used most frequently for the full range of electronic devices. It has the advantage of being readily available at low cost and has relatively low reverse saturation currents, good temperature characteristics, and excellent breakdown voltage levels. It also benefits from decades of enormous attention to the design of large-scale integrated circuits and processing technology. GaAs Since the early 1990s the interest in GaAs has grown in leaps and bounds, and it will eventually take a good share of the development from silicon devices, especially in very large scale integrated circuits. Its high-speed characteristics are in more demand every day, with the added features of low reverse saturation currents, excellent temperature sensitivities, and high breakdown voltages. More than 80% of its applications are in optoelectronics with the development of light-emitting diodes, solar cells, and other photodetector devices, but that will probably change dramatically as its manufacturing costs drop and its use in integrated circuit design continues to grow; perhaps the semiconductor material of the future. 23 Semiconductor Materials (Ge , Si, AND GaAs) Department of Electronics Engineering

24. Semiconductor Materials (Ge , Si, AND GaAs) As noted earlier, Si has the benefit of years of development, and is the leading semiconductor material for electronic components and ICs. In fact, Si is still the fundamental building block for Intel’s new line of processors . 24 Department of Electronics Engineering

25. Semiconductor Materials (Ge , Si, AND GaAs) Conductors have a positive temperature coefficient i.e. the resistance increases with an increase in heat. This is because the numbers of carriers in a conductor do not increase significantly with temperature, but their vibration pattern about a relatively fixed location makes it increasingly difficult for a sustained flow of carriers through the material While Semiconductor materials have a negative temperature coefficient i.e. they exhibit an increased level of conductivity with the application of heat . 25 Department of Electronics Engineering

26. Covalent bonding and intrinsic materials 26 Atomic structure of (a) silicon; (b) germanium; a nd (c ) gallium and arsenic. S ilicon has 14 orbiting electrons, germanium has 32 electrons, gallium has 31 electrons, and arsenic has 33 orbiting electrons. For germanium and silicon there are four electrons in the outermost shell- tetravalent , which are referred to as valence electrons. Gallium has three valence electrons- trivalent and arsenic has five valence electrons- pentavalent . Valence : T he potential (ionization potential) required to remove any one of these electrons from the atomic structure is significantly lower than that required for any other electron in the structure. Department of Electronics Engineering

27. Question No.1 27 What is the net charge of the silicon atom if it loses one of its valence electrons? If it gains an extra electron in the valence orbit? ? Department of Electronics Engineering

28. Covalent bonding and intrinsic materials 28 Covalent bonding of the silicon atom. This bonding of atoms, strengthened by the sharing of electrons, is called covalent bonding . Covalent bonding of the GaAs crystal Department of Electronics Engineering

29. Covalent bonding and intrinsic materials Intrinsic Semiconductor: A single-crystal semiconductor material with no other types of atoms within the crystal (pure). The densities of electrons and holes are equal . At room temperature, a silicon crystal acts like an insulator because it has only a few free electrons and holes produced by thermal energy. The free electrons in a material due only to external causes are referred to as intrinsic carriers . Relative mobility ( ) of the free carriers in the material, that is, the ability of the free carriers to move throughout the material.   29 Department of Electronics Engineering

30. Covalent bonding and intrinsic materials Free carriers in GaAs have more than five times the mobility of free carriers in Si - a factor that results in response times using GaAs electronic devices . F ree carriers in Ge > twice the mobility of electrons in Si - continued use of Ge in high-speed radio frequency applications. 30 Department of Electronics Engineering

31. Hole and Electrons When the ambient temperature is above absolute zero(-273°C ), the heat energy in this air causes the atoms in a silicon crystal to vibrate. The higher the ambient temperature, the stronger the mechanical vibrations become . In a silicon crystal, the vibrations of the atoms can occasionally dislodge an electron from the valence orbit. When this happens, the released electron gains enough energy to go into a larger orbit. The departure of the electron creates a vacancy in the valence orbit called a hole . At room temperature, thermal energy produces only a few holes and free electrons. To increase the number of holes and free electrons, it is necessary to dope the crystal. 31 Department of Electronics Engineering

32. Recombination and Lifetime In a pure silicon crystal, thermal (heat) energy creates an equal number of free electrons and holes. The free electrons move randomly throughout the crystal. Occasionally, a free electron will approach a hole, feel its attraction, and fall into it. Recombination is the merging of a free electron and a hole . The amount of time between the creation and disappearance of a free electron is called the lifetime . It varies from a few nanoseconds to several microseconds, depending on how perfect the crystal is and other factors. 32 Department of Electronics Engineering

33. Question No.2 If a pure silicon crystal has 1 million free electrons inside it, how many holes does it have? What happens to the number of free electrons and holes if the ambient temperature (temperature of the surrounding air) increases? 33 Department of Electronics Engineering

34. Doping a Semiconductor One way to increase conductivity of a semiconductor is by doping. This means adding impurity atoms to an intrinsic crystal to alter its electrical conductivity. A doped semiconductor is called an extrinsic semiconductor . Increasing the Free Electron (N-type) Increasing the Number of Holes (P-type) The more impurity that is added, the greater the conductivity. In this way, a semiconductor may be lightly or heavily doped. A lightly doped semiconductor has a high resistance, whereas a heavily doped semiconductor has a low resistance. 34 Department of Electronics Engineering

35. Doping a Semiconductor P-type N-Type P stands for Positive. To increase the number of holes, trivalent (3 valence electron) impurity is added to pure Si. Examples of trivalent atoms include Boron (5), Aluminum (13), and Gallium (31). Accepter impurity elements: Take electrons to form P-type semiconductors. Holes are majority carriers while electrons are minority carriers. N stands for Negative. To increase the number of free electrons, pentavalent (5 valence electron) impurity is added to pure Si. Examples of pentavalent atoms include arsenic (33), antimony (51), and phosphorus (15). Donor impurity elements: Give electrons to form N-type semiconductors Electrons are majority carriers while holes are minority carriers 35 Department of Electronics Engineering

36. Question No.3 A doped semiconductor has 10 billion silicon atoms and 15 million pentavalent atoms. If the ambient temperature is 25°C, how many free electrons and holes are there inside the semiconductor? 36 Department of Electronics Engineering

37. Energy gap 37 Now, in order to break the covalent bond, a valence electron must gain enough energy to become free electrons. The minimum energy required is known as the bandgap energy , The farther an electron is from the nucleus, the higher is the energy state, and any electron that has left its parent atom has a higher energy state than any electron in the atomic structure .   Department of Electronics Engineering

38. E nergy L evels 38 D iscrete levels in isolated atomic structures C onduction and valence bands of an insulator, a semiconductor, and a conductor. Department of Electronics Engineering

39. 39 Thank You Department of Electronics Engineering