OpenStax-CNX module: m47114 1 History of Electron Devices- Vacuum Tube Era, Solid State Era and NanoTechnologyEra(Untitled) * Bijay_Kumar Sharma This work is produced by OpenStax-CNX and licensed under the Creative Commons Attribution License 3.0 Abstract Section 2.1.1 describes the historical development through Vacuum Tube Era. 1 2.1. History of Electron Devices- Vacuum Tube Era, Solid State Era and NanoTechnologyEra. In 1897, while studying Cathode Ray in a Vacuum Tube, J.J.Thomson for the rst time identied the Cathode Ray with `Electrons'. For this J.J.Thomson was awarded Nobel Prize in Physics in 1906. * Version 1.1: Jul 23, 2013 6:13 am -0500 http://creativecommons.org/licenses/by/3.0/
OpenStax-CNX module: m47114 2 Figure 1 Thomson's Vacuum Tube was a very simple contraption. The Tube was evacuated to a vacuum of 10-3 Torr by a rotary pump. 1 Torr is 1mm Hg Vacuum. 1 Atmosphere pressure will support 76cm column of Hg. We can go to still higher vacuum using diusion pump. For MOSFET fabrication we require ion-pump which can evacuate to 10-8 Torr. This evacuated Vacuum Tube (VT) contains a lament, cathode and anode. Filament is made of tungsten which on passage of current gives a lot of thermal energy by resistive heating. This heats the Cathode. Cathode is made of Strontium-BariumCuO with a work-function W F = 1eV. The energy band diagram of Cathode is given in Figure 2.2.
OpenStax-CNX module: m47114 3 Figure 2 As seen in Figure 2.2., Cathode is heated to 1000K.Conducting electrons in the Cathode are energetic enough to cross the Surface Barrier Potential (φs) and constitute the thermoionically emitted electrons which form a space charge cloud around the cathode as shown in Figure 2.4. As emitted electrons leave the surface of the cathode, a positive charge builds up on the cathode surface because of unbalanced charge within the cathode. This positive charge holds on to emitted electrons causing space charge cloud. From this space charge cloud electrons are pulled to the anode held at positive potential of 100V. This motion of electrons from the cathode to anode constitute the Anode Current and which was interpretated as Cathode Ray until Thomson revealed the mystery of thermoionic emission of electrons from the cathode. Thomson studied this beam of electron meticulously and established the charge `q' and q/m ratio of electron where m is the mass of electron. The set up is shown in Figure 2.3.
OpenStax-CNX module: m47114 4 Figure 3 The verication of the negative charge of electron led J.J.Thomson to hypothesize the Plum-Pudding Model. He suggested that negatively charged electrons are swimming in a sea of positive charge. It was only later on that by Gold-foil experiment Rutherford decisively established the planetary model of atom. He found that 99% of the mass of the atom is concentrated in the nucleus and it was positively charged. Electrons are orbiting this nucleus and their opposite charges are equal in magnitude to make an atom neutral in charge. The discovery of electrons formally launched the Branch of Engineering we know as Electronics and Communication Engineering. This was also the start of Vacuum Tube Era.
OpenStax-CNX module: m47114 5 2 2.1.1. Vacuum Tube Era Vacuum Diode - After the discovery of electron in 1897, Vacuum Diode was invented by John Ambrose Fleming in 1903. Vacuum Diode is identical to the CRT shown in Figure 2.1 except that it does not have parallel set of deecting plates and is shown in Figure2.4. I-V characteristics of the Vacuum Diode is shown in Figure 2.5. Figure 4
OpenStax-CNX module: m47114 6 Figure 5 The Physics of current ow in Vacuum Diode is as follows: i. By Thermo-ionic emission, a space charge cloud is formed surrounding the cathode. ii. As Anode is made positive, electrons are drawn towards Anode. As long as number of electrons drawn by Anode is less than electrons emitted from the Cathode, we are in Space-Charge Limited Region and Anode Current (I A ) increases linearly with Anode Voltage(V AK ) according to 3/2 Power Law as given in Equation 2.1. iii. As soon as electrons drawn by Anode are equal to electrons emitted by Cathode, we enter Temperature- Limited Region. In Temperature Limited Region, Anode Current saturates for a constant temperature. If temperature of Cathode is increased, the saturation value of Anode Current will increase given by Equation 2.2.The Work Function of Cathode is vital in the Temperature Limited Region. At 2500 ºC, Tungsten (W F = 4.52eV) cathode gives thermo-ionic current density of 3000A/m 2 and SrBaCuO (W F =1eV) cathode gives 7.34 10 9 A/m 2. There is almost 6 orders of magnitude improvement in thermo-ionic emission. Hence with SrBaCuO we can operate at 800ºC comfortably with signicant thermoionic emission. iv. At zero Anode Voltage current in the circuit should be zero but it is not. Because of statistical uctuation in the velocity, some electrons are energetic enough to reach the Anode at 0 V even. This is known as splash current. v. Under Reverse Bias, there is no reverse diode current. But at very high voltages by quantum mechanical tunneling electrons can be pulled out from the anode to cathode and a phenomena similar to break down occurs in Vacuum Diode also. This is called high-eld emission.
OpenStax-CNX module: m47114 7 Figure 6 Figure 7 Vacuum Triode - In 1905, Lee de'forest invented Vacuum Triode. It had three electrodes: Cathode, Grid and Anode. Grid was the third electrode inserted between Cathode and Anode. This controlled the Anode Current. For zero Grid Voltage, anode current ew at zero Anode Voltage onward just as in Diode.. As Grid Voltage became more negative, anode current onset was delayed and diode I-V characteristic was displaced as shown in Figure 2.6(b). In eect Triode behaved as Voltage-Controlled-Voltage Source(VCVS). Lee de'forest used this three terminal device to assemble RC-coupled Audio Amplier the enabling technology for long-distance telephone call. Telephone had been invented in 1876 by Alexandre Grahm Bell but it could not become Public Utility Service until 1913.In Figure 2.6(a) a RC-coupled Audio Amplier is shown.
OpenStax-CNX module: m47114 8 Figure 8 In quick succession Tetrode and Pentode were invented. Tetrode and Pentode- Tetrode had four electrodes. The fourth electrode was a screen electrode held at about +70V. The Tetrode and its output characteristics are shown in Figure 2.7. As seen in Figure 2.7, we get parallel set of horizontal lines akin to Voltage Controlled Current Source but with a wiggle. This wiggle is due to secondary emission of electrons from the Anode. As the highly accelerated electrons, due to the positive screen voltage, strike the anode it causes local thermal heating which leads to secondary thermo-ionic emission. This causes the wiggle which leads to non-linear distortion and reduction in gain. To suppress this secondary emission, a fth electrode at negative voltage with respect to anode is introduced. This repels back the secondary emission and removes the wiggle. This fth electrode is known as suppressor grid connected to the cathode. This tube with ve electrodes is known as Pentode and shown in Figure 2.8. This device approximates an ideal Voltage-Controlled Current Source. These topics have been discussed in the introductory chapters of Lectures on Analog Electronics (Collection 11167 uploaded on cnx.org). The I-V characteristics of Triode and Pentode are shown in the module, m29634, and these are equivalent to controlled sources namely Voltage-Controlled-Voltage-Source and Voltage Controlled-Current source respectively. These controlled sources are discussed in the module, m33375, in the same collection of Lectures on Analog Electronics.
OpenStax-CNX module: m47114 9 Figure 9
OpenStax-CNX module: m47114 10 Figure 10 2.1.1.1. RC-coupled Amplier and Heaviside Distortion-less Transmission enables the longdistance Telephone Call across N.American Continent. In 1911, the rst long distance call was successfully made from New York to Denver, Colorado. No Ampliers had been used in this 2000miles long distance telephone call. Heaviside Distortion-less Transmission condition had been satised to enable this call. Oliver Heaviside was deeply involved with telegraphic transmission and its theorization. He treated a long transmission line as distributed parameter network as shown in Figure 2.9.
OpenStax-CNX module: m47114 11 Figure 11 By circuit analysis he showed that distortion-less transmission requires that: Figure 12 But in real life transmission telephonic cables, this balance is not achieved. The actual imbalance is:
OpenStax-CNX module: m47114 12 Figure 13 This imbalance results in serious propagation-delay distortion or dispersion of the signal. This result is phase-distortion and the voice gets jumbled up in long distance telephonic call. To correct this, Oliver Heaviside suggested Inductive-Coil loading of the transmission line and thereby achieve distortion-less transmission condition. In this remedy, single high-induction coils are connected to the transmission line in series at regular intervals. This enabled the clear reception of a call coming across 2000 miles without any kind of amplication. In 1913, periodic inductive loading as well as RC-Coupled Amplier Repeater stations were used at intervals of 100Km distance. This enabled a clearly audible long-distance telephonic call from New York to San-Francisco, California, over a distance of 4000 miles. This trans-continental telephonic call set the stage for turning Telephone into Public Utility Service. This laid the ground work for the Information Revolution. Because of the poor quality of telegraphic cables laid down under Atlantic and Pacic Ocean, trans- Atlantic or trans-pacic Telephone calls still could not be made. These trans-oceanic Calls were put through Radio-Telephony. Only in 1956 when rst Voice-quality trans-atlantic Coaxial Cables,TAT-1, were laid down that Trans-Atlantic Telephone Calls became a reality. In 1945, ENIAC (Electrical Numerical Integrator and Calculator) using 18,000 Thermo-ionic Vacuum Tubes was built up. It consumed 200kW power, it weighed 27 Metric Tonnes and it occupied a large size Hall. At the time computers were hardly user friendly.