ELECTRONIC CIRCUITS Solid State Fundamentals ■ Subtopics: 1) Semiconductor Diode, and 2) Diode Biasing and Models ■ At the end of the chapter, the learner should be able to: – Explain the parameters of a semiconductor diode – Analyze diode circuits using different equivalent models Semiconductor Diode Subtopic #1 Definition: A diode is made from a small piece of semiconductor material in which half is doped as a p region and half is doped as an n region with a pn junction and depletion region in between. • The p region is called the anode and is connected to a conductive terminal. • The n region is called the cathode and is connected to a second conductive terminal. Definition. Establishing predetermined voltages or currents at various points of an electronic circuit for the purpose of establishing proper operating conditions in electronic components. Definition. Fixed DC voltage or current applied to a terminal of an electronic component in order to establish proper operating conditions for the component. The condition that allows current through the pn junction. 1. The negative side of VBIAS is connected to the n region of the diode and the positive side is connected to the p region. 2. The bias voltage, VBIAS, must be greater than the barrier potential. The condition that essentially prevents current through the diode. Definition. The extremely small current that exists in reverse bias after the transition current dies out is caused by the minority carriers in the n and p regions that are produced by thermally generated electron-hole pairs. Also called leakage current or reverse saturation current. • Normally, the reverse current is so small that it can be neglected. • However, if the external reverse-bias voltage is increased to a value called the breakdown voltage (peak reverse voltage or PIV, peak inverse voltage), the reverse current will drastically increase. • The high reverse-bias voltage imparts energy to the free minority electrons. • They collide with atoms with enough energy to knock valence electrons out of orbit and into the conduction band. • The newly created conduction electrons are also high in energy and repeat the process. • If one electron knocks only two others out of their valence orbit during its travel through the p region, the numbers quickly multiply. • As these high-energy electrons go through the depletion region, they have enough energy to go through the n region as conduction electrons, rather than combining with holes. • The multiplication of conduction electrons just discussed is known as the avalanche effect, and reverse current can increase dramatically if steps are not taken to limit the current. • When the reverse current is not limited, the resulting heating will permanently damage the diode. Diode Biasing and Models Subtopic #2 • There is a slight increase in VF above 0.7 V as the current increases due mainly to the voltage drop across the dynamic resistance. • Point A corresponds to a zero-bias condition. • Point B corresponds to where the forward voltage is less than the barrier potential of 0.7 V. • Point C corresponds to where the forward voltage approximately equals the barrier potential. • Unlike a linear resistance, the resistance of the forward-biased diode is not constant over the entire curve. • Because the resistance changes as you move along the V-I curve, it is called dynamic or ac resistance. • The dynamic resistance of a diode is designated 𝑟′𝑑 . (Greatest below the knee) • With 0 V across the diode, there is no reverse current. • When a reverse-bias voltage is applied across a diode, there is only an extremely small reverse current (IR) through the pn junction. • As you gradually increase the reverse-bias voltage, there is a very small reverse current and the voltage across the diode increases. • When the applied bias voltage is increased to a value where the reverse voltage across the diode (VR) reaches the breakdown value (VBR), the reverse current begins to increase rapidly. • A typical rectifier diode has a breakdown voltage of greater than 50 V. • Some specialized diodes have a breakdown voltage that is only 5 V. 𝐼𝑆1 = 𝐼𝑆 𝑘(𝑇 −𝑇 ) 1 0 𝑒 where: 𝐼𝑆1 = saturation current at new temperature 𝐼𝑆 = saturation current at room temperature 𝑘 = 0.07 per degree Celsius 𝑇1 = new temperature 𝑇0 = room temperature 𝑉𝑇𝐻1 = 𝑉𝑇𝐻 + 𝑘(𝑇1 − 𝑇0 ) where: 𝑉𝑇𝐻1 = threshold voltage at new temperature 𝑉𝑇𝐻 = threshold voltage at room temperature 𝑘 = -2.5 mV/°C for Ge and -2 mV/°C for Si 𝑇1 = new temperature 𝑇0 = room temperature 1. The Ideal Diode Model 2. The Simplified (Practical) Diode Model 3. The Piecewise Linear (Complete) Diode Model  A. Malvino and D. Bates, Electronic Principles, New York: McGraw-Hill Education, 2016.  A. Agarwal and J. Lang, Foundations of Analog and Digital Electronic Circuits, San Francisco: Elsevier Inc, 2005.  M. E. Schultz, Grob's Basic Electronics, New York: McGraw-Hill Education, 2011.  S. Gibilisco, Teach Yourself Electricity and Electronics, New York: McGraw-Hill Companies, 2002.  T. L. Floyd, Electronic Devices (Conventional Current Version), New Jersey: Prentice Hall, 2012.  R. Boylestad and L. Nashelsky, Electronic Devices and Circuit Theory, New Jersey: Pearson Education, Inc., 2013 QUESTIONS?