tailieunhanh - Lecture Electrical Engineering: Lecture 8 - Dr. Nasim Zafar

Under normal operation of a diode, an applied reverse bias (voltage) will result in a small current flow through the device. However, at a particular high voltage, which is called breakdown voltage VBD, large currents start to flow. If there is no current limiting resistor, which is connected in series to the diode, the diode will be destroyed. There are two physical effects which cause this breakdown. In this chapter, you will learn about: in the bulk of the diode outside the depletion region, the semiconductor is neutral. | Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad Junction Break Down Lecture No: 8 Breakdown Characteristics * Zener Breakdown * Avalanche Breakdown Introduction: Under normal operation of a diode, an applied reverse bias (voltage) will result in a small current flow through the device. However, at a particular high voltage, which is called breakdown voltage VBD, large currents start to flow. If there is no current limiting resistor, which is connected in series to the diode, the diode will be destroyed. There are two physical effects which cause this breakdown. 4 Breakdown Mechanism: Zener Effect Occurs in heavily doping semiconductor Breakdown voltage is less than 5V Carriers generated by electric field---field ionization TC is negative Avalanche Effect Occurs in slightly doping semiconductor Breakdown voltage is more than 7V Carriers generated by collision TC is positive 5 PN Junction Under Forward-Bias Condition: The pn junction excited by a constant-current source supplying a current I in the forward direction. The depletion layer narrows and the barrier voltage decreases by V volts, which appears as an external voltage in the forward direction. Nasim Zafar 5 PN Junction Under Reverse-Bias Condition: The pn junction excited by a constant-current source I in the reverse direction. To avoid breakdown, I is kept smaller than IS. Note that the depletion layer widens and the barrier voltage increases by VR volts, which appears between the terminals as a reverse voltage. I-V Characteristic of a PN Junction: As the reverse bias voltage increases, the electric field in the depletion region increases. Eventually, it can become large enough to cause the junction to break down so that a large reverse current flows: breakdown voltage I-V Characteristic of a PN Junction: Current increases exponentially with applied forward bias voltage, and “saturates” at a relatively | Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad Junction Break Down Lecture No: 8 Breakdown Characteristics * Zener Breakdown * Avalanche Breakdown Introduction: Under normal operation of a diode, an applied reverse bias (voltage) will result in a small current flow through the device. However, at a particular high voltage, which is called breakdown voltage VBD, large currents start to flow. If there is no current limiting resistor, which is connected in series to the diode, the diode will be destroyed. There are two physical effects which cause this breakdown. 4 Breakdown Mechanism: Zener Effect Occurs in heavily doping semiconductor Breakdown voltage is less than 5V Carriers generated by electric field---field ionization TC is negative Avalanche Effect Occurs in slightly doping semiconductor Breakdown voltage is more than 7V Carriers generated by collision TC is positive 5 PN