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.
COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 Junction Break Down Lecture No: 8 v Breakdown Characteristics * Zener Breakdown * Avalanche Breakdown Kwangwoon University Semiconductor device lab Semiconductor Devices 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 Breakdown Mechanism: • • Zener Effect Ø Occurs in heavily doping semiconductor Ø Breakdown voltage is less than 5V Ø Carriers generated by electric fieldfield ionization Ø TC is negative Avalanche Effect Ø Occurs in slightly doping semiconductor Ø Breakdown voltage is more than 7V Ø PN Junction Under ForwardBias Condition: Ø Ø 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 PN Junction Under ReverseBias Condition: Ø Ø Ø Nasim Zafar The pn junction excited by a constantcurrent 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 IV 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 IV Characteristic of a PN Junction: Current increases exponentially with applied forward bias voltage, and “saturates” at a relatively small negative current level for reverse bias voltages “Ideal diode” equation: ID IS I S eVD / VT AJ S Dn Aqn N A Ln i Dp N D Lp PN Junction Under ReverseBias Condition: IV characteristic equation: i Is Independent of voltage Where Is is the saturation current, it is proportional to ni2 which is a strong function of temperature D p pn Dn n p I s qA( ) Lp Ln Dp qAni ( L p nD Nasim Zafar Dn ) Ln n A Breakdown Voltage VBD One can determine which mechanism is responsible for the breakdown based on the value of the breakdown voltage VBD : v VBD 6V Avalanche Breakdown v 4V EG/q Zener Diode Characteristics IF VR VS IR R VZ VF IZK= Zener knee current IZT= Zener test current IZM= Maximum Zener current IR 30 Zener Diode Characteristics: •The breakdown characteristics of diodes can be tailored by controlling the doping concentration Heavily doped p+ and n+ regions result in low breakdown voltage (Zener effect) Used as reference voltage in voltage regulators I Region of operation V 31 Example: Zener diode A 1N754A Zener diode has a dc power dissipation rating of 500 mW and a nominal Zener voltage of 6.8 V. What is the value of IZM for the device? I ZM = PD (max) VZ 500mW = = 73.5mA 6.8V 32 Summary ...Junction Break Down Lecture No: 8 v Breakdown Characteristics * Zener Breakdown * Avalanche Breakdown Kwangwoon... an external voltage in the forward direction PN Junction Under ReverseBias Condition: Ø Ø Ø Nasim Zafar The pn junction excited by a constantcurrent source I in the reverse direction. ... Where Is is the saturation current, it is proportional to ni2 which is a strong function of temperature D p pn Dn n p I s qA( ) Lp Ln Dp qAni ( L p nD Nasim Zafar Dn ) Ln n A Breakdown Voltage VBD One can determine which mechanism is responsible for the