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COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 Junction Junction Break Break Down Down Lecture No: 8  Breakdown Characteristics * Zener Breakdown * Avalanche Breakdown Kwangwoon University Semiconductor Devices. device lab. 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     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 4 PN Junction Under Forward-Bias Condition:  The pn junction excited by a constantcurrent 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. 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. Nasim Zafar 5 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 small negative current level for reverse bias voltages. “Ideal diode” equation:   I D I S eVD /VT  1  Dn Dp   I S AJS Aqn   N L N L  D p   A n 2 i PN Junction Under Reverse-Bias Condition: I-V characteristic equation: i  I s Independent of voltage Where Is is the saturation current, it is proportional to ni2 which is a strong function of temperature. D p pn0 Dn n p 0 I s  qA (  ) Lp Ln Dp Dn  qAn i (  ) L p n D Ln n A 2 Nasim Zafar 6 Breakdown Voltage VBD One can determine which mechanism is responsible for the breakdown based on the value of the breakdown voltage VBD :  VBD < 5 V  Tunneling Breakdown  VBD > 6V  Avalanche Breakdown  4V < VBD < 6V  both tunneling and avalanche mechanisms are responsible Energy Band Diagram of a PN Junction W EC qV E Fp EV E Fn Origin of Current Flow Reverse bias: Forward bias: W EC Ln qVbi  V  qV E Fp EC E Fn qV qVbi  V  E Fp EV E Fn EV Lp W Reverse saturation current is due to minority carriers being collected over a distance of the order of the diffusion length. Reverse Saturation Current The flow of these minorities produces the reverse saturation current but it is independent of applied reverse voltage. I(current) Forward Bias Vb I0 V(voltage) VB ; Breakdown voltage I0 ; Reverse saturation current Reverse Bias Drift current Ideal Diode I-V characteristic Real Diode I-V characteristic Real Diode – Reverse Current What’s wrong with this picture? Reverse Bias: – Current ~103 times larger than FB I0 – Reverse current doesn’t saturate – Breakdown – large current above VBbd Avalanche Breakdown Avalanche Breakdown: • Avalanche breakdown mechanism occurs when electrons and holes moving through the depletion region of a reverse biased PN junction, acquire sufficient energy from the electric field to break a bonds i.e. create electron-hole pairs by colliding with atomic electrons within the depletion region. The electric field in the depletion region of a diode can be very high. • The newly created electrons and holes move in opposite directions due to the electric field present within the depletion region and thereby add to the existing reverse bias current. This is the most important breakdown mechanism in PN junction. Avalanche Breakdown Impact Ionization Mechanism Mechanism In(w) = M * Ino Total current during avalanche multiplication Energy Band diagram; Avalanche Breakdown: Depletion width larger than mean free path lots of collisions Junction Built-In Voltage: The Junction Built-In Voltage is given as: N AN D V o  V T ln 2 ni  It depends on doping concentration and temperature Its TC is negative. 22 Junction Parameters: V bi  V A  V bi  V BR  V BR E 2 c  2q K S0  V BR  V BR  1 NB N N N A  N N AN D A ND  N A D  D One-sided junctions V BR  Current Density of an Avalanche Process: Jn J p   n J n dx dx J n   n J n dx Jp Impact ionization initiated by electrons. Jn J p   p J p dx dx J n   p J p dx Jp Impact ionization initiated by holes. dJ p dJ n  0, 0 dx dx dJ p dJ n dx dx  J  J n  J p const . Multiplication factors for electrons and holes: J p ( 0) J n (W ) Mn  , Mp  J n ( 0) J p (W ) Zener Breakdown Zener Break Down: • Zener breakdown occurs in heavily doped p-n junctions, with a tunneling mechanism. • The heavy doping makes the depletion layer extremely thin. So thin in fact, carriers cannot accelerate enough to cause impact ionization. • With the depletion layer so thin, however, quantum mechanical tunneling through the layer occurs causing the reverse current to flow. • In a heavily doped p-n junction the conduction and valance bands on opposite side of the junction become so close during the reverse-bias that the electrons on the p-side can tunnel from directly VB into the CB on the n-side. • The temperature coefficient of the Zener mechanism is negative, the breakdown voltage for a particular diode decreases with increasing temperature. Zener Breakdown Mechanism: Highly Doped Junction ( narrow W) Mechanism is termed Tunneling or Zener Breakdown Zener effect Ec n P Ef Ev h+ Doping level > 1018/Cm3 x eEc Ef Ev Semiconductor Devices Tunneling Breakdown: • Tunneling breakdown occurs in heavily-doped p-n junctions in which the depletion region width W is about 10 nm. Zero-bias band diagram: Forward-bias band diagram: EF EC W EV EFn EFp EC EV W Visualization of Tunneling: Barrier must be thin: depletion is narrow doping on both sides must be large  2K   N  N   W  S 0 A D Vbi  Vappl NDNA  q  Must have empty states to tunnel into  Vbi + VBR > EG/q 1 2 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 controlling the doping concentration by 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