Lecture 31

Lecture 31 - EEE 352: Lecture 31 Applications of p-n...

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EEE 352: Lecture 31 Applications of p-n Junctions and Bipolar Transistors * p-n junctions in reverse bias Reverse breakdown Zener and avalanche effects Capacitors * Bipolar transistors
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In REVERSE bias we have argued that current flow in a p-n junction is NEGLIGIBLE Beyond a critical voltage however REVERSE BREAKDOWN occurs, and a LARGE current flows through the reverse biased diode TWO mechanisms are responsible for the breakdown The ZENER and AVALANCHE effects p-n Junctions in Reverse Bias V a (V) I (mA) FORWARD BIAS REVERSE BIAS x n -x p No bias With reverse bias If E max exceeds E bd , then we get avalanche breakdown
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x n -x p No bias With reverse bias Under applied bias, the width changes, and the peak electric field changes. This accounts for the changed voltage drop V bi ± V a . With forward bias, the width is reduced and the peak electric field is reduced. With reverse bias, however, the width is increased and the peak electric field is increased. The crucial breakdown field (for impact ionization) is around 200-300 kV/cm in Si. Here, an incident electron (or hole) can knock a bonding electron out of the bond, creating an additional electron- hole pair. This requires the incident electron (or hole) to have an energy >E G .
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Avalanche breakdown occurs in LIGHTLY -doped diodes HIGH energy charge carriers IONIZE host atoms This creates NEW electron-hole pairs that may carry current p-n Junctions in Reverse Bias n p e - e + 2 e - AN IONIZING COLLISION PRODUCES AN ELECTRON AND A HOLE ELECTRON-HOLE PAIR CREATION IN THE AVALANCHE EFFECT THE FRESHLY CREATED ELECTRON- HOLE PAIRS GO ON, IN TURN TO GENERATE FURTHER IONIZING COLLISIONS E bd ~ 300-500 kV/cm in Si.
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p-n Junctions in Reverse Bias 300 kV/cm http://www.nanohub.org
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p-n Junctions in Reverse Bias The maximum voltage that can be applied is limited by the internal electric field, which leads to impact ionization and avalanche breakdown. Once impact ionization begins, the additional particles can create more particles, thus leading to the avalanche breakdown.
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p-n Junctions in Reverse Bias A second process can lead to “breakdown,” and this is Zener tunneling. In reverse bias, the conduction band in the n -region can be lower than the valence band in the p - region. If the junction is sufficiently narrow (very high doping), then electrons can tunnel across the gap. This is Zener tunneling. E v E c p-TYPE REGION n-TYPE REGION e(V bi -V a ) Tunneling of electrons
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E v E c p-TYPE REGION n-TYPE REGION e(V bi -V a ) Tunneling of electrons E G W x 0 Tunneling barrier In Si, the square root is about 5.4 X 10 9 m -1 , so that T ~ 0.1 at a width of 0.3 nm.
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p-n Junctions in Reverse Bias Bands begin to overlap, and W is so small, that tunneling is possible.
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Reverse bias diodes have a number of PRACTICAL applications These derive from the ability to TUNE the breakdown voltage by controlled variation of the doping DENSITY in the diode ZENER diodes can be used as VOLTAGE REGULATORS
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Lecture 31 - EEE 352: Lecture 31 Applications of p-n...

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