Chapter 5 Lecture - Part 3

The fp references ev fewer holes larger difference fn

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Unformatted text preview: ls enable us to keep track of The the majority and minority carrier distributions near the junctions. the – Fp references Ev : (fewer holes larger difference) – Fn references En : (more electrons smaller difference) Breakdown Breakdown • Reverse bias breakdown – At some critical voltage (Vbr) current will increase current sharply sharply – This can be problematic to the diode, leading to This catastrophic failure catastrophic • • • Excess current leads to overheating Recall: Power = I2R = VI As I increases dramatically, the diode is forced to As dissipate this power in the form of heat dissipate • The lattice can be irreparably damaged in the process • Current-limiting resistors are often put in Current-limiting series with diodes to prevent this series Reverse breakdown in a p-n junction. Zener Breakdown Zener • Situation: – – Pertains to heavily-doped junctions The heavy doping results in a significant offset in The the band structure at equilibrium the – Small reverse biases exacerbate this offset • Implications – Band bending leads to an alignment of filled states Band with respect to energy level with • Filled states in Ev line up with empty states in Ec • From the energy perspective, all things being equal, an From electron would prefer the conduction band Zener Breakdown (a) heavily doped junction at equilibrium; (b) reverse bias with electron tunneling from p to n; (c) I–V characteristic. Zener Breakdown Zener • Tunneling – If the band bending is significant, the potential If barrier (the energy gap) between these energy equivalent states is small enough to support a tunneling current tunneling – Electrons from Ev tunnel to Ec – Since the supply of electrons in EV is very large, this tunneling current can be very high (breakdown) (breakdown) • Requirements – Very sharp junctions – Very high doping results in small W Very Avalanche Breakdown Avalanche • Situation – – Occurs in lightly doped materials Tunneling not a factor • Breakdown Mechanism: Impact Ionization – Electrons from the p-type region entering W have Electrons sufficiently hi...
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