Breakdown and resistance of diode | Electronics | Semiconductor diode

Breakdown and resistance of diode

Breakdown of a diode 

• On applying reverse bias to the diode, two types of breakdown occurs, which are responsible to the sudden increase of current in the reverse bias condition.

Avalanche breakdown

• In this mechanism, the minority charge carriers gain enough kinetic energy from the applied reverse voltage to collide with the valence electrons of the atom and liberate them.
• In this process the covalent bond breaks and create a large number of electron-hole pairs.
• The new charge carriers so produced generate the additional electron-hole pairs.
• In this way the number of free electrons and holes increases continuously.
• This phenomenon is known as avalanche multiplication and it produces a sharp increase in the reverse current.
• The magnitude of avalanche breakdown voltage can be increased by increasing the temperature.

Zener breakdown

• If the junctions are heavily doped and have a very narrow depletion region, then the breakdown occurs is known as Zener breakdown.
• Since in this case there exist a high electric field (E = V/d), across the junction.
• This electric field is so strong to break the covalent bond and generate the electron-hole pairs.
• On further increasing the Zener voltage, the large number of carriers generate, which increase the reverse current suddenly.
• This breakdown is a field emission phenomena.
• On increasing the temperature the Zener breakdown voltage can be decreased.

Resistance of a diode 

 

Static or dc resistance 

• When only dc current flows through the diode, then the resistance offered by it is static or dc resistance.
• It is the ratio of the dc voltage across the diode to the dc current flowing through it.
• R_dc = V / I
• In forward bias the static resistance of diode is R_F and in reverse bias R_R.
• R_R >> R_F
• For Ge, R_R : R_F = 4000 : 1.

Dynamic or ac resistance

• When only ac current or a small ac current along with dc current flows through the diode, then the resistance offered by it is dynamic or ac resistance.
• It is reciprocal of the slope of the current-voltage characteristic of the diode.
• R_ac = ΔV / ΔI
• When the diode is sufficiently forward biased, then at any temperature T

• Here k = Boltzmann constant, and e = charge of electron.
• For Ge, η = 1, and at T = 300 K
• R_ac = 26 / I

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