Reverse-biased diode:
Drift : small, voltage-independent current (I0)
At a critical reverse bias voltage, a diode can become
a "perfect" conductor in the reverse direction:
"Reverse breakdown"
Vbr : breakdown voltage
Can occur through two mechanisms :
Zener effect (lower voltages)
or
Avalanche effect (higher voltages)
Zener effect
High doping levels on both sides of the junction, sharp transition
⇒ Fermi level very close to Ec /Ev on n / p sides
⇒ Small energy step between valence band on p-side.
High doping also creates a narrow transition region.
With reverse bias, p side valence band has electron potential
equal to or higher than n-side conducting band:
Electrons tunnel through the barrier
~ few volts reverse bias
Avalanche Effect
With high fields in the transition region, "drift-carriers"
gain high kinetic energies and collide with the lattice:
⇒ Collisions ionize atoms, creating new EHPs.
Let nin electrons enter the transition region:
Avalanche ⇒ nout = M · nin (multiplication)
1
M=
1"
( )
V
Vbr
x
x between 3 and 6,
(depending on the material)
Large band gap ⇒ Vbr larger
High doping ⇒ Vbr smaller
!