Reverse biased
• In this case positive terminal of the
voltage source is connected to the N
side and negative terminal to the P-
side.
• Practically no current flows through
the junction under this condition.
Reverse biased PN junction
• When PN junction is reverse biased, the holes in the P –region are attracted
towards the negative terminal of the voltage source. Thus holes leave more ions
near the junction.
• Similarly the electrons in the N region are attracted to the positive terminal of the
voltage source. Thus electrons leave ions near the junction.
• As a result the width of the depletion region increases.
• Thus there is no current due to majority carriers.
• Due to thermal energy limited no of free electrons and holes are generated in the p-
side and n-side of the PN junction respt.
Contd……..
• Electrons in the p-side move towards
right and cross the junction because
of the barrier potential.
• Similarly holes in the n side are also
drifted across the junction.
• Hence a small amount of current
flow through the reverse biased PN
junction.
• The current due to flow of minority
carriers remains same regardless of
the applied voltage increases or
decreases. The current is known as
reverse saturation current.
Reverse saturation current
• Reverse saturation current(𝐼𝑐𝑜 ) is due to thermally generated minority
carriers.
• It increases with increase in temperature.Normally, 𝐼𝑐𝑜 doubles for every
100 rise in temperature.
• The reverse saturation current (𝐼𝑐𝑜 ) is of the order of nanoamperes(nA) for
Si and microamperes (µA) for Ge PN junction.
Reverse Breakdown
The following two processes cause junction breakdown due to increase in
reverse voltage.
1. Zener breakdown: In this case, the breakdown occurs in junction, which
are heavily dopped. The heavily doped junctions have narrow depletion
layer. When the reverse voltage is increased, the electric field at the junction
also increases. A strong electric field causes a covalent bond to break. As a
result large number of minority carriers are generated and a large current
flows through the junction.
