PN Junction

By
Prof. Neeta Sahay
Institute of Engineering & Management
Salt Lake, Kolkata

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 pn Junction Review
• PN junctions are fabricated from a monocrystalline piece of semiconductor
with both a P-type and N-type region in proximity at a junction.
• The transfer of electrons from the N side of the junction to holes
annihilated on the P side of the junction produces a barrier voltage. This is
0.6 to 0.7 V in silicon, and varies with other semiconductors.
• A forward biased PN junction conducts a current once the barrier voltage is
overcome. The external applied potential forces majority carriers toward
the junction where recombinetion takes place, allowing current flow.
• A reverse biased PN junction conducts almost no current. The applied
reverse bias attracts majority carriers away from the junction. This
increases the thickness of the nonconducting depletion region.
• Reverse biased PN junctions show a temperature dependent reverse leakage
current. This is less than a µA in small silicon diodes.

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N-type

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 Band Diagram: Acceptor Dopant in Semiconductor

• For Si, add a group III element to “accept” an electron and

make p-type Si (more positive “holes”).
• “Missing” electron results in an extra “hole”, with an
acceptor energy level EA just above the valence band EV.
– Holes easily formed in valence band, greatly
increasing the electrical conductivity. EC
• Fermi level EF moves down towards EV.

EF
EA
EV
p-type Si

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P-type

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Conduction in p/n-type Semiconductors

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 Drift
Drift → Movement of charged particles in response to an external field (typically an
electric field)

E-field applies force

F = qE
which accelerates the charged particle.
However, the particle does not accelerate Current Density
indefinitely because of collisions with the lattice
(velocity saturation)
J n ,drift   n qnE
Average velocity
<vx> ≈ -µnEx electrons J p ,drift   p qpE
< vx > ≈ µpEx holes
µn → electron mobility q = 1.6×10-19 C, carrier density
→ empirical proportionality constant n = number of e-
between E and velocity p = number of h+
µp → hole mobility
µn ≈ 3µp µ↓ as T↑
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 Diffusion
Diffusion → Motion of charged particles due to a concentration gradient
• Charged particles move in random directions
• Charged particles tend to move from areas of high concentration to areas of low
concentration (entropy – Second Law of Thermodynamics)
• Net effect is a current flow (carriers moving from areas of high concentration to areas
of low concentration)

dn x 
J n ,diff  qDn q = 1.6×10-19 C, carrier density
dx D = Diffusion coefficient
dp x  n(x) = e- density at position x
J p ,diff   qDp p(x) = h+ density at position x
dx

→ The negative sign in Jp,diff is due to moving in the opposite direction

from the concentration gradient
→ The positive sign from Jn,diff is because the negative from the e-
cancels out the negative from the concentration gradient

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Total Current Densities
Summation of both drift and diffusion

J n  J n ,drift  J n ,diff
dn x 
  n qnE  qDn (1 Dimension)
dx
  n qnE  qDn n (3 Dimensions)

J p  J p ,drift  J p ,diff
dp x 
  p qpE  qDp (1 Dimension)
dx
  p qpE  qDp p (3 Dimensions)

Total current flow

J  Jn  J p
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 PN Junction: Band Diagram

• Due to diffusion, electrons move from n

to p-side and holes from p to n-side.
• Causes depletion zone at junction where n-type electrons
immobile charged ion cores remain. EC
• Results in a built-in electric field (103 to EF
105 V/cm), which opposes further
diffusion. EF
• Note: EF levels are aligned across pn
junction under equilibrium.
EV
holes p-type

pn regions in equilibrium

–––
EC +–– –
+ + –
+
EF + ++–––
+ ++––
++
EV
Depletion Zone 11
 PN Junction: Band Diagram under Bias

• Forward Bias: negative voltage on n-side promotes diffusion

of electrons by decreasing built-in junction potential  higher
current.
• Reverse Bias: positive voltage on n-side inhibits diffusion of
electrons by increasing built-in junction potential  lower
current.
Equilibrium Forward Bias Reverse Bias

p-type n-type p-type n-type p-type n-type

–V +V

e–
e– e–
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Majority Carriers Minority Carriers
Forward & Reverse Biased

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 PN Junction: IV Characteristics
• Forward and reverse currents
– pn junction current is
given approximately by
 eV  Forward
I  Is  exp  1 Bias
 ηkT 
– where I is the current,
– e is the electronic charge,
– V is the applied voltage, Reverse
– k is Boltzmann’s constant, Bias
– T is the absolute temperature
–  (Greek letter eta) is a
constant in the range 1 to 2 determined by the junction
material, for most purposes we can assume  = 1
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• Silicon diodes
– generally have a turn-on voltage of about 0.5 V
– generally have a conduction voltage of about 0.7 V
– have a breakdown voltage that depends on their
construction
• perhaps 75 V for a small-signal diode
• perhaps 400 V for a power device
– have a maximum current that depends on their
construction
• perhaps 100 mA for a small-signal diode
• perhaps many amps for a power device

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• Turn-on and breakdown voltages for a silicon device

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