Electrical Sciences EEE F111
Electrical Sciences EEE F111
Electrical Sciences EEE F111
EEE F111
EEE F111 2
Part 1. Semiconductors and Physical
operation of diodes
Linear and Nonlinear Devices
pn junction
Symbol and characteristic for the ideal
Symbol and characteristic for the ideal diode
diode
i
Anode Cathode
i ---Reverse bias--- ---Forward bias---
+ v -
0 v
(a) diode circuit symbol (b) i–v characteristic
i i
+ v - + v -
v < 0 i=0 i > 0 v =0
Intrinsic Semiconductor
Doped Semiconductor
Carriers
Diffusion, Drift
Elements and material
Periodic table
Mobility and Conductivity
EEE F111 10
Intrinsic semiconductor
Pure semiconductor
At 0 K, all bonds are intact and no free electrons
are available for current conduction
Ec
Eg = Ec - Ev @ T 0( K )
Ev
The energy bands and the states of electrons
in Si/Ge at T= 0 K
Thermal ionization
Recombination
A free electron may fill into a hole, resulting in the
disappearance of a pair of carriers ( a free electron and a
hole).
Thermal equilibrium
At a steady temperature, the recombination rate is equal to
the ionization rate thermal equilibrium
The concentration of the carriers at thermal equilibrium
does not change and can be calculated.
Carrier concentration
n p ni
where
3 EG kT (k: Boltzmann constant)
ni BT e
2
n is electron concentration
p is hole concentration
ni intrinsic carrier concentration
Important notes
Si Si Si Si Si Si Si
Free E
Si Si Si Si Si Si Si
P Donor
Si Si Si +
Si Si Si Si
bound charge
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
n type semiconductor
nn0 pn0 N D
nn 0 ni pn 0
nn 0 pn 0 ni
and
nn 0 N D
pn 0 ni / N D
2
Doped semiconductor——p type
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
Al Hole Acceptor
Si Si Si -
Si Si Si Si
bound charge
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
p type semiconductor
p p0 np0 N A
where NA is the acceptor concentration
Carrier concentration for p type
p p 0 N A
n p 0 ni / N A
2
Conclusion on the doped semiconductor
NA ND
Doping compensation
Drift
Drift velocities
where p , n are the
vdrift p E
constants called mobility of
vdrift n E
holes and electrons, respectively
J drift q(nn+p p ) E
Resistivity
1 q(n +p )
n p
Resistivity for intrinsic semiconductor
diffusion
dp( x)
J p qDp
dx
dn( x)
J n qDn
dx
where D p , Dn are the diffusion constants or diffusivities
for hole and electron, respectively
The diffusion current density is proportional to the slope
of the the concentration curve, or the concentration
gradient.
Einstein relationship
dp ( x )
J p qp p E qD p
dx
dn( x )
J n qn n E qDn
dx
EEE F111 39
pn Junction
Diffusion
Space charge
Drift
Equilibrium
Procedure of forming pn junction
Diffusion
Both the majority carriers diffuse across the boundary
between p-type and n-type semiconductor.
The direction of diffusion current is from p side to n side.
Procedure of forming pn junction
Drift
Electric field is established across the space charge
region.
Direction of electronic field is from n side to p side.
It helps minority carriers drift through the junction.
The direction of drift current is from n side to p side.
Procedure of forming pn junction
Equilibrium
Two opposite currents across the junction is equal in
magnitude.
No net current flows across the pn junction.
Equilibrium condition is maintained by the barrier
voltage.
Junction built-in voltage
The pn junction
excited by a constant-
current source
supplying a current I in
the forward direction.
The depletion layer
narrows and the barrier
voltage decreases by V
volts, which appears as
an external voltage in
the forward direction.
Carrier distribution under forward-bias
v
VT
i I s (e 1)
Nonlinear (exponential relationship)
Is (saturation current) strongly depends on temperature
η=1 or 2, in general η =1
Turn-on voltage
VD (on ) 0.5V
For silicon
VD (on ) 0.2V For germanium
UR
p-type n-type
area area
pp0 nn0
np0 pn0 x
I-V characteristic equation
Zener effect
Occurs in heavily doping semiconductor
Breakdown voltage is less than 5v.
Carriers generated by electric field---field
ionization.
TC is negative.
Avalanche effect
Occurs in slightly doping semiconductor
Breakdown voltage is more than 7v.
Carriers generated by collision.
TC is positive.
Breakdown mechanisms
Remember:
pn junction breakdown is not a destructive process,
provided that the maximum specified power dissipation is
not exceeded.
Zener Diode
Circuit symbol