65
Review of Basic Device Physics
minority carriers control the current flow in a pn-junction, the current flow in
pn-junctions can be considered as the diffusion current only. Thus, we see that
the minority carriers really control the behavior of pn-junctions.
2.3.6 pn-Junctions I–V Characteristics
We discussed in Section 2.3.2 that the drift component of the current
caused by the electric field in the depletion region is exactly balanced
out by the diffusion component of the current caused by the electron and
hole concentration gradient across the junction, resulting in zero current
flow in the pn-junction device. When an external voltage is applied, this
current component balance is upset, and current will flow in the diode.
If carriers are generated by light or some other external means, thermal
equilibrium is disturbed, and current can also flow in a pn-junction. Here,
the current flow in a pn-junction as a result of an external applied voltage
is described.
Let us consider a forward-biased pn-junction. Electrons are injected from
the n-side into the p-side, and holes are injected from the p-side to n-side.
If the generation and recombination in the depletion region are negligible,
then the hole current leaving p-side is the same as the hole current entering
the n-side. Similarly, the electron current leaving the n-side is equal to the
electron current entering the p-side. To determine the total current flowing
in the pn-junction, we need to determine either hole current entering the
p-side or electron current entering to n-side of the pn-junction.
The starting point for describing I–V characteristics of a pn-junction is the
continuity equations. From Equation 2.81, the electron continuity equation
is given by
−
∂
∂
= −
∂
∂
+
−
(
)
n
t
q
J
x
G R
n
n
n
1
(2.115)
where:
R n and G n are the electron recombination and generation rates, respectively
Equation 2.115 can be rewritten as
∂
∂
=
∂
∂
−
−
n
t
q
J
x
n n
n
o
n
1
τ
(2.116)
where τ n is the electron lifetime defined in terms of the excess electron concentration n over the thermal equilibrium value n o in Equations 2.48 and 2.53
and is given by
τ n
o
n
n
n n
R G
≡
−
−
(2.117)
Review of Basic Device Physics
minority carriers control the current flow in a pn-junction, the current flow in
pn-junctions can be considered as the diffusion current only. Thus, we see that
the minority carriers really control the behavior of pn-junctions.
2.3.6 pn-Junctions I–V Characteristics
We discussed in Section 2.3.2 that the drift component of the current
caused by the electric field in the depletion region is exactly balanced
out by the diffusion component of the current caused by the electron and
hole concentration gradient across the junction, resulting in zero current
flow in the pn-junction device. When an external voltage is applied, this
current component balance is upset, and current will flow in the diode.
If carriers are generated by light or some other external means, thermal
equilibrium is disturbed, and current can also flow in a pn-junction. Here,
the current flow in a pn-junction as a result of an external applied voltage
is described.
Let us consider a forward-biased pn-junction. Electrons are injected from
the n-side into the p-side, and holes are injected from the p-side to n-side.
If the generation and recombination in the depletion region are negligible,
then the hole current leaving p-side is the same as the hole current entering
the n-side. Similarly, the electron current leaving the n-side is equal to the
electron current entering the p-side. To determine the total current flowing
in the pn-junction, we need to determine either hole current entering the
p-side or electron current entering to n-side of the pn-junction.
The starting point for describing I–V characteristics of a pn-junction is the
continuity equations. From Equation 2.81, the electron continuity equation
is given by
−
∂
∂
= −
∂
∂
+
−
(
)
n
t
q
J
x
G R
n
n
n
1
(2.115)
where:
R n and G n are the electron recombination and generation rates, respectively
Equation 2.115 can be rewritten as
∂
∂
=
∂
∂
−
−
n
t
q
J
x
n n
n
o
n
1
τ
(2.116)
where τ n is the electron lifetime defined in terms of the excess electron concentration n over the thermal equilibrium value n o in Equations 2.48 and 2.53
and is given by
τ n
o
n
n
n n
R G
≡
−
−
(2.117)