568
21 Diodes
Fig. 21.51 Theoretical I –V characteristic of a diode at room temperature with saturation currents for the n = 1 and
n = 2 processes of I n=1
s
= 10 −12 A and I n=2
s
= 10 −9 A and resistances R s = 10 , R p = 100 M Green line: Ideal
diode with n = 1 characteristic only, Purple line: only n = 2 process, dashed red line: only parallel ohmic resistance
R p , dashed blue line: only series ohmic resistance R s , solid black line: all effects combined as in (21.159)
(a)
(b)
Fig. 21.52 a j V -characteristic of NiO/ZnO bipolar diode (cmp. Sect. 21.4.6) in half-logarithmic plot. The blue line is a
fit with ideality factor n = 1.8. b I V -characteristic in linear plot. The purple line in (b) is a linear fit (with R s = 1.06 k
for the range where the series resistance dominates
This equation is implicit with regard to I and can only be solved numerically. At high current, the
resistance of the junction becomes very small (Fig. 21.51a); then the I –V characteristic deviates from
the exponential behavior (Fig. 21.52a); finally it is dominated by the series resistance and becomes
linear (Fig. 21.52b). Sometimes the extrapolated voltage from the linear range (1.19 V in Fig. 21.52b)
is called ’threshold’ voltage, but this is a wrong term for this behavior.
The diode can also exhibit a parallel (shunt) resistance R p , e.g. due to surface conduction between
the contacts. Including the shunt resistance, the diode characteristic is
I = I s
exp
e (V − R s I )
n kT
− 1
+
V − R s I
R p
.
(21.158)
The shunt resistance can be evaluated best from the differential conductance in the reverse-voltage
regime [1725]. Due to a high surface-state density, the passivation of GaAs diodes can be difficult. Si
can be very well passivated with low leakage current and high reliability.
21 Diodes
Fig. 21.51 Theoretical I –V characteristic of a diode at room temperature with saturation currents for the n = 1 and
n = 2 processes of I n=1
s
= 10 −12 A and I n=2
s
= 10 −9 A and resistances R s = 10 , R p = 100 M Green line: Ideal
diode with n = 1 characteristic only, Purple line: only n = 2 process, dashed red line: only parallel ohmic resistance
R p , dashed blue line: only series ohmic resistance R s , solid black line: all effects combined as in (21.159)
(a)
(b)
Fig. 21.52 a j V -characteristic of NiO/ZnO bipolar diode (cmp. Sect. 21.4.6) in half-logarithmic plot. The blue line is a
fit with ideality factor n = 1.8. b I V -characteristic in linear plot. The purple line in (b) is a linear fit (with R s = 1.06 k
for the range where the series resistance dominates
This equation is implicit with regard to I and can only be solved numerically. At high current, the
resistance of the junction becomes very small (Fig. 21.51a); then the I –V characteristic deviates from
the exponential behavior (Fig. 21.52a); finally it is dominated by the series resistance and becomes
linear (Fig. 21.52b). Sometimes the extrapolated voltage from the linear range (1.19 V in Fig. 21.52b)
is called ’threshold’ voltage, but this is a wrong term for this behavior.
The diode can also exhibit a parallel (shunt) resistance R p , e.g. due to surface conduction between
the contacts. Including the shunt resistance, the diode characteristic is
I = I s
exp
e (V − R s I )
n kT
− 1
+
V − R s I
R p
.
(21.158)
The shunt resistance can be evaluated best from the differential conductance in the reverse-voltage
regime [1725]. Due to a high surface-state density, the passivation of GaAs diodes can be difficult. Si
can be very well passivated with low leakage current and high reliability.