0195136047.pdf

7.2 DIODES 343

(a) (b)

0

Maximum

specified

current

Forward

bias

10

5

−0.1 −0.05

0.05 0.1

V, V

0

−IS

I, nA

(c)

10

5

−1.0 −0.5 0.5 1.0
V, V
0

I, μA

(d)

10

5

− 10 − 5 5 10

V, V

0

I, mA

Reverse

bias

(Breakdown voltage)

−VB

Reverse

breakdown

Burnout

Burnout

−IS

V

I

Figure 7.2.4Typical static volt–ampere characteristic (dc behavior) of apn-junction diode.(a)Showing
reverse breakdown.(b), (c), (d)Omitting reverse breakdown (plotted on different scales).

in whichηdepends on the semiconductor used (2 for germanium and nearly 1 for silicon), and
VTis the thermal voltage given by

VT=

kT

q

=

T

11 , 600

(7.2.2)

wherekis Boltzmann’s constant (= 1. 381 × 10 −^23 J/K),qis the magnitude of the electronic
charge (= 1. 602 × 10 −^19 C), andTis the junction temperature in kelvins (K=°C+273.15). At
room temperature (T=293 K),VTis about 0.025 V, or 25 mV. Usingη=1, Equation (7.2.1) is
expressed by

I=IS(e^40 V− 1 ) (7.2.3)

or by the following, observing thate^4 >>1 ande−^4 <<1,

I=

{

ISe^40 V,V> 0 .1V

−IS,V<− 0 .1V

(7.2.4)

which brings out the difference between the forward-bias and reverse-bias behavior. The reverse
saturation current is typically in the range of a few nanoamperes (10−^9 A). In view of the