344 SEMICONDUCTOR DEVICES
exponential factor in Equation (7.2.3), the apparent shape of theI–Vcurve depends critically
upon the scale of the voltage and current axes. Figures 7.2.4(b), (c), and (d) illustrate this point,
takingIS=1nA= 10 −^9 A. A comparison of Figure 7.2.4(d) with Figure 7.2.1(c) suggests that
one can use the ideal diode as amodelfor a semiconductor diode whenever the forward voltage
drop and the reverse current of the semiconductor diode are unimportant.
Based on the ability of the junction to dissipate power in the form of heat, themaximum
forward currentrating is specified. Based on the maximum electric field that can exist in the
depletion region, thepeak inverse voltage(maximum instantaneous value of the reverse-bias
voltage) rating is specified.
The most apparent difference between a real diode and the ideal diode is the nonzero voltage
drop when a real diode conducts in the forward direction. The finite voltage drop across the diode
is accounted for byVon, known as theoffsetorturn-onorcut-inorthresholdvoltage, as shown
in the alternate representation of the junction diode in Figure 7.2.5(a). Typical values ofVonare
0.6 to 0.7 V for silicon devices and 0.2 to 0.3 V for germanium devices.
A closer approximation to the actual diode volt–ampere characteristic than that in Figure
7.2.5(a) is depicted in Figure 7.2.5(b), which includes the effect of theforward (dynamic)
resistance Rf, whose value is the reciprocal of the slope of the straight-line portion of the
approximate characteristic beyond the threshold voltageVon.
As an extension of the diode model of Figure 7.2.5(b), to allow for more realistic volt–ampere
characteristic slopes, the diode’sreverse resistance Rrforv<Vonis included in the model of
Figure 7.2.6.DDFigure 7.2.5Forward-biased diode models.(a)With threshold voltageVon.(b)With threshold voltageVon
and forward resistanceRf.