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Diodes II
We need to draw diode characteristic curves, on the other
hand, because they’re non-ohmic and hence, non-linear. So
to see what current passes through the device with any particular
voltage across it, it’s useful to see its characteristic
curve. This is generally true of any semiconductor device, as
we’ll see in later chapters.
Maths
Although diodes are non-ohmic, this doesn’t mean that their
operation can’t be explained mathematically (just as Ohm’s
law or, V = IR, say is a mathematical formula). Diodes, in fact,
follow a relationship every bit as mathematical as Ohm’s law.
The relationship is:
where I is the current through the diode, I s
is the saturation
reverse current, q is the magnitude of an electron’s charge, k
is Boltzmann’s constant, and T is the absolute temperature in
degrees Kelvin. As q and k are both constant and at room temperature
the absolute temperature is more or less constant,
the part of the equation q/kT is also more or less constant at
about 40 (work it out yourself if you want: q is 1.6 x 10 -19 C; k
is 1.38 x 10 -23 JK -1 and room temperature, say, 17°C is 290 K.
The equation is thus simplified to be approximately:
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