SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

4.8. DIODE APPLICATIONS: AN OVERVIEW 189

In a solar cell configuration the forward bias is not explicitly applied across the cell. It is
generated by the flow of the current across the load (which may be the resistance of a light bulb
for instance). This is shown schematically in figure 4.26 along with the equivalent circuit of the
solar cell. The total cell current goes to zero at a voltage,Voc, termed the open circuit voltage,
when the forward diode current is equal and opposite to the generated current. From equation
4.8.11

Voc=

kBT

e

ln

(

Isc

Is

+1

)

To obtain the maximum power from a cell it is desirable to have the largest product of voltage
and current possible in the fourth quadrant of theI−Vplane. The maximum power point is that
bias at which the maximum power is available from the cell, or, is the bias at which the largest
rectangle can be accommodated within the I-V curve. The power at any bias point is given by
theIVproduct

P=I·V=(Isc−ID)·V=

[

Isc−Isexp

(

eV

kBT

)]

·V

and the maximum power point is obtained by maximizing the product. This is left as an exercise.
The maximum power is also alternately represented by

P=VocIsc·F

whereFis the defined as the Fill Factor of the cell. Hence to get the maximum power from a cell
it is desirable to obtain the largest Vocand Iscpossible which is best achieved by using a tandem
cell which comprise of a series connection of cells with different bandgaps that maximize solar
absorption (while maintaining a large open circuit voltage) coupled with concentrator lenses that
maximize input photon intensity.

4.8.3 The uses of diode non-linearity (Mixers, Multipliers, Power Detec-

tors)

A mixer is a frequency translation device that translates an input signal band of frequencies to
a different band of output frequencies. There are two main uses of the mixer: down conversion
and up conversion. Down conversion, used in receivers, takes a higher input RF frequency and
shifts it down to a lower frequency where the channel selection can be performed and interfering
signals can be filtered out. Up conversion takes a lower frequency band limited signal and shifts
it to a higher frequency. This is typically thetransmitter application.
A mixer does not really “mix” or sum signals; it multiplies them. For example, the analog
multiplier performs the frequency translation function:

A=Asinω 1 tB=Bsinω 2 t

(Asinω 1 t)(Bsinω 2 t)=(AB/2) [cos(ω 1 −ω 2 )t−cos(ω 1 +ω 2 )t] (4.8.12)

Note that both sum and difference frequencies are obtained by the multiplication of the two