DIGITAL CIRCUIT DESIGN COURSE: LESSON TWO
Semiconductor Diodes
Editor's No te : The m aterial below is a serialization of th e Kurz Kasch c orrespondence course for electronics. designed speci fica lly for the coin -
operated am usement industry. Th is course is copyrighted and owned by Kurz Kasch of Dayton. Ohio and its reprinting is being sponsored j oin tly
by Kurz Kasch and Play Me ter magazine. Th is material is authorized for publication exclusively by Play Meter magazine.
This lesson is all about diodes.
Emphasis is placed on the terms
associated with diodes and what
they mean . Particular care has
been given to keying the lesson to
manufacturers' data sheets .
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Vp
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Fi g. 2-1. T he operat ional e hatac l tf la11ca of an Idea l d iode .
Note tt\11 v , • 0, 1,. .. ,~ , to Inf i nity , t,. • 0 and v 111
e xtanda to lnrl nlly .
: Avulon h.-
F ig. 2·2. The opert tl onal charact er latlcs of a l)factlca l d iode .
Comp.-re v, , 1, , l A and VA with those of ll g. 2· 1. Also , note
the limi ts Pd 1111 .. 1 and B., .
The Ideal Diode:
To achieve an understanding of
semiconductor diode character-
isics , it is best to f irst discuss an
ideal d i ode . This i mag i nary
rectifier (which exists only in our
minds) has ideal characteristics
- in other words, it is perfect . As
shown in figure 2- 1, when forward
biased it is capable of passing an
infinite amount of current (IF] and
regardless of the forward current,
it would drop zero volts (VF]. The
zero voltage drop indicates that
the res istance of the device, when
forward biased , is zero ohms.
Therefore, the power diss ipated
by the diode, regardless of the
current through it , is zero watts.
Using the power formula for
proof:
P = FxR
(if A
TeJil'tr('UII
A·
,.,
a·
= 0 then
P
= 0
When th is ideal diode is reverse
biased it would be capable of not
break ing down regardless of the
reverse voltage [VA] across it.
Also, there would be no leakage
current [IR] . The lack of IR
ind icates that the resistance of
th is imaginary d iode is infinite.
Using Ohm's Law for proof:
A = E/ 1
inf inite capability and woul d,
therefore, be the sam e. In reality,
no sem i conducto r d i od e i s
perferct. These impl icat ion s are
topics of the follow ing para-
graphs .
Figure 2-2 shows the ty pica l
behav ior of a real d iode when
forward and reverse biased. It
should be noted that with th e real
d iode the re are definite operati ng
lim its, bothg in the forward and
reverse biased states. Also , the
forward and reverse resista nces
of the real device are far from
those ofthe ideal diode: note IR
and VF.
We w ill now focus our attention
to the forward biased behav ior of
the diode curve and figure 2-3.
The test circu it shown in figure 2-
3 illustrates the mean ing of VF
and IF,wh ile the load res istan ce is
varied to change the current IF. As
AL is varied from a maximu m
res istance to a lower value, the
cu rrent drawn through the d iode
increases , say, from po int A to
point B. VF also changes from
po int A ' to point B' . Notice that
the current [IF] through the diode
determines the voltage [ VF]
dropped across the diodes.
The current through the d iode
can be increased by decreasi ng
AL until the product of the cu rrent
through the d iode and the vo ltage
across the d i ode equal the
max i mum power [Pd[ max 1 ] that
the dev i ce i s capabl e of
d iss ipating .
F l • • 2-3. T he forward b l as..S charac terl alles of a typ lc t l diode.
v,. lncrtlltt as 1,. lncraa .. a to the opera llona t li mit Pd '"' •• •·
(If I = 0 then A is without limit, or
infinite value)
Based on the above, it can be
stated that an ideal diode would
switch from a zero ohm state to an
infinite resistance state when the
source voltage is reversed .
Fit . 2-4 . The reveraa biased ch ... ac tet latlc s of a diod e. 1111
v,. IMrtaaed to the limit 8 11 .
IM rtaHa ••
8
The Practical Diode
If the diode described in the
above paragraphs existed , the
c ircu it designer would be able to
pick , at random , any diode and in-
stall it into his circu it. His choice
of diodes would require no
thought since they would all have
Once the dissipation by the
d iode exceeds the manufact ur-
ers' stated limit, the diode will
then operate on the princ ipl e of
thermal runaway. Diodes o p-
erated beyond the stated Pd [max 1
w ill , under normal circumstan ces,
be destroyed.
When the real diode is reverse
biased , as in the circu it of figure
2-4, a small current will flow . This
current is typically in the ord er of
m icroamperes. As battery voltage
in the test c ircuit is adjusted , for
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