I was surprised at some of the comments so thought I would ask a guy who has
been
responsible for probably over a billion 1N4XXX diodes .So here is his tutorial
on the subject
Not surprisingly - essentially same as Paul W9AC explanation-
Hank K7HP
PIN diodes and switching diodes are two dramatically different animals.
1. A switching diode is a simple PN junction and the doping is engineered to
give very low carrier lifetimes in order to switch fast. A switching diode is
ON as long as the forward voltage across it exceeds whatever signal voltage is
being switched.
2. A PIN diode has an "intrinsic" layer (minimal doping) between the P and the
N regions, and it is designed to have very long carrier lifetimes in that
intrinsic region. A PIN diode is a current controlled device, and it it ON as
long as the forward current exceeds the signal current essentially independent
of the voltage of the signal ... as long as the frequency of the signal is
higher than the carrier lifetimes in the intrinsic region. If the signal
frequency is too low compared to the carrier lifetimes, when it goes reverse
polarity it can outlast the carriers as reverse bias the diode (assuming the
signal voltage can exceed the control voltage supplying the control current).
Both kinds of diodes can switch fast, but in the case of the PN diode you're
switching voltage fast and counting on short carrier lifetimes, whereas in the
case of the PIN diode you're switching control current fast.
The bottom line is that replacing a PIN diode with a switching diode might work
depending upon the control voltage and the signal voltage, but it completely
changes the switching mechanism involved and it would NOT be universally
applicable.
A 1N4007 is a switching diode, period. It is NOT a PIN diode and would never in
a million years act like one ... unless the frequency being controlled was
faster than the switching speed (carrier lifetime) of the 1N4007.
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