Current-voltage characteristic of a "crystodyne" junction, a zinc oxide <a href="https://en.wikipedia.org/wiki/crystal_detector" class="extiw" title="w:crystal detector">crystal detector</a> junction diode researched beginning in 1923 by Russian radio pioneer <a href="https://en.wikipedia.org/wiki/Oleg_Losev" class="extiw" title="w:Oleg Losev">Oleg Losev</a>.   The graph shows that the junction has <a href="https://en.wikipedia.org/wiki/negative_resistance" class="extiw" title="w:negative resistance">negative resistance</a> <i>(shaded region)</i>.   An AC signal applied to the device, biased to the point labelled "wzmacniacz"  within the negative resistance region, will be amplified.    Losev found that these zinc oxide - metal junctions could amplify signals and produce oscillations, and built solid-state radio receivers and oscillators with them, 25 years before the transistor.
<a href="//commons.wikimedia.org/wiki/User:%D0%94.%D0%98%D0%BB%D1%8C%D0%B8%D0%BD" title="User:Д.Ильин">Д.Ильин</a>
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<span class="int-own-work" lang="en">Own work</span>, translstion, based on <a href="//commons.wikimedia.org/wiki/File:Krystadyna_charakterystyka_pr%C4%85dowo-napi%C4%99ciowa.svg" title="File:Krystadyna charakterystyka prądowo-napięciowa.svg">File:Krystadyna charakterystyka prądowo-napięciowa.svg</a> by <a href="//commons.wikimedia.org/wiki/User:Topory" title="User:Topory">Topory</a>