Lee de Forest, the Audion, and the Dawn of the Age of Microelectronics

June 20, 2023 by Robert Keim

A scholar, researcher, and tireless inventor, Lee de Forest made a seemingly small modification to vacuum-tube technology. The repercussions in the incipient electronics industry were immense.

Audiophiles, I’ve been told, appreciate vacuum tubes. If this is true, I’m glad that someone still respects a component that has otherwise become a symbol of obsolescence and a monument to the technological limitations that hampered humankind before the invention of the transistor.

I don’t deny that there is something almost shocking about the boldly macroscopic nature of these superseded yet historically significant components. Think of the thousands upon thousands of MOSFETs, neatly etched into silicon and housed in convenient plastic packages, that can fit inside a vacuum tube! 


MOSFETs keep getting smaller

MOSFETs keep getting smaller, so I’m not sure how many billions could currently be laid out in this same three-by-five-inch space. Image (resized) courtesy of RJB1 [CC BY 3.0]

Nevertheless, technological development is a highly incremental process, and we must remember vacuum tubes as an indispensable precursor to the semiconductor devices that eventually made tubes look so strangely primitive. And consequently, we also need to remember Lee de Forest, who opened the door to modern microelectronics through his crucial addition of a third electrode to an existing vacuum-tube design.


From Gadgets to Grad School

Lee de Forest, born in 1873, was an early example of the numerous electrical engineers whose careers began with childish curiosity and youthful tinkering. He enthusiastically observed the technological progress occurring in those heady days of the late nineteenth century—the period that led to the famous but apocryphal declaration from the United States Patent Office that “everything that can be invented has been invented.”

The young de Forest created some interesting mechanical gadgets and learned what he could from the goings-on at a railroad shop near his house. He later studied chemistry and physics as part of his education at a Massachusetts prep school, and Tesla’s inventions caught his eye while he was attending the Chicago World’s Fair in 1893.

In the 1890s, few collegiate institutions could provide a superb education in applied science. Yale was among the best, and Lee de Forest was there from his undergraduate days through the completion of his Ph.D. in 1899. His doctoral dissertation was entitled “Reflection of Hertzian Waves from the Ends of Parallel Wires,” and though nowadays this doesn’t sound particularly exhilarating, at the time, it was a pioneering investigation into radio technology.


Preliminary Inventions

The three-terminal vacuum tube, what de Forest called the Audion, was the most significant achievement of his long and dynamic career as an engineer, businessman, and inventor. But it wasn’t his only achievement, and his earlier work must have helped him attune his mind to innovative possibilities in the relatively new field of high-frequency circuitry and wireless communication.


De Forest holds two of his Audions

De Forest holds two of his Audions, a small 1-watt receiving tube (left) and a 250-watt transmitting power tube he referred to as an "oscillion" (right). Image courtesy of Wikimedia Commons

Another great figure in the early history of radio, Guglielmo Marconi, enabled RF reception by using a device called a coherer, in which metal filings situated between two electrodes cohere and form a low-impedance path under the influence of an applied RF signal. Unimpressed with the coherer’s performance, de Forest invented and patented the “responder,” a similar but superior device; he integrated the responder into an RF communication system that was successfully field-tested in 1901. Later, he improved upon the responder, modified the system’s power source, and managed to sell his updated radio equipment to the United States Navy.


The Audion   

A patent-infringement lawsuit forced de Forest to seek an alternative to his improved responder, and he found the Fleming valve. This was a two-terminal vacuum tube that functioned like a diode, and de Forest started using it for wireless detection. The historic moment arrived when he converted the Fleming valve into a three-terminal device that he called the Audion.


This 1973 postage stamp honoring de Forest's audion

This 1973 postage stamp honoring de Forest's audion is a good reminder that all “progress” is relative! Image courtesy of Wikimedia Commons

Like a diode, the Fleming valve had anode and cathode electrodes. It could rectify but not amplify. By inserting a “grid” electrode between the anode and cathode, de Forest created an active device: the three-terminal Audion, which was essentially a voltage-controlled variable resistor. The description below, written by de Forest himself, indicates the similarities between the behavior of the Audion and the linear region of MOSFET operation (the linear region is also called the triode region, a name inherited from the vacuum-tube days):

In the unlighted bulb, the resistance between the electrodes is practically infinite. When the filament is at white heat, a resistance of from 10,000 to 30,000 ohms is found between it and the cold anode. The value of this resistance depends upon a variety of conditions, such as filament temperature; size, shape, condition, and location of the two outside electrodes . . . ; and the instantaneous value and sign of the electric potential impressed, or residing, on the intermediate, or grid, electrode.

Dr. de Forest recognized (while perhaps overlooking the concept of a noise floor) the amplifying and detecting possibilities of a voltage-controlled device such as the Audion: “There appears to be no known limit of sensitiveness below which the effect of minute received energy fails to be registered”; though “exceedingly sensitive,” the device was “not microphonic, nor delicate, but reliable and free from distortion.”

Though not particularly “reliable” by the standards of twenty-first-century semiconductors, the Audion became an essential component in radio, television, radar, and computing systems and retained this privileged status until transistors started replacing vacuum tubes in the second half of the twentieth century. By cascading multiple Audions, de Forest obtained extremely high gain. By feeding an Audion output signal back to an input terminal, he created low- and high-frequency oscillators, and he could then modulate the resulting sinusoids such that they became carriers of speech and music; he successfully demonstrated this functionality in 1907. Indeed, de Forest did much to usher in the era of what he called the “wavy” electrical art—“the ladder to the final realms of the air.”


An early-twentieth-century Audion radio receiver

An early-twentieth-century Audion radio receiver. Image courtesy of Wikimedia Commons

A Telecommunications Legacy

A relentless innovator, de Forest later developed a method for recording sound onto filmstrips, incorporated the Audion into diathermy equipment, and invented a device used for automatic dialing in telephone systems. He had accumulated over 300 patents when his long career finally came to an end. The Institute of Radio Engineers awarded him their Medal of Honor, and the American Institute of Electrical Engineers honored him with the Edison Medal.

Fascinated by wireless communication, de Forest revealed his poetic side in a 1907 interview with the New York World newspaper: What he termed the “ether waves” 

are dumb to all save him who listens with the proper “responder” correctly attuned to the electric waves. He alone hears the etheric “call of the wild,” . . . and he trembles at the weirdness of it all.