FET Transistor Homemade From Cadmium Sulfide Photocell.

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FET Transistor Homemade From Cadmium Sulfide Photocell.

By Nyle Steiner K7NS May 7 2009.

Updated May 10 2009

CDS Photocell Made Into A FET Transistor

The picture above shows how transistor action was observed by improvising an insulated gate to a cadmium sulfide photo resistor. The picture was taken in normal light but the experiment had to be performed in the dark.

The photocell used is pictured above. It is a very common type which I purchased from Radio Shack many years ago.

Photo Resistor Converted Into A Field Effect Transistor.

I have long suspected that if a successful homemade transistor were to be made, it might likely happen in the form of a FET. I have read of Roger Baker's homemade FET in the June 1970 Scientific American Amateur Scientist column numerous times and have thought about the simple architecture that can make a FET. This article illustrates that a FET can be made simply by running current through a thin film of the right type of semiconductor. If a flat conductor is put very near the semiconductor film and insulated from it, voltage changes between the flat conductor and the film will cause changes in the amount of current flowing through the film.

It recently occurred to me that if this is the case, I might be able to create transistor action through a cadmium sulfide photocell (actually a photo sensitive resistor) since they consist of basically a thin semiconductor film between two electrodes. This simple experiment would be a logical first step before trying to create my own thin semiconductor films. Would this photocell act as a transistor if I put an insulated gate near it?

My first attempt was simply to observe if there is any current change through the photocell while moving a charged comb or pvc pipe near it. The excitement of seeing the current change was short lived after realizing that the light striking the photocell was also affected by the moving comb. I needed some light in the room to watch the meter. Several years ago I had also tried moving a charged comb near some catwhisker devices to see if the current changed. Seeing some current change at that time, was exciting until I realized that the electrostatic attraction from the charged comb was physically pulling on the catwhisker. I have long wondered if simply putting a charged object near a semiconductor or other type of film, could have an effect on electrical current flowing through the film. For now, that question still remains unanswered.

It was time to try improvising some kind of conductive gate near the surface of the photocell. I did this by putting a piece of scotch tape across the face of the photocell to act as the insulator. To make a conductive layer in close contact, I then put a drop of water on top of the scotch tape just big enough to cover most of the photocell area. I used water because of it's ability to conform closely to the surface of the tape. Nothing needed to be added to the water because the resistance of normal water is very low compared to the almost infinite resistance of this improvised gate. A piece of wire touching the drop of water served as the gate electrode.

From my observations, the setup described above definitely produces transistor action. This experiment had to be performed in the dark for obvious reasons although I found that a tiny bit of light falling on the photocell could sometimes improve performance. There was little or no transistor action in normal light because the photocell was saturated.

This transistor has considerable power gain but very low voltage gain because of the wide voltage excursions required at the gate to produce a significant current change through the photocell. Intuition says the gate could be made more sensitive by putting the gate closer but the thickness of the scotch tape and the clear coating on the front of the photocell precluded this. The gate input resistance is for most practical purposes, infinite. The only current that flows through the gate is whatever current can leak through a piece of scotch tape.

Battery B supplies current through the photocell and R2. Current through the photocell is measured by I2. Battery B was varied between 9 VDC to 175 VDC.

Battery A was varied between 75 VDC and 175 VDC and was connected through a switch to be able to reverse the polarity of voltage applied to the gate.

Whenever the switch was changed, the polarity of voltage across the gate would reverse, resulting in a current change through I2.

R1 and R2 were used mainly to limit current and protect the current meters in case of a high current. Since the gate impedance is so high, R1 could be anywhere between zero and 10 meg without noticing any significant difference.<br>This device acted as an enhancement-depletion insulated gate FET. A positive voltage applied to the gate caused an increase of current through I2 and a negative voltage...