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  1. Transistors
    1. Objective
    2. Motivation
    3. Transistor uses
    4. The hydraulic analogy
    5. Transistor Overview
    6. Transistor Comparison
    7. Transistor Logic
    8. Transistors Uses
      1. Amplifier
      2. Switch
      3. Logic Gates
    9. Transistor Combinations
    10. Transistor Density
    11. Nature's transistors
  2. Problems
    1. Amplify a signal
    2. Nature's transistors
  3. Activities
    1. Transistor Combinations
  4. References

1. Transistors

1.1. Objective

  • To have a basic understanding of how a transistor works

1.2. Motivation

  • The transistor is the building block of a computer. It is the equivalent of the cell to an organism, the neuron to a brain, and the atom to a molecule.

1.3. Transistor uses

  1. As an amplifier
  2. As a switch
  3. To create logic gates (and, as we will see, logic gates are used to compute)

There are many ways to create amplifiers, switches, and logic gates. Mechanical computers existed before the transistor. The transistor has special properties that make it better for computing than the alternatives.

1.4. The hydraulic analogy

A hydraulic analogy can be used to understand what a transistor does. By adding a small amount of water to the cup on the top, a large amount of water can be made to flow out the bottom. The system operates as both an on/off switch and an amplifier. The system is an on/off switch because the flow of water out the bottom is "on" when just enough water is placed in the cup at the top. The system is an amplifier because a small "signal", composed of water dripping into the top cup, can be turned into a large "signal", composed of water flowing out the bottom.

See also [1] [2] [3] [4] [5]

1.5. Transistor Overview

A transistor is shown in the image on the right. Instead of fluid flow and fluid levels, an electrical transistor operates on current flow and voltage levels.

A transistor has three components, a base, a collector, and an emitter. A small positive voltage in the base allows large current to flow from the collector to the emitter

(A transistor can also be run in reverse: Changes in current can be transformed into changes in voltage.)

From on July 04 2014 10:12:52.

1.6. Transistor Comparison

  • In a transistor, a small positive voltage in the base allows large current to flow from the collector to the emitter
  • In the toilet transistor,
    • The base is the small cup on the top.
    • The collector is the large amount of water.
    • The emitter is the pipe on the bottom.

From on July 04 2014 10:12:52.

1.7. Transistor Logic

In words, the transistor does the following:

  • If container A is not full, symbolized as 0, water does not flow through the output.
  • If container A is full, , symbolized as 1, water flows through the output.

The table to the right shows all possible combinations of inputs and outputs.

Input A Output
0 0
1 1

1.8. Transistors Uses

A transistor can be used in three ways: as an amplifier, as a switch, and to create logic gates - this is important for computing.

1.8.1. Amplifier

Suppose the small cup has a slow leak and you continually fill the cup so it is just to the point where one more drop will result in a flush. If you hold a dropper above the cup and drip water into it, there will be a large flow of water associated with each drop. If the input signal is considered to be the drops of water, then the output is an amplified version - for every small drop of water that fills the cup there will be a large amount of water that exits the pipe on the bottom.

1.8.2. Switch

If you consider "off" to be when no water is flowing out of the pipe and "on" to be when water is flowing in the pipe, then the toilet is acting as a switch. The switch is turned on by adding water to the cup and off by removing water from the cup.

1.8.3. Logic Gates

Whereas a switch takes one input that is either "on" or "off" and has an output that is either "on" or "off", a logic gate has multiple inputs and one output. Switches can be combined together to produce a logic gate. This will be discussed further in the Logic_Gates module.

1.9. Transistor Combinations

Previously, only a single transistor and its logic table was considered. Transistors can be combined. In this case, the logic table becomes more complex.

The logic table on the right is a way of summarizing the text on the left, assuming 0 means "not full" and 1 means "full".

  • If container A is full and container B is full, water flows through a pipe.
  • If container A is full and container B is not full, water does not flow through a pipe.
  • If container A is not full and container B is full, water does not flow through a pipe.
  • If container A is not full and container B is not full, water does not flow through a pipe.
Input A Input B Output
  1   1   1
  1   0   0
  0   1   0
  0   0   0

1.10. Transistor Density

In principle, you could create an iPhone by connecting a bunch of toilet transistors together to form logic gates.

A key problem computer chip manufacturers work on is how to pack more transistors into a smaller amount of space.

This image shows how the size of transistors has decreased with time: [6]

1.11. Nature's transistors

There are many systems in nature that have transistor-like properties. That is, the systems operate as a switch or an amplifier.


  • "Structure of Biological "Transistor" Detailed in Higher Organisms" [7]
  • "Organic Transistor Paves Way for New Generations of Neuro-Inspired Computers" [8]
  • "Organic transistor mimics brain synapse" [9]
  • "New Transistor Bridges Human-Machine Gap" [10]
  • Organic transistor possibilities: [11]
  • "New transistor allows humans, machines to merge. Are cyborgs imminent?" [12]
  • "The plastic processor" [13]
  • "An organic-nanoparticle transistor behaving as a biological spiking synapse" [14]
  • "MoNETA: A Mind Made from Memristors" [15]

2. Problems

2.1. Amplify a signal

Use a toilet transistor to amplify a signal. What is the input signal? What is the output signal?

2.2. Nature's transistors

Find an article that discuses biological transistors or transistors made of organic material.

  • What is the link?
  • What is the collector?
  • What is the emitter?
  • What is the base?
  • What does the transistor block or allow the flow of (In the toilet it is water and in an electronic transistor it is the flow of electrical current.)
  • Does the article mention how quickly the transistor can be turned on, off, and then on again?

3. Activities

3.1. Transistor Combinations

Create a hydraulic contraption that gives outputs given the inputs specified in the logic table.

Input A Input B Output
  0   0   0
  0   1   1
  1   0   1
  1   1   1

4. References

  • A video that compares a mechanical switch to a transistor switch: [16]
  • NPR story on Tiny DNA switches [17]

This video shows how the first electrical transistor was made and summarizes how they work.

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