4. Transistors

  • When talking about electricity and semiconductors, things can get rather nuanced and complex

  • However, the level of detail covered for this topic will be kept appropriate for the course content

  • As such, certain analogies and ideas will be used that are technically inaccurate, but sufficient for our needs

4.1. Symbols

  • For this course, software called Digital will be used to create digital circuits

  • There are a number of symbols representing components that will need to be understood

  • Although some of you may be familiar with electronic component symbols, there are some things to note

    • Certain components have different symbols depending on their context

    • Some components have different symbols in different regions of the world

  • When using Digital

    • An up arrow is used to signify a positive charge

    • A single horizontal line represents ground — something with no charge; neutral

../../_images/vcc_ground.png

Screenshot from Digital of a positive charge (up arrow) connected directly to ground (horizontal line).

  • A single horizontal line is common within computer architecture and is the symbol found in Digital

    • There are other common symbols for ground depending on the context of how it is used

../../_images/ground_symbols.png

Examples of three commonly used ground symbols. These have slightly different meanings, but all ultimately signify something with a neutral charge.

  • In the above example, a voltage source was connected directly to ground

  • In practice, this is a bad idea as that could cause damage and injury

    • Current would be too high

  • A resistor, for example, could be added to the circuit to limit its current

  • Below is an example of the simple circuit containing a resistor

../../_images/vcc_resistor_ground.png

Screenshot from Digital of a positive charge (up arrow) connected to ground (horizontal line) through a resistor (box).

  • In North America, a sawtooth symbol is used to represent a resistor, but elsewhere in the world it is typically a box

    • Like in the above figure

  • Digital was not made in North America, thus, it makes use of the box symbol

../../_images/resistor_symbols.png

Left — Typical symbol for a resistor in North America. Right — Typical symbol for a resistor outside North America.

  • One of the commonly seen symbols for transistors are actually only for a specific, but common type of transistor

    • Bipolar Junction Transistors (BJT)

  • The transistors available within the simulator Digital are Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET)

    • These transistors are common within computers

  • The symbol used in this course for transistors will be of MOSFET transistors

../../_images/transistor_symbols.png

Left — Symbol for a Bipolar Junction Transistor (BJT). Right — Symbol for a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET).

4.2. Transistors

  • Transistors may be one of the most important inventions/discoveries in human history

      1. Bardeen, W. Brattain, and W. Shockley got the 1956 Nobel Prize in Physics for creating the first transistor

  • Transistors are semiconductor devices

    • Not strictly a conductor or insulator

    • Can control the conductive properties of the component

Note

The details on how transistors physically work is outside the scope of this course. For those interested, Ben Eater have two videos describing, at a high-level, how they work.

  • Transistors perform simple tasks, but are critically important for our everyday lives

  • Typically, transistors are used for signal amplification or as a switches

  • For this course, the focus will be on transistor’s ability to be used as a switch

  • As already discussed, there are several kinds of transistors, but for this course, the focus will be on MOSFET

../../_images/transistor_model.png

Three-dimensional model of a Metal-oxide-semiconductor field-effect transistor (MOSFET). S — source, D — drain, G — gate, and B — body of the transistor. The source, drain, and gate have terminals to connect to a circuit.

Note

Within the context of digital circuits, it is common to refer to signals as 0s and 1s. The use of 0 and 1 do not signify a specific voltage, but instead 0 means a relatively low voltage and 1 means a relatively high voltage.

  • Below is an image of an N-channel MOSFET

  • This transistor can act as a switch for a signal to travel from the source to the drain

  • With this type of transistor, the switch is closed (on) when a positive charge is applied to the gate terminal

    • This would allow the signal to travel from the source to the drain

    • The signal at the source is arbitrary

  • The switch would be closed (off) is no positive charge is applied to the gate

../../_images/n_transistor_labelled.png

N-channel MOSFET as a switch. The switch is “on” when a positive charge is applied to the gate terminal, allowing a signal to travel from the source to the drain. If no positive charge is applied to the gate, the switch is “off”, not allowing a signal to pass through the transistor.

  • P-channel MOSFETs exist, which act similar to N-channel, but turn “on” when no positive charge is applied to the gate

../../_images/p_transistor_labelled.png

P-channel MOSFET as a switch. The switch is “on” when a negative charge is applied to the gate terminal.

Note

For the most part, N-channel MOSFET transistors will be used in this course.

  • Below is a circuit with an N-channel MOSFET acting as a switch on some signal that is always on (1)

  • There are several components to note

    • N-channel MOSFET

    • Voltage source providing a signal for the source

    • A current limiting resistor and ground connected to the drain

    • A toggleable square button connected to the gate labelled \(switch_{a}\)

      • Allows for changing the state of the signal applied to the gait when running a simulation

      • These are called inputs within Digital

    • A circle to read circuit output labelled \(switch_{o}\)

      • Allows for easily observing the signal at some point in the circuit

      • These are called outputs within Digital

    • A box called “Test” labelled \(switch_{tests}\)

      • These are where unit tests are written for the circuit

      • These will be discussed in more detail later

../../_images/transistor_as_switch_static.png

Screenshot of a circuit created in Digital showing an N-channel MOSFET as a switch.

  • When no positive charge (a signal of 0) is applied to the gate, the transistor does not allow a signal to pass

  • The output of this circuit in this state is therefore a signal of 0

../../_images/transistor_as_switch_off.png

Screenshot of a circuit created in Digital showing an N-channel MOSFET as a switch. Here, the switch is off, therefore the output of the circuit is 0.

  • When a positive charge (1) is applied to the gate, the transistor allows the signal at the source (1) to pass

  • Therefore, the output of the circuit in this state is 1

../../_images/transistor_as_switch_on.png

Screenshot of a circuit created in Digital showing an N-channel MOSFET as a switch. Here, the switch is on, allowing the 1 from the source to pass to the output.

  • Although this example may seem silly, remember that the actual signal at the source is arbitrary

    • In the above example, the signal at the source was always set to 1

  • Further, they become more interesting when combined with additional transistors in clever ways

4.3. For Next Time