*********** 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 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 .. figure:: vcc_ground.png :width: 50 px :align: center 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 .. figure:: ground_symbols.png :width: 333 px :align: center :target: https://en.wikipedia.org/wiki/Ground_(electricity) 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 .. figure:: vcc_resistor_ground.png :width: 50 px :align: center 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 .. figure:: resistor_symbols.png :width: 333 px :align: center 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 .. figure:: transistor_symbols.png :width: 333 px :align: center Left --- Symbol for a Bipolar Junction Transistor (BJT). Right --- Symbol for a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). Transistors =========== * Transistors may be one of the most important inventions/discoveries in human history * J. 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. * `Semiconductors `_ * `Transistors `_ * 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 .. figure:: transistor_model.png :width: 333 px :align: center :target: https://en.wikipedia.org/wiki/Transistor 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 ``0``\s and ``1``\s. 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 .. figure:: n_transistor_labelled.png :width: 333 px :align: center 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 .. figure:: p_transistor_labelled.png :width: 333 px :align: center 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 :math:`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 :math:`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 :math:`switch_{tests}` * These are where unit tests are written for the circuit * These will be discussed in more detail later .. figure:: transistor_as_switch_static.png :width: 500 px :align: center 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`` .. figure:: transistor_as_switch_off.png :width: 500 px :align: center 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`` .. figure:: transistor_as_switch_on.png :width: 500 px :align: center 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 For Next Time ============= * Check out the :download:`transistors as a switch ` schematic for Digital * Read Chapter 3 Section 2 of your text * 6 pages