Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are common, three lead semiconductor devices. The two primary purposes of MOSFETs are to act as voltage controlled switches (pretty much all digital integrated circuits (ICs) are created from MOSFETs) or as amplifiers. The three leads of a MOSFET are called the gate, source, and drain. The basic physical operation involves applying a voltage between the gate and source to control the conductivity of a channel between the source and the drain. The videos below provide a little bit more detail for the two broad categories of MOSFETs which are called depletion MOSFETs and enhancement MOSFETs.

Depletion MOSFETs

Depletion MOSFETs have a conductive channel between source and drain by default. In other words, they are “normally-ON” devices. The existing channel can be depleted by a voltage applied between gate and source to turn the channel off. Watch this video to find out more:

Enhancement MOSFETs

Enhancement MOSFETs must be “enhanced” to create a conductive channel between source and drain. In other words, E-MOSFETs are “normally-OFF”. To create this channel a voltage must be applied between source and drain.

E-MOSFET Switches

E-MOSFETs are used primarily as switches, i.e., they are devices that are either on or off. In fact, pretty much all digital circuitry is created from complementary MOSFET (CMOS) circuits which are digital circuits created using P-channel and N-channel E-MOSFETs. This video focuses more on standalone switches that control current to a load (i.e., a voltage at the gate determines whether a load is turned on or off).

JFETs, or Junction Field Effect Transistors, are three terminal devices that can be used as switches and amplifiers (amongst other things). They are not commonly used anymore but can still be found in some applications. Studying them, though, can give a good understanding of the effects of electrical fields in semiconductor devices.

The set of videos below provides a good introduction to the analysis of JFETs and circuits that use JFETs. These videos include:

• JFET Biasing
• JFET Small Signal Model
• Common Source Amplifier Configuration
• Common Drain Amplifier Configuration

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JFET Biasing

Biasing is the process of configuring the circuit around the JFET to set voltages and currents to specific values to put the JFET into a particular state (e.g., open the channel between the source and the drain). These two videos describe the different states that JFETs can be in and provide some example biasing circuits.

JFET Small Signal Models

A small signal model is a model that can be used to model only the behaviour of the AC portion of a signal applied to a circuit. The small signal model ignores biasing but assumes that the biasing is putting the device into its proper state. This video describes the AC model of a single stage JFET amplifier (including transconductance). This is a rather long video, but it shows both a graphical method and a numberic method for determining characteristics of the JFET small signal model.

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JFET Common Source Amplifier

The common source amplifier configuration has the input AC voltage applied at the gate and the output taken at the drain. It is analogous to the common emitter BJT amplifier. This video shows how to analyze a couple of different common source configurations (i.e., one with the source resistor bypassed, one without a bypass) to determine the amplifier gain, input impedance, and output impedance.

JFET Common Drain Amplifier

The common drain amplifier configuration of a JFET is analogous to the common collector (emitter follower) configuration of a BJT. It has the input applied at the gate and the output at the source. This video shows how to determine the input impedance, the output impedance and the voltage gain of three different common drain circuits (each has a different biasing configuration).

Diodes are the simplest of electronic devices. At their most basic, they are like one-way valves for electrical current, but they can be used for so many different things. Common applications include voltage rectification, voltage regulation, circuit protection, current control, and even simple (but inefficient) logic circuits. On this page you will find videos on

• PN Junctions
• Modelling Diodes
• Half Wave Rectifiers
• Full Wave Rectifiers

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PN Junctions

The way that electrons flow in doped semiconductors is very interesting, but the really interesting effects occur when you have p-type semiconductor and an n-type semiconductor created side by side to create a PN junction. The properties of the PN junction make it useful for several important electronic devices. The most basic such device is the diode which is simply a PN junction with leads on it to allow it to be used in a circuit.

Modelling Diodes

When you have a diode in a circuit, you will often want to predict its behaviour, or conversely, you want a certain behaviour in  ac circuit and want to ensure the diode will exhibit that behaviour. Diode modelling is the process of creating a mathematical or behavioural model of the diode that helps you to predict how the circuit will behave (electrically speaking) with the diode in it. Depending on the level of precision you want in your model, there are different models that you can choose from. This video describes several different models that you can use for diodes

Diode Model Examples

In case just knowing the theory behind modeling diodes is not enough, this video looks at a few simple examples and analyzes circuits using different diode models. The forward bias models used are:

1. Diode is a short
2. Diode has a small voltage drop
3. Diode has a small voltage drop and a resistance
4. Diode follows Shockley’s Diode Equation

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Half Wave Rectifiers

Half wave rectifiers can be created using a single diode in an AC circuit. Since diodes only allow current to pass in one direction, they will only allow current to flow during half of an AC circuit. Half wave rectifiers cut off half of the signal and this video shows how they work:

Full Wave Bridge Rectifiers

Full-wave rectifiers force AC current to flow through a circuit in one direction by using a clever configuration of diodes. Full-wave bridge rectifiers fulfill a very important stage in AC-DC conversion.