# Lesson Explainer: Diodes Physics

In this explainer, we will learn how to describe how diodes work and identify materials used in their construction.

A diode is an electrical circuit component that allows a current through it in one direction but not in the opposite direction.

The symbol for a diode in a circuit diagram is a triangle pointing toward a straight line perpendicular to the wire.

The orientation of the triangle in the symbol indicates the direction in which current is permitted.

Consider the simple circuit below, consisting of a cell, a diode, and a resistor.

Recall that conventional current goes from the positive terminal to the negative terminal, which is clockwise in this diagram. In this example, the current is aligned with the direction of the diode, and so there will be current in the circuit.

Now imagine that we have connected the cell the other way around, so that the current is in the opposite direction.

In this case, the direction of the current is opposite to that allowed by the diode, so there will be no current in this circuit.

### Example 1: Understanding the Circuit Symbol for a Diode

Which of the following diagrams correctly shows which way a current can flow through a diode?

In this example, we need to recall in which direction current can pass through a diode.

Here, the symbol for the diode is helpful, as the triangle points in the direction of the allowed current. In option A, the current is pointing in the opposite direction to the triangle, so the diode will not allow current in this direction.

In option B, the arrow representing the current is pointing in the same direction as the diode symbol, so current can pass in this direction.

Option B is therefore the correct answer.

Diodes have a variety of uses in electrical circuits. As we have seen, connecting the cell the other way around prevents any current if a diode is present, so diodes can be used to protect other electrical components from damage in the event that the power source is connected the wrong way around.

A diode can also be used to convert alternating current to direct current. Alternating current is used in mains power supplies, and it periodically reverses direction.

When a diode is introduced into the circuit, the negative portion of the cycle is effectively canceled, leaving only the positive current.

In the following examples, we will look at the effect that diodes have on electrical circuits.

### Example 2: Understanding Diodes in Circuit Diagrams

The diagram shows a circuit containing several diodes and bulbs. All of the bulbs are connected in parallel with the cell. Which of the bulbs, if any, are lit?

In this example, we have a circuit containing a power source, three bulbs, and five diodes. The bulbs are connected in parallel.

First, we need to recall that conventional current goes from the positive terminal to the negative terminal and that the positive terminal is represented by the longer line. In this case, that means the current is clockwise in this circuit.

The three bulbs are connected in parallel, which means we can consider each part of the circuit separately. So, let’s look at each bulb in turn.

Bulb 1 is connected to a diode that allows current in the clockwise direction. There will therefore be current through this part of the circuit, so bulb 1 will be lit.

Bulb 2 is connected to a diode that allows current in the counterclockwise direction. No current will be allowed through this part of the circuit, so this bulb will not be lit.

Bulb 3 is connected to three diodes. The upper diode allows current in the clockwise direction, but the other two only permit current in the counterclockwise direction. In order for there to be current through this part of the circuit, all three diodes would need to allow it, so bulb 3 will not be lit.

Therefore, only one bulb in this circuit is lit, and that is bulb 1.

### Example 3: Understanding Diodes in Circuit Diagrams

The diagram shows a circuit containing several diodes and resistors. All of the resistors are connected in parallel with the cell. Through which resistors, if any, is the current nonzero?

In this example, we have a circuit containing a power source, three resistors, and four diodes. The resistors are connected in parallel, so we can consider each part of the circuit individually.

Firstly, recall that conventional current goes from the positive terminal (represented by the longer line) toward the negative terminal (represented by the shorter line). In this case, that means the current is counterclockwise.

Both resistors and are connected to diodes that allow current in the downward, or clockwise, direction. This is the opposite direction to the current in this circuit, so there will be no current through or .

The third resistor, , is connected to two diodes. One of them—the lower one in the diagram—permits current in the counterclockwise direction as required. However, the upper diode only allows current in the clockwise direction. Both of these diodes would have to allow current in order for there to be nonzero current through this part of the circuit, so the current through is zero.

Therefore, none of the resistors have nonzero current through them.

Consider the following circuits in which we have a cell connected to a diode, with an ammeter to measure the current through the circuit and a voltmeter measuring the potential difference across the diode. We can vary the potential difference across the diode by using different cells. We can also attach it the opposite way around as in the right-hand diagram, which we can think of as a negative potential difference. In this case, the current will be in the opposite direction, which we can think of as negative current.

How would the measured current change as we change the potential difference across the diode? The current–potential difference, or , graph below shows the current on the vertical axis and the potential difference on the horizontal axis. The blue line shows the change in current as we adjust the potential difference.

When the potential difference is negative, so the current is in the counterclockwise direction, the diode does not allow any current. As soon as the potential difference turns positive, a large current is allowed. The diode effectively acts as a switch, which is open when the potential difference is negative and closed when it is positive.

This situation is what we refer to as an “ideal diode.” In reality, diodes are made of semiconductors that offer very high—but not infinite—resistance in one direction. This means that a small amount of current does exist when the potential difference is negative. And for very large negative potential differences, this can break down entirely and the diode will allow a large current.

When the potential difference is positive, a real diode will not react until the potential difference crosses some threshold. A more realistic diagram looks more like the one below.

In this final example, we will get some practice interpreting diagrams for diodes.

### Example 4: Finding the Resistance from an 𝐼–𝑉 Graph of a Diode

The graph shows the characteristics of a diode.

1. At which of the points marked on the graph is the resistance of the diode highest?
2. At which of the points marked on the graph is the resistance of the diode lowest?

Part 1

Here, we are given an graph of a diode that shows zero current when the potential difference is negative; a small amount of current when the potential difference is positive; and a small, ramping up quickly to a large current when there is a large positive potential difference. We need to identify the point on the graph where the resistance of the diode is highest, out of the four points marked , , , and .

Recall that resistance impedes current. Therefore, the highest resistance will coincide with the region of the graph where the magnitude of the current is lowest. This occurs when the potential difference is negative. Of the marked points, point corresponds to the highest resistance.

Part 2

We now have to identify the point where the resistance of the diode is lowest. The diode offers the least resistance when the highest current is allowed. The point corresponding to the highest current is point . Therefore, the resistance of the diode is lowest at point .

### Key Points

• A diode is an electrical circuit component that allows a current to pass through it in one direction but not in the opposite direction.
• In electrical circuit diagrams, the diode is represented by the following symbol:
• For conventional current, from the positive terminal to the negative terminal, the symbol points in the direction of the allowed current.
• An ideal diode acts as a switch, offering no resistance when the potential difference is positive and infinite resistance when it is negative.
• A real diode has a threshold voltage below which the current is small and does allow a small amount of current when the potential difference is negative. It will also break down and allow a large current when there is a large negative potential difference.