last updated: 2022-010-12
Song of this chapter: Aerosmith > Aerosmith > One way street
After the discovery of rectifying properties and the Edison effect, and the development of diodes (both vacuum-tube and semiconductor) and triodes (vacuum-tube), it was possible to get a DC voltage and current from an AC source, and to build radio receiver detectors and amplifiers like the crystal detector used in early 20th century radio receivers.
In the midst of the 20 century semiconductor diodes and transistors based on germanium and silicon replaced the vacuum tubes, consuming less energy and volume. Now it was possible to produce portable electronic devices. Today most diodes and transistors are based on silicon, but germanium and gallium arsenide are also used.
Song of this chapter: Aerosmith > Aerosmith > One way street
Diodes are non-linear components and they are always polarized!
They let current flow in only one direction! For this they have to be forward-biased, meaning there is a positive potential on one terminal, called anode and a negative potential on the other terminal called cathode. The potential difference must be greater than approximately 0.7 V for silicon diodes (0.3 V for germanium diodes and 0.2 V for Schottky diodes).
In the water analogy we have a one-way valve. A spring shows that we need a certain pressure (voltage) to open the door if the water flow is in forward direction.
The symbol clearly shows the forward direction with an arrow. The cathode is shown with a line. In cylindrical housings we have a ring showing the cathode, on surface mount diodes (SMD) it's a line.
Standard LED's have often a longer leg indicating the anode pin or a flat edge on the LED’s outer casing showing the cathode pin. An easy way is also to use the diode function on a multi-meter to test for polarity (see later).
At a greater voltage than 0.7 V the resistance of a silicon diode will get very low. With no limit for the current, the diode will eventually melt from the heat generated. So we have to include some other limiting component (e.g. resistor) to the circuit.
Silicon is basically found in sand and it's the the eighth most common element in the universe by mass. In it's pure form it's a hard and brittle crystalline solid. At -273 °C it's an insulator. But at 20 °C (+293K) we get a semiconductor because bonds in the crystal are breaking and we get free electrons.
By adding impurities to the pure silicon, different properties can be realized. Doping silicon with per example phosphorus introduces extra electrons. We get an n-type semiconductor that has extra electrons that can wander around the crystalline structure of the silicon. Doping silicon with per example boron creates a p-type semiconductor with missing electrons (holes) to complete the crystalline bonds.
Joining n-type silicon to p-type silicon creates a p-n junction. This p-n junction acts as a diode, or if the dimensions and doping are right as a photo cell.
The junction between the types creates a barrier to the electrons and holes (because they are recombining we get an insulating zone), the depletion zone. In forward-bias direction a positive voltage on the anode (p-type), pulls the electrons to the junction and a negative voltage on the cathode (n-type) the holes. The depletion zone gets smaller. When the voltage difference is big enough the depletion zone disappears. The current can flow freely across the junction.
Reverse-biasing the p-n junction only widens the depletion zone. If the reverse-bias voltage difference is too high, the junction will heat up and we get an electrical_breakdown. This can destroy the p-n-junction.
In the last decades electronics moved dramatically from analogue to digital. Two elementary digital circuits are the
OR gate and the
AND gate. We will use the positive logic, so the two digital states
false will be represented with
0V). Two inputs for the gate will give 2² = 4 possibilities or cases (3 inputs would give 2³ = 8 cases).
OR is only
false if none of the conditions are fulfilled. If I drink water
OR lemonade I won't be thirsty any more. It's not an exclusive OR because I can also drink both drinks, even mixed :). This gives us the following truth table:
AND is only
true if both conditions are fulfilled. If I have money
AND my girlfriend has time we will go to the cinema.
ORgate and an
ANDgate. The output will be visualized with a light emitting diode. Build the first circuit and document all 4 states by drawing the 4 circuits with their corresponding measured potentials and currents. Explain in detail how the circuit is working.
Do you remember the last exercise in the second chapter (Ohm's law)? We measured the I-U-diagram of three different resistors and of our diode 1N4148.
The resistor has a linear characteristic! You can see in the I=f(U) diagram that when the voltage is doubled the corresponding current doubles. This is true for both directions, as the resistor is not polarized. Here is the straight from a resistor (1kΩ) in a complete diagram. In fact we combine two diagrams, one in forward-bias and one reverse-bias.
Let's look at the diagram of a diode 1N4148. The 1N4148 is a standard silicon small signal fast switching diode. It's very often used in circuits, because of it's switching capabilities up to about 100 MHz. Because of it's popularity there are more than one manufacturer and also slightly different data-sheets.
Both diagrams are non-linear. Watch for the axis, they are not the same!! The diode is used in forward direction. Reverse biased it will break down (it will be destroyed) at 75 - 100 V.
Between the two knees of the curve (UF and UBR) the current is practically zero. If the voltage gets higher than UF = 0.7 V the diode is getting conductive and the current is very soon reaching it's maximum (about 200 mA). That's why we need a another component in series (resistor) to limit the current and prevent the diode from being destroyed.
The other knee of the curve shows at witch voltage the diode breaks down (is also destroyed except for Zener diodes (see later)). This happens at UBR = 75 V-100 V.
As our characteristic curve is non-linear, it is difficult to calculate the currents and voltages in a circuit. Also the real curve may differ from the curve in our data sheet. A simple possibility is to measure the curve, draw a diagram and read the working point of the circuit from that diagram.
Here is an example for a diode 1N4148 in series with a resistor. The curve from the resistor is mirrored to get the working point of the circuit. Point 1 to draw the straight of the resistor is on the current axes an is calculated with the total voltage of the circuit and the resistance of the resistor IP1 = U/R. Point 2 is the total voltage on the voltage axis. The intersection of the two curves (working point delivers the results (UD,UR and I).
Diodes are used to produce with the help of capacitors a DC voltage from an AC source.
Test the following circuit. The circuit is called a half-wave rectifier. Either the positive or negative half of the AC wave is passed, while the other half is blocked. Document the output voltage and the voltage on the diode (Math function!).
What does the current look like? Calculate the peak current.
We get a pulsating direct current. With the help of a big capacitor we can filter the AC frequency from the output and plane the voltage. Add a capacitor of 4700 µF (16 V) in parallel to the output. Document the output voltage with the oscilloscope.
URMS = f(Upeak).
The arrangement of four diodes is called a diode bridge or a Graetz bridge, and is often used in a single four-terminal component.
Meant for low voltage and low current use. See our 1N4148 signal diode. They often have a small glass case.
They can handle higher currents and are used per example for rectification in power supplies. Power diodes are larger and typically in heavy plastic or metal bodies.
Zener diodes have a quite precise breakdown voltage. Zener diodes are available with a large number of different breakdown voltages. You can use a reverse-biased Zener diode in series with a current-limiting resistor (acting like a constant current source) to regulate a voltage to the Zener (breakdown) voltage.
Light-emitting diodes are perhaps the best known diodes. LED's emit light (visible or otherwise) when forward-biased. The semiconductor material and the exact impurities used to create the p-n-junction determine the wavelength of the light (color). This also determines the forward voltage. This can vary from 1,6 V (red) to 4 V (white). The breakdown voltage of LED's is not very high, so pay attention if using LED's in AC circuits. Brightness of LED's can easily be changed with a PWM. High-power LED's (for illumination) are best driven with a current source circuit to achieve correct current regulation (see LED circuit).
Photodiodes will get conductive when enough photons (light) reaches the p-n-junction. The materials used to create the p-n-junction will determine at what range of the spectrum the diode is sensitive to.
The electricity available at our mains outlet is AC 230 V/50 Hz (URMS). Most electronic devices need a DC voltage from 3.3 V-24 V. Power rectification diodes in combination with transformers, capacitors and regulation circuits allow us to power electronic circuits from mains outlets.
When the power is turned off from inductors (electromagnets, solenoids, motors) they induce a reverse current that can generate a very high voltage. A diode is used to drain that current off and reduce that voltage, so the controlling circuit (typically a transistor) will not be destroyed. This is called "back EMF protection" and the used diodes "flyback diode".
Diodes are protecting circuits from connecting power with incorrect polarity (e.g.batteries). This reverse polarity protection can be done with one single diode in series to the circuit.
If you want make sure that a signal never exceeds a certain voltage, diodes can be used as a voltage clamp (or clamper).
Song of this chapter: Darren Korb > Transistor (original Soundtrack of the game)
In the year 1947 three researchers at Bell Labs invented the "transistor" (from "transresistance"). Before this fragile, big and power-hungry vacuum tubes were used for signal amplification and switching. Without the transistor the revolution in electronics and more specific in digital electronics that took place would not have been possible.
Transistors have three terminals. One terminal is used to control with a small current or a voltage a large current flowing between two other terminals. This makes the device capable of amplification or switching.
In this course we will only look at the transistor as switch.
There are two types of transistors: Bipolar Junction Transistors (
BJT), and Field-Effect Transistors (
FET). The FET's can be divides in two groups, the Junction Field-Effect Transistor (
JFET) and the Metal-Oxide-Semiconductor Field-Effect Transistor (
Some symbols of different transistors:
Bipolar Junction Transistor (BJT) NPN, BJT PNP and Junction Field Effect Transistor (JFET) depletion type N-channel
Metal-Oxide-Semiconducter FET (MOSFET) enhancment type N-channel 4 terminal, MOSFET depletion type P-channel without substrate and MOSFET enhancment type N-channel subtrate connected internally with gate
Bipolar junction transistor (BJT) use electrons and electron holes as charge carriers (unipolar transistor (e.g. FET) use only one kind of charge carrier).
As seen above, joining n-type silicon to p-type silicon creates a p-n junction. This p-n junction acts as a diode. BJT use two junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material.
They exist as
PNP types. The e.g. p-doped region in the middle for the NPN transistor is very thin to make the transistor work. So it is not possible to build a transistor with 2 diodes.
The three terminals are called Base, Collector and Emitter. The base current is controlling the main current from Collector to Emitter (resp. Emitter to Collector).
The water analogy can help to understand how a BJT is working:
The field-effect transistor (
FET) uses an electric field to control the flow of current.
The terminals for the FET are called: Gate, Source and Drain. The gate voltage controls the current. In limits we can look at the FET as a voltage controlled resistor.
Field effect transistors have a very high input impedance at low frequencies. The most widely used field-effect transistor is the Metal-Oxide-Semiconductor Field-Effect Transistor (
MOSFET or IGFET).
In micro-controller circuits we often have to switch bigger loads than the controller can manage. Classical Arduino chips (ATmega328 or ATmega32u4) can deliver up to 40 mA per pin, ESP32 only 12 mA. Also the voltage that's needed by relays, motors, lamps etc. is not always 3.3 V or 5 V.
With transistors this problem can easily be managed.
Let's begin with an NPN BJT. It's important to choose the right voltages and currents for the transistor, so we have always to study the data sheet.
The circuit itself is quite simple:
Files > Examples > 01. Basics). Calculate the collector current
ICwhen the forward voltage of the LED is 1.7 V (UCE from data sheet). Measure the current
IC, the voltage on R2
UR2, on the LED
UDand on the transistor
Now we do the same thing with a N-Channel MOSFET 2N7000:
If we need to change the direction from a current through a device, we can use 4 transistor forming a H-bridge (so called because of the shape).
Here an example where the H-bridge is used to change the direction of a motor: