Semiconductor diode – type of diode, which contains “p-n junction” made from differently doped semiconductor materials. It is double-ended, nonlinear electronic component, where the end attached to the “p” layer (+) is called anode and “n” layer (–) cathode. This electronic component is mainly used because of it’s ability of making electric current flow only in one direction (from anode to cathode) after forward-biasing the aforementioned “p-n junction” with the positive electric voltage.
However, in the opposite direction (reverse bias of the “p-n” junction with the negative electric voltage) we can say that in ideal semiconductor diode electric current won’t flow. This is why semiconductor diode is often described as the “electric valve”, which can pass or block the flow of the electric current.
Semiconductor diode consists of two, differently doped semiconductor crystals – “p” and “n” types. Together, they form so-called “p-n junction”, where the “n” layer (with electron donor dopants) has an excess amount of electrons, which are the majority carriers there (we have more electrons (-) than electron holes (+)). However, in the “p” layer (electron acceptor dopants) the majority carriers are electron holes (+) rather than electrons (-), so we have more holes “to fill”, than electrons available. The electron hole is a vacancy created by the electron “travelling” from its initial place to some other location in that crystal. In reality, there is no such thing as “a hole”, but that lack of electron kind of makes it a positively charged particle, which attracts negative electrons to form a pair again (holes can move too).
After they combine, a proportional distribution of electrons begins. Electrons, which previously lacked in “p” layer are transferred there from “n” layer, where were too many of them. So, “n” layer is a good friend for “p” layer, right? 🙂 And this is where so-called depletion region is formed, which prevents the flow of the electric current (thermodynamic equilibrium).
To allow the flow of the electric current through the “p-n junction” (electric valve on), external positive electric voltage must be applied to “push” and help large group of electrons and holes to meet together (forward bias of the diode). After they are “pushed” through the depletion region with enough force (Vf = 0,7V) diode starts conducting current, so it starts to flow through it.
To make sure, that the electric current won’t flow (electric valve off), it is needed to apply external negative voltage to the semiconductor diode (reverse bias) to make depletion region even larger (illustration below).
With passing time, technological requirements were increasing what resulted in development of new types of diodes. When a semiconductor is combined with the corresponding metal, we acquire MS junction (metal-semiconductor), which also possesses rectifying properties (current conduction in one direction) – it is used for example in fast Schottky diodes.
MS junctions can have one of two current-voltage characteristics:
- Unsymmetrical non-linear
- Symmetrical, linear
MS junction properties depend mainly on the surface state of semiconductor and on the output work difference of electrons from metal and semiconductor itself. Schottky diode is mainly used in systems that require fast switching time (small junction capacitance Cj of the diode has a decisive impact) with frequencies up to several tens of GHz.
Semiconductor diode characteristic
The graph below shows the current-voltage characteristic of the semiconductor diode. This is a typical characteristic for semiconductor diodes used in electronics (Vf = 0,7V). The semiconductor diode starts conducting current after exceeding the threshold of the forward voltage value specified by the manufacturer in the data sheet. Semi-thermal diodes are mainly used to protect other electronic components.
How to determine where is the anode and where is the cathode?
Simple multimeter can be used to determine the polarity of a diode. There are at least three ways to do this but I will show here two most popular ways that can be done even with the cheapest multimeters (GET Handheld multimeter Digital VOLTCRAFT VC-11 Calibrated to: Manufacturer’s standards CAT III 250 V Disp):
a) Using ohmmeter (2kΩ range):
You can also use “diode check” function (diode symbol on the multimeter) but the result will be the same as the above with using ohmmeter.
b) Using VDC measurement function:
Experiment for self-execution
This experiment will allow you to visualize the principle of operation of a semiconductor diode, whether current conducts or not. Because you will do it by yourself, you will better remember this lesson.
- breadboard (GET Breadboard slidable Total number of pins 840 (L x W x H) 172.7),
- 9V battery with wires,
- one LED diode (remember, LED’s are also in the family of semiconductor diodes!) (GET LED wired Red Rectangular 2 x 5 mm 8 mcd),
- one 1kΩ resistor (GET 1k Resistor)
We will be using two circuit schematics you’ve seen earlier:
Below you can see pictures showing the circuit mounted on the breadboard and visualization of two opposite positions of the LED diode (reverse polarity).
On the first picture, the LED was put into conducting state. Voltage potential at the anode was higher (+) than at the cathode (-), so the flow of the current was possible. In our experiment we used the 9V battery, so the current flowing through the diode will be about 9mA (calculated from the Ohm’s law).
On the second picture, the diode was inserted in the opposite way (the voltage potential at the cathode was higher (+) than at the anode (-)), so the diode was behaving like the closed electric valve, which prevented the flow of the current – LED isn’t lit.
Common types of semiconductor diodes (and their main functions):
- Rectifying diode – alternating current rectification,
- Stabilizing diode (Zener diode) – stabilization of voltage and current in electronic systems,
- Light Emitting Diode (LED) – emits light in the infrared or visible zone,
- Variable capacitance diode – its capacity depends on the voltage applied to it in the reverse bias,
- Switching diode – used in pulse electronic systems that require very fast switching times,
- Tunnel diode – specially designed diode characterized by the negative dynamic resistance region,
- Photodiode – diode that works as photodetector – it reacts to light radiation (visible, infrared or ultraviolet),
- Gunn Diode – the component used in high-frequency electronics.