Semiconductor diode – type of diode, which contains “p-n junction” made of semiconductor materials. It’s double-ended, nonlinear element where the end attached to the “p” layer (+) is called anode and “n” layer (–) – cathode. The main advantage of this electronic component is current flowing only in one direction (from anode to the cathode) after forward-biasing the p-n junction.
However, in the opposite direction (reverse bias) current doesn’t flow and we could call this as the electric valve. The diode is constructed from two semiconductors, which were doped in different proportions – “p” and “n” types. They together form the p-n junction. The “n” layer with donor dopants has an excess of electrons where the “p” layer (acceptor dopants) – have a deficiency of them. After combining these on semiconductor diode together, a proportional distribution of electrons begins. Electrons, which previously lacked in “p” layer are transferred there from “n” layer. In this way region called depletion region that prevents the electric flow is formed. With passing time, technological requirements were increasing what resulted in the formation 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 Schottky diode. 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 short switching time (small junction capacitance Cj of the diode has a decisive impact) with frequencies up to several tens of GHz.
Semiconductor diode characteristics
The graph below shows the current-voltage characteristics of a semiconductor diode. The graph is standard characteristic for electronic semiconductors. The semiconductor diode is switched to the active state after exceeding the forward voltage value specified by the manufacturer in the data sheet. Semi-thermal diodes are mainly used to protect other electronic components.
Illustration of differently biased P-N junctions:
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 two most popular that can be used with cheap 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)
- The 9V battery with wires
- LED diode (remember, LED’s are also in the family of semiconductor diodes!) (GET LED wired Red Rectangular 2 x 5 mm 8 mcd)
- 1kΩ resistor (GET 1k Resistor)
We will be using two schemes you’ve seen earlier:
Below you can see pictures showing the circuit mounted on the breadboard and visualization in two opposite positions of the LED diode (reverse polarity).
On the first picture, the LED was put into conducting state. Voltage on her anode was higher (+) than at the cathode (-), so the flow of current was possible. In our experiment we used a 9V battery, so the current flowing through the diode will be about 9mA (from Ohm’s law).
On the second picture, the diode was inserted in the opposite way (the voltage at the cathode was higher (+) than at the anode (-)), so the diode entered a blocking state, 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 systems, also used in circuits
- Light Emitting Diode (LED) – emits light in the infrared or visible zone,
- Variable capacitance diode – its capacity depends on the voltage applied in the reverse bias,
- Switching diode – used in pulse systems that are requiring very short switching times,
- Tunnel diode – specially designed diode characterized by a section of negative dynamic resistance,
- Photodiode – diode that works as photodetector – it reacts to light radiation (visible, infrared or ultraviolet),
- Gunn Diode – the component used in high-frequency electronics.