Multimeter: Diode (semiconductor) testing mode

The diode testing mode in a multimeter is designed to determine the voltage drop across a semiconductor junction.
This mode is indicated by a diode icon on the multimeter panel.
As is known, at a semiconductor junction in direct connection (positive potential to the anode of the diode, and negative to the cathode - see Fig. 1.) there is a small voltage drop (residual voltage).


Fig. 1 Connecting a diode to a multimeter in the forward direction (upcoming)

The value of the voltage drop across the diode when connected directly depends on the material of the semiconductor junction of the diode. For a silicon diode this value (typical) is 0.6-0.7 volts, and for germanium diodes it is 0.3-0.4 volts. There are other diodes (so-called Schottky diodes) for which this voltage drop is less - 0.2-0.4 volts.
Functionally, the diode testing mode in a multimeter is a voltmeter mode, but with an internal voltage source (battery) connected to the input. As a result, when the test leads are open or when connected to a working semiconductor (diode), when turned back on, the multimeter indicator shows an overload (OL).


Fig.2. Fragment of a multimeter circuit in diode testing mode (upcoming)

The voltage on the test leads in diode testing mode depends on the voltage of the multimeter's internal battery.
An interesting feature is obtained when dialing a white or blue LED. White LEDs are built on the basis of a blue InGaN crystal with a large voltage drop across the semiconductor junction - up to 3.4. volts and when the internal battery charge level is low (less than 3 volts), the working LED will light up (i.e. the LED is working), while the multimeter scale will show overload (OL), i.e. the subconductor junction is closed.
An example of such LED testing is given in the YouTube Video (2 minute mark)