How does EFD108 work?
Because they have very small capacity (tenths of pF) and very low opening voltage, are mainly used for "recovery" (detection) of high frequency (up to hundreds of MHz) and low level signals.
Unlike rectifier diodes, Schottky diodes (with silicon) they have no PN junctions, but works based on the unidirectional conduction effect at the metal-semiconductor contact.
Constructively, they contain an N-type doped silicon plate, on which a metal layer of aluminum is deposited. However, they have a much lower breakdown voltage (reverse), of the order of tens of volts, compared to more than 1000V in the case of diodes with PN junction.
What are the advantages of using EFD108 diodes?
Their main advantage is saturated-locked switching time much smaller than in the case of ordinary diodes, as well as a lower voltage drop in direct conduction.
That is why germanium diodes are preferred in the rectification of pulses with high frequencies (as in the case of switching stabilizers), but also for the detection of signals with very high frequencies, up to the microwave range.
The differentiation of the two fields of application is made by the geometry of the structure, including the physical dimensions of the metal-semiconductor contact, but also by the nature of the "impurities" with which it is doped with Si-N.
Where and how were the EFD108 diodes made?
Diodes EFD108 were manufactured at IPRS Baneasa, Bucharest, Romania in the 70's. The original manufacturing patent is owned by the company SGS-THOMSON Microelectronics, now STMicroelectronics, and the original name is SFD108.
They are made on a plate of germanium, but after encapsulation undergoes a process of "formation" by applying short pulses of high current, after which atoms in the metal wire (tungsten) diffuse into germanium and thus form a "micro-junction", much dimensional smaller than one made by alloying.
That is why its capacity is much smaller. And how the mobility of charge carriers in germanium is higher than in silicon, we understand why point diodes with germanium can work up to such high frequencies.
The structure of a diode with point contact
For those who still have such diodes in "dowries", here they have the marking code with colored rings, although they all work the same, the only difference being the maximum voltage supported:
Encoding by colored rings at IPRS point diodes (read from cathode to anode):
- AA112 white - red 25V, 20mA;
- AA114 orange - 25V, 20mA;
- AA117 black - yellow 100V, 20mA;
- AA118 black - 100V orange, 20mA;
- AA131 green - 40V orange, 20mA;
- EFD103 red - blue - yellow 30V, 20mA;
- EFD105 green - red 30V, 20mA;
- EFD106 gray - red 25V, 20mA;
- EFD107 white - yellow 15V, 20mA;
- EFD108 orange - yellow 100V, 20mA;
- EFD109 orange - yellow - black 100V, 20mA;
- EFD110 orange - 35V blue, 20mA;
- EFD115 gray (gray) 45V, 20mA;
- 1N54A black - green - yellow 80V, 20mA;
- 1N541 green - yellow - brown 45V, 20mA;
- 1N542 green - yellow - red 45V, 20mA.
Detailed picture of the EFD115 diode, 45V, 20mA, made by IPRS Baneasa
There are not many recent manufacturing equivalents. Most diodes today are made of silicon. Alternatively you can use a junction in a transistor EFT307, EFT317, EFT319 if the purpose of the application is for RF detection. But EFTs have been around for a long time. But you can get it from an older radio.
The closest to the features are 1N63, 1N34A, or uses a Schottky diode, because it has an internal fall close to the diodes on germanium. E.g 1N5817. You can definitely find this one.
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