What is the 1N34A crystal diode?
The 1N34A is a point contact germanium diode that was first introduced in the 1940s. It is often used by hobbyists in the detector stage of a crystal radio. Recall that the crystal radio does not contain a battery or other power source. Instead, the energy to drive the headphones is derived from the long-wire antenna. Consequently, a diode detector with good sensitivity (low threshold voltage) such as the 1N34A was required.
The 1N34A was once common but has become increasingly scarce. Devices such as the ones pictured in Figure 1 (background) could be purchased at the local radio shop. Today, germanium-based semiconductors are hard to find as silicon is the preferred technology. Most are listed as obsolete. The only exception at DigiKey is offered by NTE Electronics. It has zero stock at the time of this writing.
In this engineering brief we will identify a modern, readily available, alternative for the classic 1N34A. We will classify the replacement diode based on threshold voltage as measured by the Digilent Analog Discovery and the Digilent transistor tester as shown in Figure 2.
You may also be interested in the galena cat whisker detector. The galena detector predates the 1N34A with roots back to the earliest days of radio.
Figure 1 Image of BAT46 diodes (foreground) and antique 1N34A (background).
Tech Tip: The older germanium devices are heat sensitive as the crystal will melt at a low temperature relative to silicon. The risk of melting the crystal is high when soldering. Be sure to use a heatsink such as a hemostat clamped close to the body of your germanium diodes and transistors.
Figure 2: The Digilent Analog Discovery with Transistor Tester (sold separately) is used to characterize a BAT46 diode.
What are the consideration for selecting a 1N34A substitution for a crystal radio?
There are many alternatives to the 1N34A. However, like the 1N34A, most of those germanium substitutions are also obsolete. This includes out-of-the-box solutions like using the PN junction (2 legs) of a germanium transistor. You may be able to locate small quantities on online auction and hamfests and radio conventions. At the same time, there are advantages to ordering diodes and other supplies from DigiKey.
Silicon-based devices such as the venerable 1N4148 small signal diode are readily available. However, as will be shown, the 1N4148 does not have the sensitivity of the 1N34A. Consequently, the performance will be subpar in a crystal radio. Weak and distant radio stations are unlikely to be heard, as the received signal may not have sufficient voltage to reach the 1N4148 relative high threshold.
Schottky diodes are readily available and have low threshold voltages. The BAT46 as pictured in Figures 1 and 2 appears to be a good candidate.
Diode characteristics
A diode may be characterized via its curve on a Current Voltage (IV) plot as shown in in Figure 3. Here, the current is on the vertical axis and voltage on the horizontal.
Setup
The setup is trivial as the diode is simply inserted into the Digilent transistor tester as shown in Figure 2. The Digilent WaveForms software is very easy to use. The various settings are shown on the top portion of Figure 3.
Results
In Figure 3, the 1N34A gold standard is shown in green. We can see that germanium diode starts to conduct in the 0.2 VDC region. This is very different than the 1N4148 silicon diode which starts to conduct at about 0.5 VDC.
The BAT46 diode curve (blue) suggests that it is a good substitution for the 1N34A.
We also include the curve for the 1N5711. This is another Schottky diode that is occasionally mentioned as a substitution for the 1N34A. While its threshold in the 0.3 VDC region is better than the 1N4148, it is not as good as the BAT46.
Figure 3: Representative diode curves for the BAT46 (blue), 1N34A (green), 1N5711 (purple), and 1N4148 (red). The curve for the BAT46 is nearly identical to the 1N34A.
Tech Tip: Each curve in Figure 3 represents the data from a single diode. Please use care when interpreting this data, as there will be variation between devices and between manufacturers. This caution is especially important in critical application such as the crystal radios, where performance is highly corelated with the threshold voltage.
Bias for increased sensitivity
Before departing, we should mention that the curves in Figure 3 may be shifted to the left for increased sensitivity. This requires a departure from the purity of the crystal radio by adding a battery. However, with a battery you can bias the diode. For example, with a 0.4 VDC bias the 1N4148 detector has the potential to be more sensitive than the 1N34A.
Then again, if you have the source, why not use a transistor for increased amplification? If it helps ease your mind, you could make your own batteries. 
Parting thoughts
Please share your crystal radio diode replacement experiences in the space below. Let us know if you were successful with the BAT46? Also, include pictures of your handywork and ingenuity as you follow the traditions of crystal radio construction.
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.