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How to Identify an Old Ge Transistor

Pure germanium and pure silicon form perfect covalent bonds, giving way to semiconductor electron behavior. When these compounds are modified with intentional impurities, they can be used to create transistors, or electrical components used as switches and amplifiers. The first transistors were made from germanium, but were replaced with silicon transistors largely because of reduced cost and higher temperature stability. However, salvaged germanium transistors are still available if you have a device or project in which it would be a better transistor choice than silicon.

Part Numbers
- You can identify all transistors by their part number. In the standard European system of component numbers, a part number that begins with the letter "A" denotes that the component is made of germanium. If the second letter is "C," "D" or "U," the component is a transistor. In the Joint Electron Device Engineering Council numbering system, all transistors begin with the number "2," followed by the letter "N." Beyond this, there is no numeric distinction between germanium and silicon, so you must look at the data sheet for each part number to determine its material. Among the JEDEC germanium transistors are 2N404, 2N527, 2N388 and 2N1309.
Bias Voltage
- Transistors can function as electronic switches. Once the transistor is supplied with a certain voltage, current flows; otherwise, the transistor does not conduct. The voltage required to turn a transistor "on" is called the bias voltage. Silicon transistors have a bias voltage of 0.7 volts, whereas germanium transistors have a bias voltage of 0.3 volts. Therefore, if you have a transistor of unknown origin in a circuit, test the bias voltage to see if current flowing through the transistor is at 0.3 volts.
Manufacturing History
- The first junction-based transistors were developed in the late 1940s at Bell Labs. These transistors were made of germanium. The silicon transistor was developed by Texas Instruments in 1954. Between the mid 1950s and late 1960s, transistors were made from both materials. Eventually, silicon transistors became more popular due to the difficulties of obtaining germanium and the higher heat threshold of silicon. However, most popular electronic devices from the 1960s still used germanium transistors. This is especially the case for musical equipment such as effect pedals and electric organs. Therefore, if you are inspecting the transistors of devices such as these, it is likely that they are made from germanium.
Characteristics
- While both germanium and silicon transistors work based on the same principles of atomic theory and electricity, they have distinct differences, especially in the realm of audio circuitry. When used in overdrive and fuzz effect pedals, germanium transistors give a warmer output than their silicon counterparts. In addition, for some engineers, musicians and other circuit enthusiasts, the temperature instability of germanium transistors is actually an asset because it gives each transistor a different character.

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