Germanium diodes were used for Radar during WW2. Germanium transistors appeared about a decade later. Most early transistors were only specified for audio use, like the OC71. The specified RF types like the OC44 were only good for a few MHz. Why was there such a speed difference?
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6\$\begingroup\$ I feel that focusing specifically on germanium in this question is a mistake. For example, photodiodes are still much faster than phototransistors for any given material used. Perhaps the question should be "why are the fastest diodes faster than the fastest transistors?" \$\endgroup\$– DKNguyenOct 10, 2019 at 3:56
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8\$\begingroup\$ Fabrication techniques were quite different in those early days so a transistor was quite an elaborate structure, much more complex and larger than a diode. Local diffusion techniques did not exist yet so my guess would be that transistors were much larger and therefore slower than diodes. Diodes could be made using a point contact which results in a small PN-junction. For some information on how semiconductors were made in the past read: sites.google.com/site/transistorhistory/Home/… This is my guess I could be wrong. \$\endgroup\$– BimpelrekkieOct 10, 2019 at 6:51
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3\$\begingroup\$ It's not "germanium transistors" that were slow. Rather, early transistors were. \$\endgroup\$– Dmitry GrigoryevOct 11, 2019 at 12:19
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Major factors affecting bipolar transistor frequency response include:-
- minority carrier transit time
- junction resistance
- junction capacitance
These can all be reduced by reducing the physical dimensions of the transistor. However in the early days this was not so easy.
The first transistors used point-contact junctions, which themselves had small dimensions (which is why point-contact diodes work well at high frequencies). However carrier transit time between Collector and Emitter was proportional to the cube of their separation, so performance diminished rapidly as distance increased. To get a cutoff frequency of 20MHz required that the contact points be only 1 mil (0.025mm) apart, which was difficult to achieve and maintain.
Inside 2N110 Point Contact Transistor:-
Diffused alloy junction transistors were more robust and easier to manufacture, but had larger junctions that increased capacitance, and risked shorting out if the diffusions got too close together. Again, high frequency performance could be improved by reducing dimensions, but there was still a limit to how small the structure could be made.
The photo below shows the insides of two popular Germanium transistors. In the AC128 (an audio frequency transistor rated at 1A) we can clearly see the rather large Emitter and Collector pellets on either side of the Base. On the AF106 (an RF transistor used in TV tuners) the semiconductor part (hidden under a protective coating) is much smaller. This transistor uses 'mesa' construction to reduce the active area. It can only handle 10mA, but maximum oscillation frequency is 1.2GHz.
answered Oct 10, 2019 at 8:54
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8\$\begingroup\$ Thank you for the good question! I learned a lot about the history of transistors today. \$\endgroup\$ Oct 10, 2019 at 10:00
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1\$\begingroup\$ Also, transit frequency does not tell the whole story - a BC548 has 300MHz, but try to use it at VHF given the high miller capacitance.... \$\endgroup\$ Oct 10, 2019 at 13:10
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6\$\begingroup\$ @rackandboneman True, but... in the early 1970's I designed my own 27MHz radio control transmitter (since as a poor teenager I couldn't afford to buy one). Read various books that said I needed expensive rf transistors. Imagine my surprise to find that the lowly BC209 had an ft of 150MHz! I got ~1/2 Watt of rf and a mile of range out it with a super-regenerative receiver using the same transistor - all built on Veroboard (another no-no according to the experts). \$\endgroup\$ Oct 10, 2019 at 16:57