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A project log for Germanium ECL

How fast can germanium transistors compute ? And how much current will that draw ?

Yann Guidon / YGDESYann Guidon / YGDES 01/19/2021 at 03:210 Comments

The previous log found a weird and unexpected behaviour, a sort of parasitic diode, in a high-speed low-voltage low-power germanium transistor.

So let's see if this hypothesis holds for other types of transistors and I can test other high speed parts I collected in the last years.

Let's start with the classic MPSH81 : 600MHz, PNP, 50mA, 20V, hFE>60 (@10V 50mA=>Ib=1mA), Nothing to see here but it's a sort of baseline and you get the idea.


hFEVbe (mV)Ic (mA)
11197311.1
21277351.2
31287271.2
41217281.1
51297271.2

Current is recommended in the 1 to 30mA range, and frequencies up to 250MHz.

Collector-Emitter Breakdown Voltage is 20V, which is probably out of range of the tester.

I only have about a hundred pieces but it's enough for simple tests.


Then we have the BF970: Silicon PNP, Ft: 900M to 1GHz, 35V, 30mA, hFE>25. At this voltage, no breakdown should be visible:


hFEVbe (mV)Ic (mA)
1427070.39
2426970.39
3416980.38
4416970.38
5446970.41

That sample looks well binned :-)


So far, nothing unusual, the leakage is not detected and the operating voltage is high enough. So let's turn to Germanium with parts in the A series.

AF439 is a Germanium PNP rated at 15V, 10mA, GBWP=400MHz (or 800 depending on the sources) and hFE>10. Bingo !


hFEVbe (mV)Ic (mA)Drop (V)Ice0
11044261.96
21043561.93
31043861.57
494345.91.69
594345.91.76
694375.91.64
784315.91.65
81143161.94
91043361.80
101143361.67

Same behaviour with a pair of AF280 ! (15V, 10mA, 60mW, 550MHz)


hFEVbe (mV)Ic (mA)Drop (V)Ice0 (mA)
11134261.520.12
21246561.380.001

As the Vbe decreases, the Ice0 increases...

I don't want to bend more TO-50 pins for a reference where I have so few parts, but I think I made my point.


Back to some old good AF138:
PNP germanium diffusion alloy ... 12V, 10mA, hFE>60


hFEVbe (mV)Ic (mA) Drop (V)Ice0 (mA) Ices
1
404
258
4.3
2.03
0.27

2
198
249
2.3
1.99
0.36

3
222
244
2.9
2.20
0.7
4
44
270
6.6
2.13
0.002

5
322
254
4.1
2.44
0.86

6
66
314
6.6
2.6
0.11

7
277
257
3.3
2.48
0.5

8
265
243
3.6
2.1
0.92
9
321
264
3.8
2.19
0,63

10
312
259
3.8
2.17
0.66

As noted before, higher leakage could be mistaken for higher gain.

I wonder what Ices is... Oh and number 4 is out of spec ;-)

Despite its leakage and the inability to work as pass transistor, I kind of like this reference, of which I have 1.6K which allows me to build some interesting digital devices, probably slow and medium I/O ports and interfaces, probably using DCTL.


And now, the AF306:

GBW: 100 to 500MHz (?), 15mA, 18V


hFEVbe (mV)Ic (mA) Drop (V)Ice0 (mA)
1322930.342.1214µ
2162770.182.4621µ
3163936.22.83
435
2980.362.5618µ
5332880.322,0331µ
6133916.12.91

In their TO-92-like package and with only 50 of them, I don't know what I'd do with them, as they are less consistent than the other references.


And AF178 :

PNP 25V, 0.01A, 80-200MHz, hFE>20


hFEVbe (mV)Ic (mA) Drop (V)Ice0 (mA)
1
122
307
1.3
2.190.72
2
248
308
2.6
2.110.11
3
122
302
1.3
2.380.11
4
117
299
1.2
2.330.041
5
79
278
0.85
2.170.058

I don't see how to best use them, though I have a hundred of them... Their current is too low, only the voltage is higher.


The AF200 might be a precursor to the AF240. I can't find its frequency but it's a mesa type.

10mA, 25V, hFE > 30, ft=10MHz ??


hFEVbe (mV)Ic (mA) Drop (V)
Ice0 (mA)
1
16
416
6.2
2.28
2
14
278
0.15
2.38
3
20
293
0.2
2.28
4
12
259
0.13
2.47
5
22
287
0.23
2.52
6
13
283
0.14
2.34
7
13
269
0.14
2.57
8
20
409
6.2
2.73
9
18
294
0.18
2.36
10
20
288
0.2
2.39

This generation starts to solve some problems of the previous Ge generations, in particular very low leakage without raising Vbe too much. But the gain drops...

.

My hypothesis that the "parasitic diode" is structural with a type of Ge transistors seems to hold water. It might be due to a very low breakdown voltage, a non-controller Zener effect that could be due to the need for higher speeds and thinner junctions...

The reverse drop depends on the family so it should be related to the fabrication process.

Now, I don't have more Ge trannies but some interesting ultrafast silicon ones, such as the BFG425 which is rated at 4.5V max for 25GHz@20mA. I wonder if the same diode appears again... But does it matter for pure ECL topologies ?

-------------------------------------------------

This mystery is solved and as usual, I should have asked Google first, and it's always a matter of asking

the right questions....

https://northcoastsynthesis.com/news/transistors-for-the-perplexed/ explains this very well : this value is called Veb0 and this is inherent in the construction of all transistors, though it is more apparent in Ge because the bandgap is narrower than silicon : Silicon is in the 5V+ range where Ge is in 1.5-3V range. This is random avalanche effect often used in noise generators, such as explained at https://electronics.stackexchange.com/questions/207192/white-noise-generator-transistor-voltage-question.

Of course, even Wikipedia mentions it.

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