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Transistors - die pictures

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Yeah, they definitely had some problems getting the bonds robust.  :palm:

I had one minor mistake:
The BSY34 and the SSY20 were designed for ferrite core memory, not for magnetic bubble memory.


The 2N2646 is a unijunction transistor. In contrast to the so-called programmable unijunction transistors such as the 2N6027 (https://www.richis-lab.de/Bipolar14.htm), the 2N2646 is a real unijunction transistor, i.e. it contains just one junction. A manufacturer cannot be determined. The numbers 0248 could be a date code. The component would therefore have been manufactured in 2002.

The pictures above are taken from the book "Beyond the Transistor: 133 Electronics Projects" and show, among other things, the structure of a unijunction transistor. It consists of an n-doped elongated element. The ends of this element are labelled base1 and base2. A p-type doping is inserted and contacted at the side. This connection is called the emitter. The n-doped element represents two resistors, between which a diode is formed at the emitter connection. If a voltage is applied between the two base connections, a certain potential is established at the emitter depending on the position of the p-doping.

The characteristic curve is shown in simplified form. It shows the voltage Ve between the emitter and the lower base as a function of the emitter current. The behaviour is easier to understand if Ve is varied and the resulting current is observed. As long as the potential applied to the emitter from the outside is lower than the potential set by the voltage divider between base 1 and base 2, the pn junction is isolating the emitter. Just a very small leakage current (1-2) flows into the emitter. If the pn junction becomes conductive, more current can flow into the emitter. At first glance, the behaviour of the voltage Ve is unusual, as it decreases in this area as the current increases (2-3).

There is only n-doped material between the base contacts. The current flow in the first part of the characteristic curve can therefore only take place via free electrons. As soon as current flows into the emitter, additional positive charge carriers ("holes") flow from the p-doped area into the unijunction transistor. The additional transport mechanism reduces the resistance between the base contacts.

The unijunction transistor can therefore represent a negative resistance. This feature was used in the past to build oscillators with little component effort. Another advantage was certainly that the structures are less complicated than those of a transistor and were therefore probably easier to manufacture.

In February 1968 (Volume 41), the journal Electronics describes the development of the design. The first unijunction transistors consisted of an n-doped cuboid into which the emitter contact and thus a p-doping was alloyed at the same time. This principle was further improved until the unijunction transistors could be manufactured using planar technology and finally integrated with other circuit components.

This 2N2646 contains a modern-looking die. What is unusual is that the bond connections on the pins have been additionally protected with a type of varnish.

The edge length of the die is 0,45 mm. A manufacturer cannot be determined here either. Just the Cyrillic letters КБB (KBV) in the top right-hand corner provide a clue. For the simple design of a unijunction transistor, the structures on the die are surprisingly complex.

It is certain that the emitter is contacted from the left and the base1 from the right. The potential of base 2 is tapped via the housing, i.e. via the substrate. The reddish area under the emitter contact must contain a p-type doping. It appears that three possible emitters have been integrated here. There must be a strong n-doping under the smaller central base contact, which ensures a low-resistance connection of the n-doped area. On closer inspection, you can see that the three possible emitters have different distances to the base 1 contact. If the emitter bondwire is placed differently, the properties of the component can be modified.

The contact in the bottom left-hand corner appears to be relatively deep. It is most likely a connection to the substrate and therefore to base2. Thin lines lead to the emitter areas. Probably the lines makes it possible to connect unused emitter areas to the base2 potential. The low base2 potential guarantees that no charge carriers flow from the inactive emitter surfaces into the active area.




The BC109 is a low-noise transistor from the first generation of the BCxxx family. The maximum collector emitter voltage is 25V. Collector currents of up to 200mA are permissible. The cut-off frequency is specified at more than 100MHz. The amplification factor of the transistors lies between 200 and 800, with index B limiting the range to 200-450. The manufacturer of this transistor cannot be identified.

The dimensions of the die are 0,40mm x 0,39mm. The base contact only covers half of the emitter area.




The КT117 is a unijunction transistor, as described in more detail in the 2N2646. The logo belongs to the Russian Ulyanovsk radio tube plant and had to be rotated 90° anti-clockwise on the small housing. The index, here a B (Latin V), shows which resistance can be expected between the two base connections. There are a total of four bins with two different resistance values and two different factors by which the resistance value can change.

The housing contains a die with an edge length of 1,13 mm. A transparent potting protects the semiconductor.

This is the classic structure of a unijunction transistor, as documented in the 2N2646. The bottom of the n-doped substrate represents the base contact B1. The right-hand bondpad forms the second base contact. The left-hand bondwire transmits the emitter potential.

At first glance, one might have thought that the right-hand area was the p-doped emitter. This is obviously not the case here. No edge can be recognised around the emitter bondpad. It can therefore be assumed that the entire area around the bondpad B2 contains a p-type doping. Two frame structures can be recognised under and next to the base bondpad. Low-resistance contacting of an n-doped surface requires a local strong n-doping. The inside of the two edges shows the area that contains this stronger n-doping. The outer edge is a window within the emitter surface and exposes the n-doped substrate. The outer frame most likely contains an n-doping and thus represents a clean termination of the emitter surface.




The PNP germanium transistor AF201 built by Siemens is a high-frequency transistor with a cut-off frequency of at least 100 MHz. The blocking voltage is 25V. The maximum permissible collector current is 10mA. The AF201 can dissipate up to 225mW. The junction temperature must not exceed 90°C.

The AF200 is also described on the same datasheet. The AF200 and the AF201 are obviously two grades of the same transistor type. The guaranteed current amplification of the AF201 is slightly lower than that of the AF200 at a factor of 20 and the feedback capacitance is slightly higher (maximum 0,7pF).

The housing contains a white compound that protects the structures from environmental conditions and improves heat dissipation towards the housing.

The AF201 has four pins. In addition to the collector, base and emitter, one pin contacts the housing exclusively. The transistor is located on a carrier insulated from the housing, which is welded to the collector pin.

A kind of protective lacquer is applied to the die. Due to the varying thickness of the lacquer, the interferences in the layer create a repeating rainbow pattern. The edge length of the die is 0,5mm. The structure of this type of high-frequency transistor is described in more detail in the 2N1561 (https://www.richis-lab.de/BipolarA47.htm). The area around the active area has been etched down so that just a pedestal with an edge length of 0,1 mm remains. This measure reduces the parasitic capacitances. The base is diffused into the pedestal. On top of the pedestal is a base contact and the emitter, both alloyed into the base.




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