Electronics > Projects, Designs, and Technical Stuff
Transistors - die pictures
Noopy:
The 2N2894 is a PNP transistor from National Semiconductor optimized for fast switching in the saturation region. Here you can see the bin with the index A, which is still a bit faster than the 2N2894. The A variant offers a cutoff frequency of at least 800MHz. The maximum turn-off time of 25ns is also worth mentioning. The blocking voltage is 12V. The current gain is typically 120 (150 for the 2N2894). The collector current must not exceed 200mA.
In the Discrete Databook from National Semiconductor there is a note that the 2N2984 is based on process 64. This is a process with a gold doping. Only transistors from process 65 (among others 2N4208) are faster, but with a maximum collector current of 50mA they are much less powerful.
If a bipolar transistor is operated in saturation, it contains very many free charge carriers. With an interruption of the base current, the number of free charge carriers is reduced continuously, but relatively slowly. As long as free charge carriers are still present, a collector current continues to flow. If you want to switch off bipolar transistors coming from the saturation region and you want to do it fast, you have to take additional measures to discharge free charge carriers. In the simplest case, the base and emitter are connected with a resistor through which the free charge carriers can exit the transistor. Such resistors are often found in Darlington transistors. Alternatively, the bipolar transistor can be driven with a push-pull stage to actively discharge the free charge carriers. In some cases it may be advantageous not to operate the transistor in saturation.
While manufacturing a transistor, one can optimize its intrinsic turn-off time by introducing gold doping. The gold atoms act as recombination points, which reduces the average lifetime of the free charge carriers. The IEEE article "Parasitic Effects in Microelectronic Circuits" deals with this technique. In this paper the influence of gold doping on parasitic effects is mainly considered, but the tables you can see here also show how the basic specifications are improved.
Process C contains gold doping and can be compared with process B, which has no gold doping. Process A lacks a buried collector. The lower table shows the reverse recovery times of the transistor junctions. The times are significantly shorter with gold doping. As a side effect, the leakage current into the substrate is also reduced, which is documented in the upper table. Due to the shorter lifetime, fewer free charge carriers reach the substrate.
It turns out that the structures on the die correspond to the illustration in National Semiconductor's Discrete Databook.
The transistor contains two emitter areas. The geometries could be interpreted as follows: The green area in the center contains the n-doping, which represents the base region. Within it are the two heavily p-doped emitter areas, which are hidden under the metal layer. The orange-red area represents the p-doped collector area, which is connected with low impedance to the substrate and to the package via a stronger, violet p-doping.
https://www.richis-lab.de/BipolarA41.htm
:-/O
Noopy:
The Siemens BSY34 is a fast switching transistor that was used to drive magnetic bubble memory. The dielectric strength is specified as 50V. The maximum collector current is 600mA. Besides a cutoff frequency of 400MHz, switching times of 30ns and 50ns respectively can be expected (tone/off). The BSY34 datasheet additionally lists the BSY58, which seems to be a slightly worse bin.
The die of the transistor is placed rotated by 45° between the pins of base and emitter. Neither the ball-wedge nor the wedge-wedge technique was used for the electrical connection. On the die, the bonding wires apparently already had the appropriate length before the bonding process. The wire was then welded to the metal surfaces on both sides. On the pins, the wire was processed in a similar way, but welded twice. It seems like the wires were cut off over the edges of the pins.
The edge length of the die is 0,60mm. The emitter surface describes the shape of a U. The fanned out base contact surrounds the emitter area and thus ensures a low-resistance connection to the active base area. In the lower right corner there are auxiliary structures which allow to evaluate the alignment of the masks against each other.
It can be assumed that heavily doped silicon was used in the BSY34 to achieve the high switching speeds. The low emitter base blocking voltage fits to this, which the datasheet specifies with 5V.
Here, the breakdown of the emitter base junction occurs at -6V and the well-known glow of the avalanche breakdown occurs. With increasing current, individual breakdown points change to a uniform glow over the whole junction. The current flow is 10mA, 20mA, 30mA and 50mA.
As described in detail in the context of the SF137 (https://www.richis-lab.de/Bipolar75.htm), the recombination of charge carriers can be observed in normal operation with the help of infrared imaging. The glow allows just a subjective comparison, yet it appears to be much more limited in the BSY34 than in the SF137. In the SF137, without collector current, the light extends far beyond the base contact.
With increasing collector current, the geometry of the glow appearance changes just slightly in the BSY34. However, there seems to be a slight shift towards the emitter, which would fit the shift of the recombination center of grarvity.
The optimization towards fast switching times could be an explanation for the more concentrated glow. If the free charge carriers are more concentrated in the active area, they probably can be discharged faster.
https://www.richis-lab.de/BipolarA42.htm
:-/O
T3sl4co1l:
Hmm, not so much heavily doped, given the modest breakdown voltages; gold doping likely though?
Could well be higher doping in the connection layers (by extension, substrate -- being the collector connecting layer -- if epitaxial type), forcing recombination near the junctions and reducing switching resistance.
(...Ah yes, "double diffused epitaxial", so that could be. That is, n+ substrate; n- epitaxy for collector drift region; p (base) then n+ (emitter) diffusions; and, maybe p+ base contact diffusion?, but would that be triple, or can it be done at the same time as emitter? Not sure. ...Nah, don't think there's a base contact layer. That checks out then.)
Also I guess an extra "emitter" ring around everything, for guard ring? I... forget exactly how those work, but that looks like what they've done anyway.
Tim
Wolfgang:
Unglaublich saubere Fertigung. für das Alter. Haben wir Restströme von dem Ding ?
In English, sorry: Very clean manufacturing, indeed. Do we have leakage currents for this part ?
Noopy:
--- Quote from: T3sl4co1l on October 30, 2023, 07:42:56 pm ---Hmm, not so much heavily doped, given the modest breakdown voltages; gold doping likely though?
--- End quote ---
A modest Vce but the Veb is quite low with 5V. Nevertheless, of course there are HF transistors with higher doping concentration.
Gold doping is at least possible but you don´t find a hint in the datasheet.
--- Quote from: T3sl4co1l on October 30, 2023, 07:42:56 pm ---Could well be higher doping in the connection layers (by extension, substrate -- being the collector connecting layer -- if epitaxial type), forcing recombination near the junctions and reducing switching resistance.
(...Ah yes, "double diffused epitaxial", so that could be. That is, n+ substrate; n- epitaxy for collector drift region; p (base) then n+ (emitter) diffusions; and, maybe p+ base contact diffusion?, but would that be triple, or can it be done at the same time as emitter? Not sure. ...Nah, don't think there's a base contact layer. That checks out then.)
--- End quote ---
I agree with your explanation.
--- Quote from: T3sl4co1l on October 30, 2023, 07:42:56 pm ---Also I guess an extra "emitter" ring around everything, for guard ring? I... forget exactly how those work, but that looks like what they've done anyway.
--- End quote ---
Since the "emitter ring" is in the collector area it generates no real junction. But probably it hat a positive effect...
--- Quote from: Wolfgang on October 30, 2023, 07:48:35 pm ---Unglaublich saubere Fertigung. für das Alter. Haben wir Restströme von dem Ding ?
--- End quote ---
Hello Wolfgang! Some german words? ;)
The cutoff current is at room temperature <70nA (Vcbo=50V).
Really a nice transistor. 8)
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