Elemental analysis of the insides of the simplest microcircuit - ULN2003

    In previous articles with photographs of chip crystals ( 1 , 2 , 3 ) - in the comments they wrote that it would be nice to parse a simple chip in detail - so that it is clear "what is what" at the lowest level, and where there is "magic smoke "Hiding. For a long time I could not choose a chip that could be figured out in a few minutes - but finally a solution was found: ULN2003 is an array of Darlington transistors .

    Despite its simplicity, the chip is still widely used and manufactured. ULN2003 consists of 21 resistors, 14 transistors and 7 diodes. It is used to control a relatively powerful load (up to 50 volts / 0.5 amperes) from the foot of a microcontroller (or other digital circuits). Canonical application - for controlling powerful 7-segment LED indicators.

    Crystal itself

    As follows. The colors are somewhat enhanced relative to natural, under the pads the metal is damaged by acid (and acquired such a brown color): As you can see, 7 channels are absolutely identical, therefore we will consider only one. Fortunately for us, the scheme of each channel is known to us - and we can peek into it: And now 1 channel with marked elements. Comparison of specific elements to the scheme - I leave it as a homework for the reader. But how is the transistor itself made? It is known that the internal structure of a planar bipolar npn transistor during production is as follows:















    Thin base - “dives” under the emitter. Despite the fact that both the collector and the emitter have silicon doped in type n, the concentration of the dopant and thickness are different: this is done in order to optimize the transistor for “amplifying the current” in one direction.

    Knowing this, we can take a closer look at 1 transistor, and understand where that is. Silicon alloyed in different types - slightly different in color. To the naked eye, this is practically not noticeable - but here the color saturation and contrast are twisted almost to the maximum. Let 2 emitters connected in parallel do not bother you - they work as 1 large area.

    So that the compounds do not "short-circuit" what is not needed - the silicon surface is covered with a layer of transparent glass (SiO 2), in which there are holes directly above the places where the output is connected to the desired location on the transistor. This is clearly visible in the next photo, because the depth of field on this lens is less, and for example, the connection to the base is no longer in focus, because located above a layer of glass.

    The collectors of both transistors are in fact a single whole, because according to the scheme they are connected. The adjacent channels are isolated by a pn junction; you can see a rectangle of slightly different color around each channel in the general high-resolution photo . As you can see, there is no magic inside :-)




    Also popular now: