Allegro 1391 Hall Effect Sensor

(I was debating posting this as the depot quality is not at all good, despite trying multiple acid baths and boiling acetone baths I could not remove all the plastic mold.  But it is what it is and I figured there is still good stuff to look at.)

This is not a vintage die, but a currently available device. I thought it would be interesting to see what a modern Hall Effect Sensor looks like.

Hall Effect Sensors measure magnetic field strength and are found in a wide variety of applications such as camshaft and crankshaft position sensors in vehicles, and all sorts of other position sensing applications.  The Hall Effect was discovered by Dr Edwin Hall in 1879 who found that when a magnet was placed so that its field was perpendicular to one face of a thin rectangle of gold through which current was flowing, a difference in potential appeared at the opposite edges. He found that this voltage was proportional to the current flowing through the conductor, and the flux density or magnetic induction perpendicular to the conductor. The Hall Voltage is very small (micro Volts) so high precision amplifiers are required to make them practical.  Thus it was a good application for silicon and it took until the silicon era before Hall Effect sensors became common.

Here is the die photo

It is a tiny die* just 1.16mm x 1.16mm!


Die Markings – Allegro symbol – “1391”  (A die that corresponds to the package device that’s helpful).  “2009” design year and “ST02”



Looking around the die starting off with the hall sensor, which is the pink square in the middle measuring 150μm x150μm with a metal connection in each corner – unfortunately it’s a tad boring 🙁

Hall Sensor Cross Section – courtesy of Allegro website

This diagram from Allegro explains the sensor. You have a piece on n-type epitaxial silicon, and you have current flow diagonal (as a resistor), and the change in Voltage is sensed on the other diagonal and input into a high precision amplifier.





Looking around a bit more and I realized this is a BiCMOS process with a fair number of Bipolar transistors used – the smallest structures I could find were poly lines that measured ~1.22 μm so this is probably a 1.25 μm BiCMOS. It also has two levels of metal which in places is quite dense. Bipolar transistors are excellent for high precision op amps so this makes sense for this application that requires amplification of ultra low voltages and precision comparators. Here are the three types of bipolar I found (All NPN transistors).  On the right image (A double emitter transistor) the rectangle surrounding the transistor with corners cut is the outline of the highly doped (N++) buried layer region (Buried under the epitaxial silicon) that is used to lower the collector resistance of the transistor (Which in turns dramatically improves the transistor characteristics).








This is the block diagram from Allegro’s datasheet

The metallization is pretty dense with large areas of metal 2 making it difficult to impossible to trace where signals are going so I cannot identify much of the circuit blocks, but I did manage to figure out the pin out:





*Working with such tiny die is very very difficult.  I need a magnifying glass just to see the die in the chemical beaker from ~6″ away.  If you touch the die with tweezers too fast they can jump off the table onto the floor never to be seen again.  I have found I can clean some die by scrubbing them with a Q-tip dipped in acetone, but with such small die this is not possible as they are a fraction of the size of a Q-tip head.  If anyone reading this has experience working with tiny die and can share some tips I would truly appreciate it.

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