"fT (Current Gain bandwidth Product): This is directly related to device gain and also to the device physical base width (wb). Most of the audio transistors in the industry have high fT (~30MHz), the trade-off is SOA performance with high voltage conditions. ON Semi Power Base Technology (which is unique in the market) has low/medium fT devices (8 to 12MHz) devices like the MJL21193/94 which have excellent SOA above 100V, these devices have wider Bases and also some unique "base spreading resistor" design which make them extremely rugged, used by most high end audio manufacturers." http://sound.westhost.com/soa.htm
For LAPT, Sanken calls it "Multi-emitter for high-frequency transistor", while "Ring Emitter" is a somewhat generic term, also referred to in many patents.
So, it doesn't look like it's all the same, there are quite different goals.
@Mihai: Your Amp (RMI-FC100) looks very interesting, I'm about to sim it over the holidays.
Anybody tried the Fairchild outputs, like FJA4213/4313? Those look quite promising, high fT, low Cob...
Motorola introduced their PowerBase as an answer to RCA's hometaxial base technology, I believe.
RCA's hometaxial line was the best in SOA for a few years - but had limited fT. Many amplifier manufacturers used their 2N3055 (including Quad, (it is believed); Sugden and others?).
The PowerBase technology presumably had a thinner base than the old 2N3055- as most fT's were 2 MHz (min). Examples are MJ15015/MJ15003 etc. How was the second breakdown spec. accomplished with a thinner base?
Part of the answer is that the emitters could have been ballasted: even RCA developed ballasting technology to improve Is/b on some of their devices. Part of the answer also is (probably) that the collector has as much to do with SOA as the base. RCA's process built a very deep graded collector junction, so would have had spreading resistance of its own as well as the base. Many newer transistors have very high BVceo (250V) that this probably is used to improve Is/b.
The Ring Emitter technology is a newer development, in that the emitter shapes are more dominated by perimeter than area. THis can be done, for example, by placing extra base contacts where an emitter area would have been. This helps to sustain current gain at high currents by minimising base resistance effects.
But to achieve 30 MHz fT means that the base should be much thinner than the old 2N3055. Perhaps only the collector, and maybe emitter ballasting, are used to improve Is/b.
In summary, what ON Semi, Toshiba and Sharp have done is to optimise the epi layers to achieve high current gain sustain with high Is/b. My guess is that they have at least two collector epi layers, and two base epi layers, one to suppress high current injection and one to provide some spreading resistance.
And it should be possible to measure the various internal resistances by using high frequency small-signal measurements. This is what is routinely used for RF devices; for audio similar measurements ought to be possible at lower frequencies (1-30 MHz).