# When do you want an ESBT (emitter switched bipolar transistor)?

I just found out about ESBTs, which seem to be a hybrid of BJT and MOSFET:

When I googled it most links led to STMicroelectronics, so I think currently they're the only manufacturer.
I noticed that many devices are high voltage (1000V to more than 2000V), and some devices come in rather large packages,

despite being relatively low current (this one is 7A). Must have to do with their application in high voltage (2200V) circuits.

Has anyone used one of these yet? What are the advantages over a MOSFET (besides maybe the higher voltage)?

Traditionally, MOSFETs are able to switch fast, but are available for voltages up to ca. 800 V or 1000 V only. Power BJTs can take > 1000 V but are not as fast.

The ESBT is available as a single-package part from ST, but can also be made using two discrete transistors. It takes advantage of the cascode configuration, which combines the low-voltage device's ability of being very fast and the high voltage device's ability to block a large voltage. The base of the BJT is held at a moderate DC voltage, causing its emitter to be just a bit less than 1 V below it. This low emitter voltage is the maximum voltage the MOSFET has to block.

The concept is best illustrated when thinking about the turn-off process: The MOSFET has to take only a bit less than the small base voltage of the BJT when it is switched off and thereby cuts off the current through the BJT's collector and its own drain very fast. Once the current is cut off by the MOSFET, the BJT's collector could take its time to rise to whatever high voltage it needs to block off (and actually doesn't take much time any more because the current is zero already), and the slowing effect of its Miller capacitance (collector-to-base) doesn't show.

Typical applications are flyback converters that work off a rectified 400 V (ac) bus, which relates to a design for 600...800 V (dc) and requires a transistor's blocking voltage of 800 V + n * Vout, with n being the pri:sec winding ratio of the transformer and Vout being the DC output voltage of the converter. Whenever a single high-voltage MOSFET is enough to get the job done in a switching application, this will very likely be the more economical way to go -- however elegant the concept of using the typical advantages two different devices in a cascode configuration may be. ESBTs or similar MOSFET-and-BJT circuits are a niche topology, from my experience.

NOTE (edit, August 2012): It seems that all of ST's ESBT devices are now marked as NRND (not recommended for new design). Source. Really not a long time since they were presented/marketed at PCIM Europe 2008.

• They don't look like just the BJT added to the MOSFET (like you would create with discrete components). The ESBT seems to have the low $V_{CS(ON)}$ (collector to source voltage) of a MOSFET, without the voltage drop you would expect between collector and emitter. – stevenvh Jun 20 '11 at 14:21
• @stevenvh It look like it is just that: st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/… – mazurnification Jun 20 '11 at 15:01
• @stevenvh - more details: st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/… – mazurnification Jun 20 '11 at 15:05
• @mazurnification - Are you referring to the (equivalent) symbol they use? I was thinking about the low value of 0.07$\Omega$ for $R_{CS(ON)}$, a value which you can expect for a good MOSFET, but which should become much larger with the BJT in series. – stevenvh Jun 20 '11 at 15:14
• @stevenvh - on the second link they showed internal structure. They also mention that device could be "hybrid" that is two separate structures in the single package. Also in the given DS they show VCS(ON)=0.4V@3.5A and 0.5V@7A which is consistent with BJT saturation voltage + series resistance. RCS(ON) parameter have to be probably taken with "marketing" grain of salt - note wording "equivalent series resistance". – mazurnification Jun 20 '11 at 15:27

Very interesting. I didn't know about these devices before. From a quick look, it seems they are a bipolar run in common base configuration with the FET in series with the emitter doing the current switching. The point seems to be you get the high voltage operation of the BJT with the speed of the FET. Since high voltage BJTs tend to have low gain, it means the base supply must supply significant current, and must be pretty solid to keep the base at just the right voltage to minimize the voltage drop but still keep the BJT operating as a transistor.

It's interesting to note that for many applications the emitter transistor could be a faster switching low voltage BJT too. In fact I did this once to make a carrier line AM transmitter at 1MHz. This was in college, and I didn't have transistors with the right combination of voltage, speed, and gain.

• You knew about that stuff in college? Crap... what am I doing with my life? – NickHalden Jun 20 '11 at 19:00
• @JGord: I learned about the common base configuration in college, but I was a EE major (M.eng. EE RPI May 1980) so there would have been something wrong if I didn't. I hadn't heard of emitter switched bipolar transistors until this thread. @stevenvh thanks for pointing those out. – Olin Lathrop Jun 20 '11 at 20:10
• We were taught about cascode circuits in college too (ca. 1993 for me), but in a linear sense (not switching sense), where the configuration helps reduce the effect of parasitic capacitance. – Jason S Jun 21 '11 at 10:59