IR gets hot on powerFETs with CoolMOS equivalent

IR gets hot on powerFETs with CoolMOS equivalentSteve Bush
International Rectifier is to introduce high-voltage powerFETs based on very similar technology to CoolMOS, announced by Siemens Semiconductor last year.
CoolMOS cut the on-resistance of 500V powerFETs by a factor of five, a remarkable achievement in a component area that has seen only incremental improvements over a number of years, far short of the dramatic technological advances made in the sub-60V power-FET arena. Making it
Unlike most power-FETs, IR’s CoolMOS-like technology is made by multiple epitaxial growth stages. It requires deep P-type structures which are being made by a step-and-repeat process.
The diagram shows the columns built-up as six ‘slices’. This needs 15 or 16 mask steps. Conventional high voltage powerFETs require six masks at the most. The extra complexity gains a lot of performance. Comparing device on-resistance for a TO-247 outline package, IR’s latest conventional generation is 130mž. IRsays samples of 70mž CoolMOS-like devices are available now with 45mž devices to follow.  
 
Siemens, now called Infineon, has a TO-220 190m omega 20A 600V part in production, and will be producing 70m omega TO-247 part in future. Having seen CoolMOS when it was first announced, IR – a past-master at manufacturing powerFETs – has developed its own CoolMOS-like technology. Our CoolMOS-equivalent was fully developed within IR, with knowledge of the CoolMOS structure. Knowing what structure to uses is not necessarily the biggest battle, learning how to make it effectively can be far more challenging, said Gabe Gotthard, v-p of marketing at IR.
Existing cross-license agreements with Infineon, said Gotthard, allow IR to sell devices including the CoolMOS-like structure.
Having two sources of devices should be of mutual benefit to Infineon and IR as customers will be less nervous about buying components that they can get from more than one source.
The processing required to manufacture the CoolMOS-like structures is more complex than other powerFETs. As a result, devices will be aimed at applications that have space constraints and will be able to take the price premium.
For the more demanding switch-mode power supply applications where performance has a high value to the customer, said Gotthard, Perhaps when they are trying to get the next level of performance without increasing size.
CoolMOS-equivalent and CoolMOS-type is all that anyone in IR will call its new FETs at the moment. A proper name is, apparently, in the pipeline.
The company will initially be offering devices in its Super220 and Super247 high-dissipation leaded packages which it claims will be shipping towards the late summer. How does CoolMOS work?
High voltage power-FETs need a thick epitaxial-layer with a reduced doping concentration to block voltage. Both these measures increase on-resistance. CoolMos effectively increases doping levels when the device is on and reduces them when it is off, reducing on-resistance while increasing off-state blocking voltage. The highly doped (low-resistance) N-type column under the gate of the device is where current flows when the device is on. To increase off-state blocking voltage, highly-doped P-type material is trenched around the column. In the off-state, carriers from the N-column and the P-trenches migrate into each other’s areas creating a depletion region, making the column ‘disappear’ and increasing blocking voltage.


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