Patent ID: 6380569
Filing Date: 2002-04-30
Classification: H01L

Abstract:
A unipolar field-effect transistor (FET) switch having an associated rated breakdown voltage, comprising:an Nâˆ’ drift layer; a first N+ layer on said Nâˆ’ drift layer which provides an ohmic contact to said Nâˆ’ drift layer; a first layer of metal on said first N+ layer which provides a drain connection for said FET switch; a pair of trenches recessed into said Nâˆ’ drift layer opposite said N+ layer, said trenches separated by a mesa region comprised of that portion of said Nâˆ’ drift layer found between said pair of trenches; a layer of oxide lining the sides and bottom of each of said trenches to form oxide side-walls and respective oxide bottoms in each trench; a conductive material in each of said trenches; a second layer of metal connecting the conductive material in each of said trenches together which provides a gate connection for said FET switch; respective shallow P regions extending from the oxide bottoms of respective trenches into said Nâˆ’ drift layer, each of said shallow P regions extending around the corners formed at the intersections of its respective trench's oxide side-walls and its oxide bottom, said shallow P regions doped to provide an activated charge sufficient to prevent said shallow P regions from becoming completely depleted when subjected to said rated breakdown voltage; an epitaxial N type layer which completely envelops said shallow P regions and extends between said shallow P regions and which is-more heavily doped than said Nâˆ’ drift layer; a second N+ layer on said Nâˆ’ drift layer within said mesa region which provides an ohmic contact to said mesa region; and a third layer of metal contacting said second N+ layer which provides a source connection for said FET switch; the width and doping concentration of said mesa region arranged such that with no voltage applied to said gate connection said mesa region is completely depleted by the potentials created by the work function difference between said conductive material and the Nâˆ’ material in said mesa region, thereby making said switch normally-off, and such that a positive voltage applied to said gate connection undepletes said mesa region, creates accumulation channels adjacent to said oxide side-walls, and modulates said Nâˆ’ drift layer within said mesa region, thereby turning said switch on and enabling current to flow between said drain and said source connections via said mesa region and said accumulation channels, said accumulation channels, said ohmic contacts, and said mesa region modulation combining to provide a low on-resistance for said switch.