High power unipolar FET switch

DETAILED DESCRIPTION A high power unipolar field-effect transistor (FET) switch is presented which overcomes the problems noted above.
The switch is particularly well-suited to high power switching applications, providing a very low on-resistance, a high blocking voltage, and negligible switching loss.
An N- drift layer is on an N+ layer which provides an ohmic contact to the drift layer (X+ denotes a carrier concentration of at least 1.
times.
10.
sup.
18 /cm.
sup.
3, X- denotes a carrier concentration of less than 5.
times.
10.
sup.
16 /cm.
sup.
3).
A layer of metal on the N+ layer provides a drain connection for the FET.
A pair of trenches are recessed into the drift layer opposite the N+ layer; the trenches are separated by a mesa region comprised of that portion of the N- drift layer found between the trenches.
Oxide layers line the walls and bottom of each trench, which are each filled with a conductive material.
A second layer of metal connects the conductive material in each of the trenches together to provide a gate connection for the FET.
A shallow P region extends from the bottom of each trench into the drift layer and around the corners formed at the intersections of its respective trench's oxide side-walls and its oxide bottom.
A second N+ layer is on the N- drift layer within the mesa region which provides an ohmic contact to the mesa region, and a third layer of metal contacts the second N+ layer to provide a source connection for the FET.
The structure is preferably arranged so that the switch operates as a "normally-off" device; i.
e.
, current is prohibited from flowing between drain and source when the voltage applied to the gate connection is zero.
This is accomplished by making the width and doping concentration of the mesa region such that, with no voltage applied to the gate, the mesa region is completely depleted by the potentials created by the work function difference between the conductive material and the N+ material in the mesa region.
The structure can also be arranged to operate as a "normally-on" device by making the mesa region so wide or its doping concentration so high that a negative gate voltage is required to completely deplete the mesa region