Insulated gate turn-off device with turn-off Schottky-Barrier MOSFET

Abstract

An insulated gate turn-off (IGTO) device, formed as a die, has a layered structure including a p+ layer (e.g., a substrate), an n epi layer, a p-well, vertical insulated gate electrodes formed in the p-well, and n+ regions between the gate electrodes, so that vertical npn and pnp transistors are formed. The device is formed of a matrix of cells. To turn the device on, a positive voltage is applied to the gate electrodes, referenced to the cathode. The cells further contain a vertical p-channel MOSFET, for shorting the base of the npn transistor to its emitter, to turn the npn transistor off when the p-channel MOSFET is turned on by a slight negative voltage applied to the gate. The p-channel MOSFET includes a Schottky source formed in the top surface of the npn transistor emitter.

Claims

1. An insulated gate turn-off (IGTO) device formed as a die comprising: a first semiconductor layer of a first conductivity type; a second semiconductor layer of a second conductivity type overlying the first semiconductor layer; a third semiconductor layer of the first conductivity type overlying at least a portion of the second semiconductor layer; an array of cells comprising a plurality of insulated gate electrodes within trenches not extending through the third semiconductor layer; at least some of the cells comprising: a first semiconductor region of the second conductivity type overlying the third semiconductor layer and adjacent to an insulated gate electrode; a second semiconductor region of the second conductivity type overlying at least part of the first semiconductor region and being more highly doped than the first semiconductor region; and a Schottky conductor overlying and contacting at least a top surface of the first semiconductor region adjacent to the insulated gate electrode, wherein the Schottky conductor, the first semiconductor region, and the third semiconductor layer form a MOSFET, wherein a voltage applied to the insulated gate electrode greater than a threshold voltage of the MOSFET inverts the first semiconductor region adjacent to the insulated gate electrode to form a lower resistance path between the Schottky conductor and the third semiconductor layer to reduce a beta of a bipolar transistor formed by the first semiconductor region, the third semiconductor layer, and the second semiconductor layer to turn off the IGTO device.

2. The device of claim 1 wherein the first conductivity type is a p-type, the second conductivity type is an n-type, and the Schottky conductor provides holes for conducting through a channel induced in the first semiconductor region adjacent to the insulated gate electrode.

3. The device of claim 1 wherein the first conductivity type is an n-type, the second conductivity type is a p-type, and the Schottky conductor provides electrons for conducting through a channel induced in the first semiconductor region adjacent to the insulated gate electrode.

4. The device of claim 1 further comprising a cathode electrode electrically contacting the Schottky conductor and the second semiconductor region.

5. The device of claim 1 wherein the cathode electrode is in ohmic contact with the second semiconductor region.

6. The device of claim 1 wherein the Schottky conductor is a silicide formed in the first semiconductor region.

7. The device of claim 1 wherein the Schottky conductor also contacts a top surface of the second semiconductor region and forms an ohmic contact with the second semiconductor region.

8. The device of claim 1 wherein the first semiconductor layer is a growth substrate.

9. The device of claim 1 wherein the third semiconductor layer is formed as a well.

10. The device of claim 1 wherein the first semiconductor region is formed as an epitaxial layer.

11. The device of claim 1 wherein the first semiconductor region is formed as a doped region.

12. The device of claim 1 further comprising an anode electrode electrically contacting the first semiconductor layer.

13. The device of claim 1 wherein the first semiconductor region extends between adjacent insulated gate electrodes, and the second semiconductor region is formed in a center area between the adjacent insulated gate electrodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view of a single cell in an IGTO device disclosed in the inventors' U.S. Pat. No. 9,391,184.

(2) FIG. 2 is an equivalent circuit of the cell of FIG. 1.

(3) FIG. 3 illustrates the cell of FIG. 1 but where the p+ source of the p-channel turn-off MOSFET has been replaced with a Schottky source.

(4) FIG. 4 illustrates the dopant profile of the IGTO device of FIG. 3.

(5) FIG. 5 is an equivalent circuit of the cell of FIG. 3.

(6) Elements that are the same or equivalent are labelled with the same numerals.

DETAILED DESCRIPTION

(7) Since the inventive device may be identical to that of FIG. 1 except for the formation of the p-channel turn-off MOSFET, the below description focuses mainly on the differences, and the description of the common features is described above with reference to FIGS. 1 and 2 and the inventors' U.S. Pat. No. 9,391,184.

(8) FIG. 3 is a cross-sectional view of a single cell in an array of identical cells. The gate electrodes 12 may be formed in parallel strips, with the doped regions between adjacent gate electrodes. All of the gate electrodes 12 are connected in parallel.

(9) As previously described, the IGTO device forms an npnp vertical structure comprising a vertical npn bipolar transistor and a vertical pnp bipolar transistor. When the cathode and anode electrodes are forward biased and the gate electrode 12 voltage is above the threshold voltage, the base of the npn transistor is narrowed, which increases its beta so that the product of the betas of the npn and pnp transistors is greater than one. At that point, regenerative action begins to fully turn the IGTO device on.

(10) To force the IGTO off under all conditions (including latch-up), a p-channel MOSFET, using a Schottky source, is formed to essentially short together the base-emitter of the npn transistor to turn the npn transistor off. The p-channel MOSFET threshold voltage is only slightly negative and very repeatable.

(11) FIG. 3 is a cross-sectional view of a single cell of an IGTO device, formed as a single die, in accordance with one embodiment of the invention. FIG. 4 is a dopant profile from the top silicon surface through to the n-epi layer 32 along the gate sidewall. FIG. 5 is an equivalent circuit. Elements that are the same as in FIGS. 1 and 2 are equivalent and their description is not repeated in detail.

(12) It is known to form a MOSFET with a Schottky source and drain, and such a MOSFET is referred to as a Schottky-barrier (SB) MOSFET. Short channels may be fabricated since there is no diffusion of p-type dopants to form the source and drain. In such SB MOSFETs, both the source and drains are formed of the Schottky conductor. Many papers describe forming SB MOSFETs including, Overview and Status of Metal S/D Schottky-Barrier MOSFET Technology, John Larson et al., IEEE Trans. on Elec. Devices, vol. 53, no. 5, May 2006; and Schottky-Barrier S/D MOSFETs with High-K Gate Dielectrics and Metal-Gate Electrode, Shiyang Zhu et al., IEEE Elec. Device Letters, vol. 25, no. 5, May 2004, both incorporated herein by reference.

(13) Instead of using a p+ region 54 (FIG. 1) as the source of the vertical p-channel MOSFET, a Schottky source is used. In FIG. 3, a p-type Schottky conductor 70, such as a metal or a metal silicide, is deposited on or formed in the top surface of the n-layer 50 to form the Schottky source. The Schottky conductor 70 is used to supply holes. Possible Schottky conductors include PtSi, PdSi, or other suitable silicides or metals. The carrier concentration at the surface of the n-type surface may first need to be adjusted to lower the effective n-type dopant concentration, for instance using ion implantation of boron, to obtain more efficient Schottky diode performance. Accordingly, the p-channel MOSFET uses a Schottky conductor as the source and the p-well 14 as the drain.

(14) In one embodiment, a platinum layer is deposited over the surface, and the platinum-silicon interface is annealed to form PtSi (the Schottky conductor 70). The excess platinum is removed. A thin TiN barrier layer is then deposited to electrically contact the PtSi and any exposed silicon. An aluminum layer is then deposited over the TiN barrier layer to complete the cathode electrode 20.

(15) It is optional whether the Schottky conductor 70 is also formed over the n+ contact 52, since it has no effect either way. Due to the high dopant concentration of the n+ contact 52 and its depth, the silicide will just form an ohmic contact with the n+ contact 52. The Schottky conductor 70 near the gate electrode 12 only has an effect as a Schottky source due to the relatively low dopant concentration of the n-layer 50.

(16) FIG. 4 illustrates the dopant profile extending from the surface of the Schottky conductor 70 down to the n-epi layer 32. The depth of the gate electrode trench is shown within the p-well 14.

(17) FIG. 5 shows an equivalent circuit. All components and its operation are the same as described with respect to FIG. 2 except for the operation of the p-channel MOSFET 72. To turn off the IGTO device, the gate electrode 12 is made negative with respect to the cathode electrode 20. Holes from the Schottky conductor 70, as a result of the negative potential on the gate electrode 12, can then flow along a p-type inversion layer formed along the vertical sidewall of the gate electrode 12, which forms a conducting channel between the base (p-well 14) and emitter (n regions 52/50) of the npn transistor 60/64 to turn it off.

(18) Eliminating the p+ region 54 in FIG. 1 reduces the number of processing steps. Further, the surface area required by the Schottky source can be very small since it is not limited by the diffusion of p-type dopants in the silicon. Further, the same Schottky conductor 70 acts as a source as well as provides good ohmic contact with the n+ contact 52. Additionally, the use of the Schottky source instead of a doped source results in faster switching.

(19) This technique of using a vertical MOSFET with a Schottky conductor source and a doped drain to turn off an IGTO device may be employed in various other designs of IGTO devices and is not limited to only the embodiments shown.

(20) The conductivities of the various semiconductor regions may be reversed, and the Schottky conductor may then be an n-type Schottky conductor, such as dysprosium silicide (DiSi) or erbium silicide (ErSi), for providing electrons for inverting the region adjacent to the insulated gate electrode. The turn-off MOSFET will then be an n-channel type.

(21) While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.