Tuning capacitance to enhance FET stack voltage withstand

• US 20080265978A1
• Filed: 04/26/2007
• Published: 10/30/2008
• Est. Priority Date: 04/26/2007
• Status: Active Grant

• Current Assignee: pSemi Corporation (Murata Manufacturing Co Limited)
• Original Assignee: Peregrine Semiconductor Corporation (Murata Manufacturing Co Limited)
• Inventors: Englekirk, Robert Mark
• Granted Patent: US 7,960,772 B2
• Time in Patent Office: Days
• US Class Current: 327/427
• CPC Class Codes: H01H 11/00 Apparatus or processes specially adapted for the manufacture of electric switches processes specially adapted for manufacture of rectilinearly movable switches having a plurality of operating members associated with different sets of contacts, e.g. keyboards, H01H13/88
• CPC Class Codes: H03K 17/102 in field-effect transistor switches
• CPC Class Codes: H03K 17/161 in field-effect transistor switches
• CPC Class Codes: H03K 17/6871 the output circuit comprising more than one controlled field-effect transistor
• CPC Class Codes: H03K 17/693 Switching arrangements with several input- or output-terminals, e.g. multiplexers, distributors logic circuits H03K19/00; code converters H03M5/00, H03M7/00
• CPC Class Codes: H03K 2017/066 Maximizing the OFF-resistance instead of minimizing the ON-resistance
• CPC Class Codes: Y10T 29/49105 Switch making

Abstract

An RF switch to controllably withstand an applied RF voltage Vsw, or a method of fabricating such a switch, which includes a string of series-connected constituent FETs with a node of the string between each pair of adjacent FETs. The method includes controlling capacitances between different nodes of the string to effectively tune the string capacitively, which will reduce the variance in the RF switch voltage distributed across each constituent FET, thereby enhancing switch breakdown voltage. Capacitances are controlled, for example, by disposing capacitive features between nodes of the string, and/or by varying design parameters of different constituent FETs. For each node, a sum of products of each significant capacitor by a proportion of Vsw appearing across it may be controlled to approximately zero.

Claims

1. A stacked transistor RF switch apparatus, comprising:

a) a transistor stack having a multiplicity of constituent transistors all coupled in series connection drain to source to form a series string for which internal nodes are those between adjacent transistors; and

b) a total effective drain-source capacitance Cds for each constituent transistor, wherein values of Cds for two constituent transistors differ from each other by at least 2%.

2. The RF switch of claim 1, wherein values of Cds for two constituent transistors differ from each other by at least 5%.

3. The RF switch of claim 1, wherein values of Cds for two constituent transistors differ from each other by at least 10%.

4. The RF switch of claim 1, where in high impedance operation a first switch endnode has a voltage V1 and a second switch endnode has a voltage V2, and each drain parasitic capacitance (Cpd) is not encompassed within a Cds, but is coupled at a terminal end to a Cpd terminal node having an ac voltage of A*V1+B*V2;

and a total Cpd for a particular internal node is a sum of values of all Cpd having a non-terminal end coupled to the particular internal node;

wherein values of Cds for adjacent pairs of constituent transistors differ from each other by an amount greater than the total Cpd for a node between the adjacent transistors, for at least half of adjacent transistor pairs.

5. The RF switch of claim 1, wherein differing Cds values for constituent transistors are effective to tune the stack to improve voltage distribution in that a variance of the magnitude of Vds-off as distributed across all the constituent transistors increases when the Cds values are made substantially more equal.

6. The RF switch of claim 1, wherein, for most adjacent constituent transistor pairs, values of Cds are higher for a transistor closer to a particular RF switch endnode than for the other transistor of the pair.

7. The RF switch of claim 6, wherein values of Cds for two constituent transistors differ from each other by at least 20%, and differing Cds values for constituent transistors are effective to tune the stack to improve voltage distribution in that a variance of the magnitude of Vds-off as distributed across all the constituent transistors increases when the Cds values are made substantially more equal.

8. The RF switch of claim 1, further comprising a distinct capacitive feature that has a predominantly capacitive impedance at a primary frequency of a signal ordinarily controlled by the RF switch and is coupled to an internal node of the stack series string.

9. The RF switch of claim 1, wherein differing Cds values between constituent transistors are due at least in part to design differences between the transistors.

10. The RF switch of claim 1, wherein differing Cds values between constituent transistors are due at least in part to a discrete capacitive feature disposed in parallel to one of the Cds.

11. A stacked transistor RF switch apparatus, comprising:

a) a transistor stack having a multiplicity of constituent transistors all coupled in series connection drain to source to form a series string for which internal nodes are those between adjacent transistors; and

b) at least one discrete physical capacitor element Ccomp coupled an internal node of the series string of the stack, Ccomp having an impedance that is predominantly capacitive at primary frequencies of signals ordinarily controlled by the RF switch.

12. The RF switch of claim 11, wherein capacitances coupled to internal nodes of the stack series string are configured to effectively tune voltage distribution, in that a variance of Vds-off as distributed across all constituent transistors would increase if all distinct capacitor elements coupled to internal nodes were omitted.

13. The RF switch of claim 12, further comprising a MIM (metal-insulator-metal) Ccomp capacitor.

14. The RF switch of claim 11, further comprising a MIM (metal-insulator-metal) Ccomp capacitor.

15. The RF switch of claim 11, further comprising a plurality of Ccomp capacitors.

16. The RF switch of claim 15, wherein the plurality of Ccomp capacitors are coupled to different internal nodes of the stack.

17. A method of fabricating a stacked RF switch that includes a multiplicity of series connected constituent transistors in a series string for which internal nodes are those between each pair of adjacent transistors, the method comprising a step of establishing total effective drain-source capacitance Cds values that are significantly different for different transistors in the stack, wherein:

a) such difference is at least 2% between a maximum Cds value and a minimum Cds value of the constituent transistors of the stacks;

or

b) for at least half of adjacent transistor pairs, a difference between Cds values of each pair differs by an amount greater than a total Cpd of an internal node between such pair, where the total Cpd of a node is a sum of all parasitic capacitance elements coupled to the node that are not part of a Cds.

18. The method of claim 17 wherein the significantly different Cds between constituent transistors is effective to more uniformly distribute voltage across the constituent transistors, in that a variance of the magnitude of Vds-off as distributed across all the constituent transistors would increase if the differences between Cds values are substantially reduced.

19. The method of claim 17 wherein difference (a) is at least 5%, and alternative difference (b) is satisfied only if a sum of Cds differences between each pair of adjacent constituent transistors exceeds a sum of the total Cpd for the internal nodes between each pair.

20. The method of claim 17 wherein difference (a) is at least 10%, and alternative difference (b) is satisfied only if adjacent pair Cds values differ by more than the total Cpd of a node between such pair, for a majority of adjacent constituent transistor pairs.

21. The method of claim 17, further comprising balancing charge injection for a particular internal node by estimating values of all significant capacitive elements coupled to the particular internal node, estimating a voltage across each such significant capacitive element, and controlling capacitance coupled to the particular node as necessary to approximately zero a sum of all such capacitance values, as multiplied to reflect the corresponding estimated voltage.

22. The method of claim 21, further comprising approximately balancing charge injection for each of a majority of internal nodes of the RF switch stack.

23. The method of claim 22, further comprising approximately balancing charge injection for each internal node of the RF switch stack.

24. The method of claim 17, further comprising coupling a discrete capacitive feature to one or more internal nodes of the stack, where a discrete capacitive feature is a distinct element having an impedance that is predominantly capacitive at a primary frequency of a signal ordinarily switched by the RF switch.

25. A method of fabricating a stacked RF switch that includes a multiplicity of series connected constituent transistors in a series string for which internal nodes are those between each pair of adjacent transistors, the method comprising coupling a discrete capacitive feature to one or more internal nodes of the stack, where a discrete capacitive feature is a distinct element having an impedance that is predominantly capacitive at a primary frequency of a signal ordinarily switched by the RF switch.

26. The method of claim 25, comprising coupling a plurality of discrete capacitive features to the one or more internal nodes of the stack.

27. The method of claim 25, further comprising balancing voltage distribution by controlling any discrete capacitive features coupled to internal nodes of the stack such that a variance of magnitude in Vds-off, as distributed across all constituent transistors due to a voltage Vsw applied across the RF switch, would increase if all such discrete capacitive features were omitted.

28. The method of claim 25, further comprising balancing charge injection for a particular internal node by estimating values of all significant capacitive elements coupled to the particular internal node, estimating a voltage across each such significant capacitive element, and controlling capacitance coupled to the particular node as necessary to approximately zero a sum of all such capacitance values, as multiplied to reflect the corresponding estimated voltage.

29. The method of claim 28, further comprising approximately balancing charge injection for each of a majority of internal nodes of the RF switch stack.

30. The method of claim 29, further comprising approximately balancing charge injection for each internal node of the RF switch stack.