Tuning capacitance to enhance FET stack voltage withstand
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.
1. A radio frequency (RF) switch comprising a stack of N series coupled transistors, N being an integer equal to 2 or more, wherein
a first transistor of the N series coupled stacked transistors has a first width and a first length; and
a second transistor of the N series of coupled stacked transistors has a second width smaller than the first width and a second length, wherein a difference between the first width and the second width is set at a predetermined amount.
2. The RF switch of claim 1 , wherein the first transistor is above the second transistor in the stack.
3. The RF switch of claim 1 , further comprising a first endnode at a first end transistor of the stack and a second endnode at a second end transistor of the stack, the first endnode configured to receive an RF voltage.
4. The RF switch of claim 3 , wherein the first end transistor is a top transistor of the stack and the second end transistor is a bottom transistor of the stack.
5. The RF switch of claim 3 , wherein the second endnode is connected to a reference voltage.
6. The RF switch of claim 5 , wherein the reference voltage is ground.
7. The RF switch of claim 3 , wherein the second endnode is connected to an input node or an output node.
8. The RF switch of claim 1 , wherein the difference between the first width and the second width is set at the predetermined amount to balance voltage distribution within the stack of N series coupled transistors.
9. The RF switch of claim 1 , wherein a third transistor of the series of coupled stacked transistors has a third width smaller than the second width, wherein a difference between the second width and the third width is set at a predetermined amount.
10. The RF switch of claim 9 , wherein the third transistor is below the second transistor.
11. The RF switch of claim 9 , wherein the predetermined amount is selected to reduce an area of the series of coupled stacked transistors.
12. The RF switch of claim 9 , further comprising at least one transistor added to the stack of N series coupled transistors, thus forming a stack of at least N+1 series coupled transistors within an area of the stack of N series coupled transistors.
13. A radio frequency (RF) switch comprising a stack of series coupled transistors arranged between a first endnode and a second endnode from a first endnode transistor in the stack to a second endnode transistor in the stack, wherein
a plurality of transistors of the stack have a width depending on a location of the plurality of transistors in the stack, and
a width of a second transistor of the plurality of transistors is set at a predetermined amount to be smaller than a width of a first transistor of the plurality of transistors.
14. The RF switch of claim 13 , wherein the plurality of transistors of the stack are an entirety of the transistors of the stack.
15. The RF switch of claim 13 , wherein the first endnode transistor is connected to a reference voltage.
16. The RF switch of claim 15 , wherein the reference voltage is ground.
17. The RF switch of claim 13 , wherein the first endnode transistor is connected to an input or an output node.
18. The RF switch of claim 13 , wherein the second endnode is connected to an input node or an output node.
19. A voltage distribution-balanced transistor arrangement for radio frequency (RF) operation, comprising:
a stack of series coupled field emission (FET) transistors comprising a first transistor having a first width and a second transistor having a second width smaller than the first width by at least a predetermined amount to voltage balance the transistor arrangement.
20. The voltage distribution-balanced stack of claim 19 , further comprising:
a first endnode at a top of the stack of series coupled FET transistors;
a second endnode at a bottom of the stack of series coupled FET transistors;
a plurality of internal nodes between pairs of adjacent transistors of the stack of series coupled FET transistors; and
at least one compensation capacitor coupled across: i) the first endnode and at least one node of the plurality of internal nodes, ii) at least two nodes of the plurality of internal nodes, iii) at least one node of the plurality of internal nodes and the second endnode, iv) the first endnode and the second endnode, or v) at least one of the plurality of internal nodes and ground.