Tuning capacitance to enhance FET stack voltage withstand
View Patent ↗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 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,
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.
2. The method of claim 1 , further comprising approximately balancing charge injection for each of a majority of internal nodes of the RF switch stack.
3. The method of claim 2 , further comprising approximately balancing charge injection for each internal node of the RF switch stack.