Digital circuits having improved transistors, and methods therefor
Digital circuits are disclosed that may include multiple transistors having controllable current paths coupled between first and second logic nodes. One or more of the transistors may have a deeply depleted channel formed below its gate that includes a substantially undoped channel region formed over a relatively highly doped screen layer formed over a doped body region. Resulting reductions in threshold voltage variation may improve digital circuit performance. Logic circuit, static random access memory (SRAM) cell, and passgate embodiments are disclosed.
1. A circuit, comprising:
a plurality of transistors having controllable current paths coupled between a first logic node and a second logic node, the transistors configured to selectively couple an output node to the first or second logic node in response to at least one input signal;
the plurality of transistors forming at least one static random access memory (SRAM) cell, each SRAM cell including
at least one latch, and
at least one access transistor having a source-drain path coupled between a bit line and the output node: wherein
at least one of the transistors of the SRAM cell has a deeply depleted channel formed below its gate that includes a substantially undoped channel region formed fully over a relatively highly doped screening layer formed fully over and in contact with a doped body region.
2. The circuit of claim 1 , wherein:
the transistors are formed in a same integrated circuit substrate and include transistors of different conductivity types.
3. The circuit of claim 1 , wherein:
SRAM latch includes at least one latch that provides a binary value and its complement on a data and the output node, respectively, each latch including
a first driver transistor having a source-drain path coupled between the output node and the first logic node, and a gate coupled to the data node,
a second driver transistor having a source-drain path coupled between the data node and the first logic node, and a gate coupled to the output node, and
first and second load devices coupled between the output and data nodes, respectively, and the second logic node.
4. The circuit of claim 1 , wherein:
at least one of the transistors has a non-stacked channel formed below its gate that does not include relatively highly doped screening layer formed over a doped body region.
5. The circuit of claim 1 , wherein:
the at least one stacked channel transistor has a body coupled to a body bias voltage different from a voltage applied at the first or second logic nodes.
6. An integrated circuit device, comprising:
a digital circuit comprising a plurality of deeply depleted channel (DDC) transistors, each having a screening layer fully covering and in contact with a doped body region and positioned under and fully covered by a substantially undoped channel, the screening layer having a doping concentration greater than a body of the at least one transistor, the deeply depleted channel transistors configured to drive an output node between at least two voltage levels in response to at least one input signal that varies between the two voltage levels, the digital circuit includes at least one latch comprising
a first driver DDC transistor having a source-drain path coupled between the output node and a first voltage level, and a gate coupled to a complementary node,
a second driver DDC transistor having a source-drain path coupled between the complementary node and the first voltage level, and a gate coupled to the output node, and
first and second load devices coupled between the output node and complementary node, respectively, and the second voltage level.
7. The integrated circuit device of claim 6 , wherein:
at least one of the deeply depleted channel transistors has a gate length of no more than about 32 nanometers.
8. The integrated circuit device of claim 6 , wherein:
the at least one latch is coupled between a power supply voltage (Vsupp) of no more than one volt, and
at least one of the deeply depleted channel transistors has a threshold voltage less than 0.6*Vsupp.
9. The integrated circuit device of claim 6 , wherein:
a plurality of the deeply depleted channel transistors are of substantially the same size; wherein
a sigma Vt threshold voltage variance value for the plurality of stacked channel transistors is less than that equivalent to 15 millivolts in transistor having a gate length of 45 nanometers and a gate width of 90 nanometers.
10. The integrated circuit device of claim 6 , wherein:
the at least one deeply depleted channel transistor includes a stacked n-channel transistor that provides a body bias threshold voltage (Vt) modulation of greater than about 220 mV change in Vt per volt of body bias.
11. A method, comprising:
controlling a current in a digital circuit with at least one deeply depleted channel transistor having a substantially undoped channel region fully formed over a relatively highly doped screening layer fully formed over and in contact with a doped body region, controlling the current including providing matched deeply depleted channel transistors as cross-coupled driver transistors of a latch circuit.
12. The method of claim 11 , wherein:
incorporating deeply depleted channel transistors into the latch circuit to reduce switching threshold voltage variation as compared to such a signal path composed of doped channel transistors of equivalent sizes.
13. The method of claim 11 , wherein:
controlling the current includes
including the latch in a static random access memory cell, and
providing matched stacked channel access transistors that couple the drain of each driver transistor to a corresponding bit line.
14. The method of claim 11 , wherein:
controlling the current includes
including the latch in a static random access memory cell, and
providing matched stacked channel load transistors that couple the drain of each driver transistor to a corresponding power supply node.
15. The circuit of claim 1 , wherein:
at least one of the deeply depleted channel transistors has a gate length of no more than about 32 nanometers.
16. The circuit device of claim 1 , wherein:
the at least one SRAM cell is coupled between a power supply voltage (Vsupp) of no more than one volt, and
at least one of the deeply depleted channel transistors has a threshold voltage less than 0.6*Vsupp.
17. The circuit of claim 1 , wherein:
a plurality of the deeply depleted channel transistors are of substantially the same size; wherein
a sigma Vt threshold voltage variance value for the plurality of deeply depleted channel transistors is less than that equivalent to 15 millivolts in transistor having a gate length of 45 nanometers and a gate width of 90 nanometers.
18. The circuit device of claim 1 , wherein:
the at least one deeply depleted channel transistor includes a stacked n-channel transistor that provides a body bias threshold voltage (Vt) modulation of greater than about 220 mV change in Vt per volt of body bias.
19. The circuit device of claim 1 , wherein:
the at least one access transistor is a deeply depleted channel transistor.
20. The integrated circuit device of claim 6 , wherein:
the DDC transistors are formed in a same integrated circuit substrate and include transistors of different conductivity types.
21. The circuit of claim 6 , wherein:
at least one of the transistors has a non-stacked channel formed below its gate that does not include relatively highly doped screening layer formed over a doped body region.
22. The circuit of claim 6 , wherein:
the digital circuit further includes
a first access DDC transistor having a source-drain path coupled between the output node and a first bit line, and
a second access DDC transistor having a source-drain path coupled between the complementary node and a second bit line.