Semiconductor device

Abstract

To stably control a threshold voltage of a functional circuit using an oxide semiconductor. A variable bias circuit, a monitoring oxide semiconductor transistor including a back gate, a current source, a differential amplifier, a reference voltage source, and a functional circuit which includes an oxide semiconductor transistor including a back gate are provided. The current source supplies current between a source and a drain of the monitoring oxide semiconductor transistor to generate a gate-source voltage in accordance with the current. The differential amplifier compares the voltage with a voltage of the reference voltage source, amplifies a difference, and outputs a resulting voltage to the variable bias circuit. The variable bias circuit is controlled by an output of the differential amplifier and supplies voltage to the back gate of the monitoring oxide semiconductor transistor and the back gate of the oxide semiconductor transistor included in the functional circuit.

1 . (canceled) 
     
     
         2 . A semiconductor device comprising:
 a variable bias circuit;   a first transistor including a back gate;   a current source;   a differential amplifier;   a reference voltage source; and   a functional circuit including a second transistor,   wherein the second transistor includes a back gate on and in contact with an insulating film,   wherein each of the first transistor and the second transistor includes an oxide semiconductor,   wherein the current source is capable of supplying current between a source and a drain of the first transistor to generate a gate-source voltage in accordance with the current,   wherein the differential amplifier is capable of comparing the gate-source voltage of the first transistor and a voltage of the reference voltage source, amplifying and outputting a difference, and   wherein the variable bias circuit is capable of being controlled by the output of the differential amplifier and supplying a voltage to the back gate of the first transistor and the back gate of the second transistor.   
     
     
         3 . The semiconductor device according to  claim 2 , wherein the functional circuit is a memory circuit. 
     
     
         4 . The semiconductor device according to  claim 2 , wherein the functional circuit is a register circuit. 
     
     
         5 . The semiconductor device according to  claim 2 , wherein the output of the differential amplifier is an output voltage or an output current. 
     
     
         6 . The semiconductor device according to  claim 2 , wherein the current source is a constant current source. 
     
     
         7 . The semiconductor device according to  claim 2 , wherein the oxide semiconductor includes indium or zinc. 
     
     
         8 . The semiconductor device according to  claim 2 , wherein the oxide semiconductor includes a nanocrystal. 
     
     
         9 . The semiconductor device according to  claim 2 , wherein a gate length and a gate width of the first transistor are similar to a gate length and a gate width of the second transistor. 
     
     
         10 . A semiconductor device comprising:
 a variable bias circuit;   a first transistor including a back gate;   a current source;   a differential amplifier;   a reference voltage source; and   a functional circuit including a second transistor and a third transistor, the second transistor being stacked over the third transistor,   wherein the second transistor includes a back gate on and in contact with an insulating film,   wherein each of the first transistor and the second transistor includes an oxide semiconductor,   wherein the third transistor includes silicon,   wherein the current source is capable of supplying current between a source and a drain of the first transistor to generate a gate-source voltage in accordance with the current,   wherein the differential amplifier is capable of comparing the gate-source voltage of the first transistor and a voltage of the reference voltage source, amplifying and outputting a difference, and   wherein the variable bias circuit is capable of being controlled by the output of the differential amplifier and supplying a voltage to the back gate of the first transistor and the back gate of the second transistor.   
     
     
         11 . The semiconductor device according to  claim 10 , wherein the functional circuit is a memory circuit. 
     
     
         12 . The semiconductor device according to  claim 10 , wherein the functional circuit is a register circuit. 
     
     
         13 . The semiconductor device according to  claim 10 , wherein the output of the differential amplifier is an output voltage or an output current. 
     
     
         14 . The semiconductor device according to  claim 10 , wherein the current source is a constant current source. 
     
     
         15 . The semiconductor device according to  claim 10 , wherein the oxide semiconductor includes indium or zinc. 
     
     
         16 . The semiconductor device according to  claim 10 , wherein the oxide semiconductor includes a nanocrystal. 
     
     
         17 . The semiconductor device according to  claim 10 , wherein a gate length and a gate width of the first transistor are similar to a gate length and a gate width of the second transistor.