[Alt-photo] sensor and curve

[jusdado]

Greetings.
We know how to design a characteristic curve of sensitive material (film 
or paper), base + veil, heel and shoulder, line, depending on the values 
​​of light exposure. But someone could draw or define the 
"characteristic curve" of a digital sensor?. The digital sensor has a 
linear response to increases in exposure. There would be "heel", not 
"shoulder", just straight?. Thank you.