cribbed from diysb but kept in a txt file so I don't have to remember it ...
"Example #1
G+S - D = normal silicon diode
Correct. This is the intrinsic body diode. Notice that in an N channel, the G+S must be positive with respect to the drain to get the diode to conduct. The anode is the source, the cathode is the drain, and the gate actually plays no part other than being tied to the source. It is a silicon diode with a forward voltage determined by the silicon doping of the semiconductor substrate. Typical Vf is 0.5 to 0.7V when the diode is clearly into conduction. All diodes begin conducting tiny currents well before they nominally turn on.
Notice that if you had a P channel MOSFET, the polarities are reversed and the anode of the diode is the drain and the cathode is the G+S.
Example #2
G+D - S = diode connected mosfet = very soft knee = as used in the Shaka B.
This is the different one. The gate shorted to drain means that the "diode" doesn't start to conduct until the gate/drain is more than the threshold voltage more positive than the source (positive for N-channels, negative for P channels that is). So the forward voltage of these diodes is usually 1.5 to 3V. The knee of conduction is quite soft, and so diodes using this don't go from full off to full on, they gradually turn on. This happens to be quite good for soft distortion. This is the only FET diode connection worthy of any special study or pursuit IMHO. The others are just too similar to ordinary diodes.
It's this connection that needs the series ordinary diode to keep the body diode (connection 1) from conducting when the signal reverses.
Example #3
D+S - G = low leakage, high res diode
This does not exist for MOSFETs. MOSFET gates are insulated by 20 volts thickness of high purity glass. This connection exists for JFETs, where you are using the gate-channel junction as a diode forward biased, instead of reverse biased as it is in normal JFET operation. The junction is typically lightly doped, low leakage, and will show typical silicon diode drops (0.5-0.7V) as above. The only reason to do this or the connection above is to get a slightly different silicon diode junction, hoping that a diode not normally intended for use as a diode is different a little bit."
Because I'm in a highly detailed, very technical environment at work and in my hobbies, I keep stuff like this in notes for future reference. Ask me to remember which leg of the LED is the cathode or I die and I'll start digging my own grave because I'll forget it often enough but I have a nice pinout diagram I can reference when that is the case.
