Bunalmish
This circuit is a non practical trick circuit (i.e. academic)
There is a current loop where a current source injects a current to n-ch transistor's Source and Substrate, Drain is at lower potential connected to current source to close the current loop.
Fact 1: Gate is at a higher potential than Source. Since Fact 1 there is an active channel between Source and Drain i.e. channel is on so it acts like a resistor between Source and drain (Vgs > VT).
Note: Gate potantial is higher than Source and Drain potentials. Source potential is higher than Drain potential since current flowing throuh Source to Drain. In this configuration Drain acts as Source and Source acts as Drain. So actual Vgs is between Gate and Drain. Vgs is actualy Vgs + Von (details details...).
Fact 2: Vgs voltage source just supply a gate potential but no current flows throug it (ignoring Gate leakage here. Even if we consider it, it is very small unless current source is in the same magnitude we can ignore it).
If we visualize 3-D layout with relevant parasitics what we see is that n+ Source shorted to P+ Substrate externally (so no substrate to Source diode action) and there are two current paths to Drain (where current source connected so that current can flow through)
First path is a pn diode between Substrate(Source) to Drain and second path is through an active n+ channel between Source(Substrate) and Drain.
External diode also paralel connected to internal Source Drain pn diode. Now since channel is a Resistor so unless IxR > Vt of pn diode (or external diode) all the current flows through the channel (voltagedrop through channel will be low). (Id/Is=0, Id takes all the current)
If IxR > Vt of internal pn diode or external diode than most current flow through diodes and assuming that they have same Vt, current amount in each diode will be determined by internal diode resistors acting as ballasts. (Id/Is depends. channel, internal diode and external diode all conduct. Since part of the Id goes through internal diode. If we assume internal and external diodes have same Vt and their resistances are the same -- including connection resistances--- than Ichannel+Iinternal_diode=Id and I internal_diode=Is)
If IxR=Vt than diodes and channel will all conduct. ( Id and Is need to be calculated and for this two diode's and channel's equivalent circuits must be considered. Two diodes and channel re all parallel connected)
I think questioner means to ask the first case where IxR < Vt of diodes (other cases complicated and need more variables being determined).
I hope this explanations will take you out of bunalmis situation. I felt sorry.
If you have any question please ask.
regards,
tekno1