Let's look at N-MOSFET band diagram in a different perspective again. This is the MOSFET, this is the gate, and this is the source, and this is the drain, and this is the p-type silicon. As you can see in this graph, unlike in the previous 3D N-MOSFET band diagram, source is stopped only at top and drain is the same, other place is the p-type semiconductor. So if you look at this MOSFET in the cross-section here, so source, drain, and gate, and p-type semiconductor, then we can flat band the condition where our surface potential is at zero and the drain voltage is zero, then there's a lot of electron in source region and drain region, and a lot of a majority hole in p-type semiconductor. Current of a electron cannot flow in from source to drain because of the built-in contact potential. If you look at the band diagram of this direction, this is the only EC of the semiconductor, only redrawing the EC, EV, EI and EV below then this. There are a lot of electron in the source region and drain region, and they are blocked by the built-in potential of a PN junction and PN junction. Since this is the flat band condition, then if you look at the band diagram from here to here, then there's always throughout the flat from here to here or here to here. This is the flat band condition, means that there's no inversion. However, you're applying the V_DS with a high voltage. These cases, even though you're applying the drain voltage, the current is not flowing because there's no inversion charge. So what happen in flat band condition is this, current cannot flow in because of the built-in potential of the PN junction, PN junction. Now you're applying the hybrid drain voltage. Then if you're applying positive drain voltage, that is the reverse bias. Reverse bias is lowering the potential of the drain region and a huge electric field is formed. Then source to drain is positive voltage, however, there is still the potential barrier in here and here, therefore, electron cannot flow in, especially in reverse bias PN junction. Now, inversion condition, where our surface potential is above the two Pi F, but the drain voltage is zero. Since drain voltage is zero, although you'll form the inversion charge, current is not flowing. So inversion charge, then there's an electron channel is formed to source to drain, then your built-in potential barrier is lower, therefore, huge electron carriers can go to the P region that forms electron channels from source to drain. However, you didn't apply in drain potential, current is not flowing. This is the same thing, source to drain and then inversion is formed only about 10 nanometer underneath the silicon oxide layer. These are 10 nanometers. Underneath inversion layer there is, let's say that one micron of the depletion layer, where the negative charge of a fixed charge of boron. Then this depletion layer rocking down the combination between the inversion charge and majority hole in p-type region. This area is p-type region, neutral region. If you look at the band diagram from here to here, only the 10 nanometer thickness of the inversion layer is formed and then depletion region, and [inaudible] p-type region. So a channel is formed from the N plus and M plus. However, potential is not applied for the drain region, current is not flowing. Now you're applying a small VD. Then if you're apply in small VD, you lower the potential of the drain region, then inversion of the electron channel from this direction of electron carrier flowing in source to drain. Same thing. In this region, small VD, current is linearly flowing. Inversion charge after the pinch up, then if the inversion after pinch up region go deeper into the depletion region, this region inversion charge is formed, but the depleted region, there is no inversion. So a lot of huge inversion electron charge in the source region and the inversion layer, but there is no charge in depletion region, but huge reverse bias of a diverse electric field, those electron in this region slip down to the depletion region. Then current flowing, but there is no inversion at the depletion region, current are saturated.
