The variable resistor depicts the inversion layer formed under the gate. Simulate this circuit – Schematic created using CircuitLab Their construction looks like this (N-MOSFET): And your transistor will not behave as same as what is specified in the datasheet.Īll MOSFETs are fundamentally blocking in both directions. However, if you apply higher voltage to the predefined source as mentioned in the datasheet, the threshold voltages will not be same as mentioned in datasheet. In that case, the above is still true that higher voltage terminal is drain and lower voltage terminal is source for nmos. This makes the mosfet pins predefine as mentioned in the datasheet. Nevertheless, be careful when buying/dealing with discrete 3 pin Mosfets (i.e SiHG47N60EF) where internal bulk is already connected to the source(for nmos) or to the drain (for pmos) internally. So, whichever (out of the two terminals on sides of nmos) terminal has higher voltage than the other terminal, that becomes your drain (for NMOS) and the other terminal with lower voltage becomes the source (for nmos). You can see that the cross section of the Mosfet is EVEN from the center vertical line. I am sure you are familiar with the cross section of the CMOS transistor. To simplify it even further, I would like to add a bit to just what Lawrence has mentioned. Yes, the current can flow from drain to source and vice-versa. The q-point signifies the current-voltage (I-V) operating point of the transistor. In reality there is a phenomenon known as the body-effect which introduces additional complexity in calculating the quiescent operating point of a transistor. A simplified view of the transistor turns it into a 3-terminal device ignoring the body terminal. The transistor is a four-terminal device, gate, drain, source, and body. Operationally MOS transistors have the same basic operating behavior in analog and digital applications but they differ in how they're applied and where their current-voltage behavior is utilized. The PMOS transistor will instead have a bubble at the gate terminal while the NMOS won't have a bubble. Conversely, simplified schematic drawings of digital transistors typically do not have arrows. It is conventional to see arrows on the "source" terminal of the transistor. The P signifies that the channel forms on P-type semiconductor, while the N signifies an N-type semiconductor. P-channel refers to the type of channel that forms underneath the "gate". With PMOS, current flows from Source-to-drain (arrow points to the device at the Source) With NMOS, current flows from Drain-to-source (arrow points away from device at the Source) The channel can be widened to a certain extent by decreasing the gate-to-source voltage below 0. P-channel depletion devices also have a channel by default, and need a voltage on the gate higher than the source in order to turn the channel off. The channel can be widened to a certain extent by increasing the gate-to-source voltage above 0. N-channel delpetion devices have a channel by default, and need a voltage on the gate lower than the source in order to turn the channel off. (Enhancement devices don't have a channel automatically, and need gate voltage to create one - because it's N-channel \$V_\$ for this to happen.) N-channel enhancement devices need a voltage on the gate higher than the source in order to create a conduction channel. This symbol shows you the inherent diode between drain and source. Your picture does not show the intrinsic diode in the devices - the arrow point towards or away from the gate is an indication of the channel type (N-channel points towards the gate, P-channel points away from the gate). In all of these cases, current can flow from source to drain as well as from drain to source - it's just a matter of how the device is connected in the circuit. When the channel is off, the diode is in circuit and will either conduct or block depending on the drain-source current polarity.Īs your picture shows, there are both N-channel and P-channel devices, as well as enhancement mode and depletion mode devices. When the conduction channel is present, the diode is shunted and current flows through the path of least resistance (the channel). The intrinsic body diode inside the MOSFET is in parallel with the conduction channel, however. The conduction channel has no intrinsic polarity - it's kind of like a resistor in that regard. When a channel exists in a MOSFET, current can flow from drain to source or from source to drain - it's a function of how the device is connected in the circuit.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |