Hello, I learned this from my electronics course. To turn on or turn off a transistor, you have to cater for the charges drawn from/driven into the junction. Now, in one of my books, it illustrates this concept with a slant line, with more charges on the taller side, where the shorter side has less. Does the concept of charge density profile really exist in electronics or physics? I ask because I couldn't find such terms in my other reference books... Thanks Jack
There are two main types of transistors: the BJT, and MOSFET, It sounds like you are talking about the BJT. Both of these have negative, and positive analogies. BJT has NPN, and PNP, while MOSFETs have NMOS, and PMOS.
Here is the NMOS:
MOSFET: Voltage controlled current amplifier (use voltage to control the amount of current the device will sink)
There are three terminals: Source, Drain, and Gate.
the gate is like a faucet control knob, which controls the current across the source to drain. The gate is also one side of a capacitor, the other side of the capacitor is the substrate, or path for current to flow from source to drain. Since the gate is a capacitor there is a separation (or dielectric) between the substrate, and the gate. The only way for the gate to control the flow of current is with an electric field, which is controlled by the charge on the gate. The reason why it is a charge density, as apposed to just a charge is because there are many different types of NMOS's which allow for larger or smaller amount of current. If you are doing some heavy lifting (like running an amplifier, or driving a motor for an RC car) you need a large gate, that can force the substrate to make a very low resistive path, but if you have something small like, logic transistors, then you really don't want to be wasting your energy pushing and pulling a lot of charge into and out of the gate of a FET. So the density is relative to the amount of current, and in turn the amount of power flowing through the transistor.
The reason why there is a NMOS, and PMOS is because to say we have 1V would mean nothing if there was nothing to refer it to. Normally when it is said to be 1V, it is implied 1V with respect to ground. An NMOS means that you need to generate a positive voltage, which will push a charge into the gate, with respect to the drain of the transistor.
in other words here is a narative.
I have a NMOS I wish to use to control current flowing from the source to the drain, I must supply a voltage to the gate relative to the drain in order to allow a voltage to flow through. If the MOSFET is made for high current, it is going to need more charge on the gate to raise the voltage to control the output.
BJT's on the other hand are current controlled current amplifiers (use current to control the amount of current it sinks), and the three terminals include: collector, emitter, and base. if you are looking at an npn, it has some similarities to the NMOS. The npn needs to generate a threshold voltage across the base and emitter, like the gate, and drain of a FET, but this is where it changes. The BJT will amplify the amount of current that you send through the base and emitter, by some amount (usually 20). The charge density, in this case, is related to how much charge is needed to overcome that initial threshold voltage which will turn on the BJT. Same principle as before, if you want to sink a lot of current with the NPN then you need a BJT with a large charge density because the diode will be larger, and need more charge to make the inherent diode be forward bias.
I am pretty sure I am correct on all this, but if I am wrong, I would love to learn the correct answer.
There are two main types of transistors: the BJT, and MOSFET, It sounds like you are talking about the BJT. Both of these have negative, and positive analogies. BJT has NPN, and PNP, while MOSFETs have NMOS, and PMOS.
Here is the NMOS:
MOSFET: Voltage controlled current amplifier (use voltage to control the amount of current the device will sink)
There are three terminals: Source, Drain, and Gate.
the gate is like a faucet control knob, which controls the current across the source to drain. The gate is also one side of a capacitor, the other side of the capacitor is the substrate, or path for current to flow from source to drain. Since the gate is a capacitor there is a separation (or dielectric) between the substrate, and the gate. The only way for the gate to control the flow of current is with an electric field, which is controlled by the charge on the gate. The reason why it is a charge density, as apposed to just a charge is because there are many different types of NMOS's which allow for larger or smaller amount of current. If you are doing some heavy lifting (like running an amplifier, or driving a motor for an RC car) you need a large gate, that can force the substrate to make a very low resistive path, but if you have something small like, logic transistors, then you really don't want to be wasting your energy pushing and pulling a lot of charge into and out of the gate of a FET. So the density is relative to the amount of current, and in turn the amount of power flowing through the transistor.
The reason why there is a NMOS, and PMOS is because to say we have 1V would mean nothing if there was nothing to refer it to. Normally when it is said to be 1V, it is implied 1V with respect to ground. An NMOS means that you need to generate a positive voltage, which will push a charge into the gate, with respect to the drain of the transistor.
in other words here is a narative.
I have a NMOS I wish to use to control current flowing from the source to the drain, I must supply a voltage to the gate relative to the drain in order to allow a voltage to flow through. If the MOSFET is made for high current, it is going to need more charge on the gate to raise the voltage to control the output.
BJT's on the other hand are current controlled current amplifiers (use current to control the amount of current it sinks), and the three terminals include: collector, emitter, and base. if you are looking at an npn, it has some similarities to the NMOS. The npn needs to generate a threshold voltage across the base and emitter, like the gate, and drain of a FET, but this is where it changes. The BJT will amplify the amount of current that you send through the base and emitter, by some amount (usually 20). The charge density, in this case, is related to how much charge is needed to overcome that initial threshold voltage which will turn on the BJT. Same principle as before, if you want to sink a lot of current with the NPN then you need a BJT with a large charge density because the diode will be larger, and need more charge to make the inherent diode be forward bias.
I am pretty sure I am correct on all this, but if I am wrong, I would love to learn the correct answer.
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