# Concept of Charge Density profile

Discussion in 'Electronic Basics' started by Jack, Dec 20, 2008.

1. ### JackGuest

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

2. ### neon

1,325
0
Oct 21, 2006
If you want to understand transistor at that level then yes a phisic book will be much better. Actualy manufacture of transistors are a better cleaner source. read their specs.

3. ### CospanGuest

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.

Dave

4. ### CospanGuest

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.

Dave