Comparison of BJT, JFET, D-MOSFET, E-MOSFET construction and operation

 Here the fabrication construction of Bipolar Junction Transistor(BJT), Junction Field Effect Transistor(JFET), Depletion type Metal Oxide Semiconductor Field Effect Transistor(D-MOSFET) and Enhancement type Metal Oxide Semiconductor Field Effect Transistor(E-MOSFET) are compared and their operation explained.

BJT(Bipolar Junction Transistor)

The following shows fabrication construction of a NPN Bipolar Junction Transistor(BJT).

BJT is a three-layer semiconductor device. In BJT transistors, a substrate is doped layer by layer by N-type, P-type and N-type material. The doping concentration is uniform, that is the emitter is highly doped, followed by collector and then base. That is base is least doped and narrower. The electrical contact to the emitter, collector and base is via metallic or ohmic contact. A thin layer of silicon dioxide is deposited at the top. In BJT there are two PN junctions formed between the emitter and base and between collector and base. Whether the PN junction is forward biased or reversed biased depends upon the biasing method used. 

In common emitter for example, the collector is +ve with respect to emitter(-ve) and the base is connected to the emitter. Thus the base is -ve with respect to collector. The base is more +ve with respect to the emitter by the PN junction voltage which is approximately 0.7V for silicon based BJT. This is illustrated in the following diagram.

NPN BJT circuit

As can be seen from the above picture, the collector and base PN junction is reversed biased and the emitter and base PN junction is forward biased.

Application example of BJT are different types of BJT amplifier such as base biased BJT amplifier, self biased BJT amplifier, Collector-Emitter Feedback Bias BJT Amplifier,


JFET(Junction Field Effect Transistor)

 N-channel JFET fabrication construction is shown below.

 An N-type material is used as the base P-type material is embedded into the base N-type material. Opposite side of the N-type is drain and source with direct ohmic contact. The p-type material is the gate which also has direct ohmic contact. The region between the two p-type region is the n-type material channel. Thus in JFET there are two PN junctions with depletion regions(gray colored).

When some voltage Vdd is applied between the drain(+ve) and source(-ve) and the gate is connected to the source(-ve), gate is -ve. Hence there are two PN junctions between the drain(+ve) and gate(-ve). Further the PN junctions are reversed biased as the gate which is P-type is is at ground(-ve) and the drain which is N-type is at +ve. Conventional current called drain current will flow from drain to source via the N-channel. When the drain to source voltage(Vdd=Vds) is increased then more drain current will flow and at the same time the depletion region will expand proportional to the increase in drain to source voltage. Thus increment in Vds will cause increment of Id but also decrease in the channel width. Eventually, a voltage Vds=Vp(pinch off voltage) is reached where the two depletion region touch each other and the channel is essential closed. At this pinch off voltage Vp the drain current stops to flow because the channel is closed. Any further increase in Vds after Vp the drain current is essential constant and is called drain to source saturation current Idss.

 When negative gate voltage applied to the gate with respect to the source, then depletion layer region is increased and hence the channel is narrow and less drain current flows. Thus saturated drain current is reached faster than when the gate is shorted(grounded Vgs=0V). That also means for the same drain to source voltage(Vds), applying negative bias voltage on the gate saturated drain current is reached quicker.

Application example of JFET are JFET Shunt and Series Switch, source follower, JFET amplifier etc.


Depletion Type MOSFET(D-Metal Oxide Semiconductor Field Effect Transistor)

The following picture shows fabrication construction of N-type Depletion type MOSFET or D-MOSFET.

fabrication construction of Depletion Mode MOSFET or D-MOSFET

In depletion type N channel MOSFET, a  two N-type material along with a narrow N-type channel between the two N-type are doped into a P-type material substrate. The two N-type material have a direct metallic connect which forms the drain and source terminal. N-type channel already exists between these two N-type material. The gate with formed above the P-Type material where there is narrow N-channel and there exist now a dielectric silicon dioxide(SiO2) between the N-channel and the gate. That is there is no direct metallic contact for the gate. In some D-MOSFET, there exist a substrate terminal SS but most of the time, this is internally connected to the source terminal.

When a positive voltage between drain(+ve) and source(-ve) is applied and the gate is shorted(Vgs=0V), that is gate is connected to the source, drain current starts to flows. This is because the drain terminal being +ve attracts the electrons from the N channel. Like in JFET, this drain current when Vgs=0V is called drain to source current with gate shorted Idss. However unlike JFET, the drain current can be higher than drain to source current with gate shorted Idss because the depletion mode MOSFET can also be applied with positive gate voltage unlike in case of JFET. 

When the D-MOSFET gate is applied with negative voltage relative to source, that is Vgs is made negative, the gate will repel the free electrons in the N channel and attract the holes in the P substrate which causes recombination of the electrons with holes and depletion region starts to form. This process causes the channel width to decrease and hence drain current is reduced. If this increase in reverse bias of the gate is continued and eventually the depletion region stops the flow of the electrons in the channel. The gate voltage Vgs at which the drain current is stopped(little will flow) is called the gate to source cutoff voltage Vgs(off). 

When the depletion MOSFET is applied with positive gate to source voltage, it attracts electrons from the P substrate into the channel and thereby drain current is increased. In the drain curve the region where positive Vgs is applied is called the enhancement region.

Enhancement Type MOSFET(E-Metal Oxide Semiconductor Field Effect Transistor)

An N channel enhancement type MOSFET or E-MOSFET fabrication construction is shown below.

N channel enhancement type MOSFET or E-MOSFET fabrication construction

The fabrication of enhancement type mosfet transistor is similar to the depletion type mosfet transistor expect that there is no channel. 

When gate is grounded(connected to source) and voltage is applied between the drain(+ve) and the source(-ve) there is no drain current(unlike in depletion type MOSFET) because there is no channel and no electrons which are attracted to the drain. Even if electrons from p subtrate are attracted towards the drain it will be still insufficient for current or electrons to flow from the source to drain. In this case there are two PN junction between the drain N-type and substrate and between source N-type and substrate. These two pn junction will oppose the current to flow.

With drain to source voltage, when gate is applied a positive voltage, the +ve gate potential repels the hole between the N-type material, that is holes are repelled from the channel and minority electrons from the substrate are attracted into the channel. With enough positive gate bias called threshold voltage Vgs(th), there will be enough electrons in the channel and N-type channel is formed and drain current starts to flow. With constant gate to source voltage, Vgs, if drain to source voltage Vds is increased and reaches a pinch off value Vp the drain current will get saturated. This is because with Vgs in some constant voltage, when the drain to source voltage is increased, the (negative)drain to gate voltage will decrease, that is, gate will be become less and less with respect to drain. This causes reduction in channel width and hence drain current. At this point there is no further increase in drain current and the transistor is in saturation. Applying drain to source voltage high enough the transistor will enter into the cutoff or breakdown region.


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