BJT's Flashcards
How do we achieve forward active mode?
Forward bias Emitter-Base-Junction (EBJ) - Set Base to be at a higher potential than the collector and ensure this voltage VBE is above 0.7V
Reverse bias Collector-Base-Junction (CBJ) - Set VCB to a voltage in which the Collector is at a higher potential than the Base
Describe how the bias conditions are created in forward active mode of a npn BJT (potential).
The voltage Vbe causes the p-type base to be higher in potential than the n-type emitter, thus forward-biasing the emitter-base junction. The collector-base voltage Vcb causes the n-type collector to be at a higher potential than the p-type base, thus reverse-biasing the collector-base junction.
In the forward active mode of a npn BJT, which region is forward-biased?
The emitter-base junction (EBJ)
In the forward active mode of a npn BJT, which region is reverse-biased?
The collector-base junction (CBJ)
How do we reverse bias a pn junction?
A negative voltage is applied to the p-type material and positive voltage is applied to the n-type material
What does current consist of in a npn BJT?
Electrons injected from the emitter into the base. Holes injected from the base into the emitter.
What is iE?
The emitter current, which is the current that flows across the emitter-base junction.
What is the direction of the emitter current?
The direction of emitter current is “out of” the emitter lead, which is in the direction of the hole current and opposite to the electron current, with iE equal to the sum of these two components.
What kind of current dominates the emitter current?
The electron component is much larger than the hole component, the emitter current will be dominated by electron component.
What is a minority carrier?
These are the type of carriers that are in the minority in equilibrium in a semiconductor: holes in n-type material and electrons in p-type material.
What is a majority carrier?
These are the type of carriers that are in the majority in equilibrium in a semiconductor: electrons in n-type material, holes in p-type material
What are donors?
These are pentavalent atoms, usually phosphorous, arsenic or antimony added to the semiconductor to produce and n-type material (i.e containing an excess of electrons in equilibrium). Nd = donor density.
What are acceptors?
These are trivalent atoms, usually aluminium, boron, indium and gallium added to the semiconductor to produce p-type material containing an excess of holes in equilibrium. Na = acceptor density.
What is the equation for collector current?
What is ß?
The common-emitter current gain
This is defined as the ratio of change in collector current to the change in base current.
What is the relationship between base and collector current involving ß?
What does the emitter current consist of in equation form?
What is α?
The common-base current gain, which is defined as the change in collector current divided by the change in emitter current. In a well-designed BJT this value is close to unity.
What is the relationship between α and ß?
What is the relationship between emitter and collector current as a function of beta and alpha?
What is the correlation between alpha and beta
A small change in alpha results in a large change in beta
What factors is beta influenced by? And how do we achieve a high beta with this?
The width of the base - the width of the base should be thin (W small).
The relative dopings of the base and emitter - base should be lightly doped and the emitter should be heavily doped.
What is the ic-VBE characteristic of a BJT?
The BJT shows diode like behaviour in which a forward voltage of approximately 0.7V is needed across VBE before current is allowed to flow. After 0.7V we see exponential growth in current flow as VBE is increased.
How does temperature affect the iC-VBE relationship of a diode
The voltage across the emitter-base junction decreases by about 2mV for each 1°C in temperature, provided that the junction is operating at the constant current I.
How do we forward bias a pn junction?
By applying a positive voltage on the p-type material and a negative voltage on the n-type material
What is the IC-VCB characteristic of a BJT?
Each of the characteristic curves intersect the vertical axis at a current level equal to αIE, where IE is the constant emitter current at which the particular curve is measured.
α = iC/iE, where iC and iE denote total collector and emitter currents
What is the equation for ic involving the early current?
Describe the ic-VCE characteristic
At low values of VCE, as the collector voltage goes below that of the base by more than 0.4V, the CBJ becomes forward biased and the transistor leaves the active mode and enters the saturation mode.
We observe that the characteristic curve, though still straight lines, have finite slopes. When extrapolated, the characteristic lines meet at a point in the negative VCE axis at VCE = - VA
The voltage VA, is a positive number and a parameter for a particular BJT, known as the Early Voltage.
What are typical values for the Early Voltage?
In the range of 50V to 100V
