BJT's

Question 1

How do we achieve forward active mode?

Answer 1

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

Question 2

Describe how the bias conditions are created in forward active mode of a npn BJT (potential).

Answer 2

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.

Question 3

In the forward active mode of a npn BJT, which region is forward-biased?

Answer 3

The emitter-base junction (EBJ)

Question 4

In the forward active mode of a npn BJT, which region is reverse-biased?

Answer 4

The collector-base junction (CBJ)

Question 5

How do we reverse bias a pn junction?

Answer 5

A negative voltage is applied to the p-type material and positive voltage is applied to the n-type material

Question 6

What does current consist of in a npn BJT?

Answer 6

Electrons injected from the emitter into the base. Holes injected from the base into the emitter.

Question 7

What is iE?

Answer 7

The emitter current, which is the current that flows across the emitter-base junction.

Question 8

What is the direction of the emitter current?

Answer 8

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.

Question 9

What kind of current dominates the emitter current?

Answer 9

The electron component is much larger than the hole component, the emitter current will be dominated by electron component.

Question 10

What is a minority carrier?

Answer 10

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.

Question 11

What is a majority carrier?

Answer 11

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

Question 12

What are donors?

Answer 12

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.

Question 13

What are acceptors?

Answer 13

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.

Question 14

What is the equation for collector current?

Answer 14

Question 15

What is ß?

Answer 15

The common-emitter current gain

This is defined as the ratio of change in collector current to the change in base current.

Question 16

What is the relationship between base and collector current involving ß?

Answer 16

Question 17

What does the emitter current consist of in equation form?

Answer 17

Question 18

What is α?

Answer 18

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.

Question 19

What is the relationship between α and ß?

Answer 19

Question 20

What is the relationship between emitter and collector current as a function of beta and alpha?

Answer 20

Question 21

What is the correlation between alpha and beta

Answer 21

A small change in alpha results in a large change in beta

Question 22

What factors is beta influenced by? And how do we achieve a high beta with this?

Answer 22

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.

Question 23

What is the i_c-V_BE characteristic of a BJT?

Answer 23

The BJT shows diode like behaviour in which a forward voltage of approximately 0.7V is needed across V_BE before current is allowed to flow. After 0.7V we see exponential growth in current flow as V_BE is increased.

Question 24

How does temperature affect the i_C-V_BE relationship of a diode

Answer 24

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.

Question 25

How do we forward bias a pn junction?

Answer 25

By applying a positive voltage on the p-type material and a negative voltage on the n-type material

Question 26

What is the I_C-V_CB characteristic of a BJT?

Answer 26

Each of the characteristic curves intersect the vertical axis at a current level equal to αI_E, where I_E is the constant emitter current at which the particular curve is measured.

α = i_C/i_E, where i_C and i_E denote total collector and emitter currents

Question 27

What is the equation for ic involving the early current?

Question 28

Describe the ic-V_CE characteristic

Answer 28

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.

Question 29

What are typical values for the Early Voltage?

Answer 29

In the range of 50V to 100V