Practice Questions: Bipolar Junction Transistor (BJT) - 1 - Electronics and Communication Engineering (ECE) MCQ

Redraw the circuit considering Zener diode operating at the breakdown region;

Doping in semiconductor:

• We usually add impurities to semiconductors, to increase their conductivity.
• When a small amount say a few parts per million (ppm), of a suitable impurity is added to the pure semiconductor, the conductivity of the semiconductor is increased manifold.
• The planned addition of a desirable impurity is called doping and the impurity atoms are called dopants.
• The dopant has to be such that it does not distort the original pure semiconductor lattice.
• It occupies only a very few of the original semiconductor atom sites in the crystal.

Transistors:

• Transistors are semiconductor devices, which are commonly used in amplifiers or electrically controlled switches
• Transistors are the basic building block that regulates the operation of computers, mobile phones, and all the other modern electronic circuits
• The most important transistor function is as a current amplifier and as a switch
• Transistors as current amplifiers are often used in amplifiers while transistors as switches are often found in automatic light circuits
• The functions of a transistor are variable resistor and switching device

Current amplification factor: It is defined as the ratio of the output current to the input current. In the common-base configuration, the output current is emitter current IC, whereas the input current is base current IE.
Thus, the ratio of change in collector current to the change in the emitter current is known as the current amplification factor. It is expressed by the α.

Calculation:
Given,
I_E = 2 mA
I_B = 20 μA = 0.02 mA
From above concept,
I_C = 2 mA - 0.02 mA = 1.98 mA
Current amplification factor is given as,

BJT stands for Bipolar junction transistor

• B = Bipolar (because conduction is due to two opposite type of carriers Holes and electrons)
• J = Junction refers to the two PN junctions between emitter and base, and collector and base.
• BJT's are current-driven devices i.e. It is a current-controlled device.
• The current through the two terminals is controlled by a current at the third terminal (base).
• It is a bipolar device (current conduction by both types of carriers, i.e. majority and minority electrons and holes)
• It has a low input impedance.

Symbol

I_CO = It is the reverse saturation current across reverse biased collector junction.

I_CEO = Base cut-off current

I_CEO = (1 +β) I_CO

Consider a npn transistor

In cutoff region:
Collector base junction is reversed biased: V_B < V_C
Emitter base junction is reversed biased: V_B < V_E

In Saturation region:
Collector base junction is forward biased: V_B > V_C
Emitter base junction is forward biased: V_B > V_E

In Active region:
Collector base junction is reversed biased: V_B < V_C
Emitter base junction is forward biased: V_B > V_E

A transistor is a 3 layered 2 junction device as shown below.

I_E = I_B + I_C
I_C = βI_B
I_E > I_C > I_B

Concept:

• As the N-type material has lost electrons and the P-type has lost holes, the N-type material has become positive with respect to the P-type.
• Then the presence of impurity ions on both sides of the junction causes an electric field to be established across this region with the N-side at a positive voltage relative to the P-side.
• The problem now is that a free charge requires some extra energy to overcome the barrier that now exists for it to be able to cross the depletion region junction.
• This electric field created by the diffusion process has created a “built-in potential difference” across the junction with an open-circuit (zero bias) potential

Calculation:
Given built-in voltage of the diode is 0.7 V
Now doping concentrations on both sides are doubled. i.e, N_A’ = 2N_A and N_D’ = 2N_D
Newly built-in potential is:

A power transistor is a three-terminal semiconductor electronic device
It has a four-layer structure of alternating P and N-type doping as shown below

• The thickness of the drift region determines the breakdown voltage of the power transistor which is normally very high when compared to signal level transistors
• This doping provides a capability to the transistors for higher current and higher voltage operability resulting in the application in high power application