Leakage current:

• When the supply at the emitter-base junction is open-circuited, there is only reverse biasing in the base-collector junction.
• Therefore, this sets up a small amount of current called the leakage current.
•  It is highly temperature-dependent because it depends upon a number of minority charge carriers that are thermally generated and in turn depend upon temperature.
• The main source of the leakage current in a transistor is thermally generated minority carrier. consider common emitter configuration.

• When the switch is open, the emitter-base junction is an open circuit and so the value of input or base current is zero.
• But a leakage current I_CEO flows between collector and emitter. this leakage current I_CEO is not only due to the thermally generated minority carriers (I_CBO) across the C - B junction but also due to the movement of holes that flow across the base-emitter junction.

Current components of NPN transistor are shown below

The arrow on the symbol for bipolar transistors indicates the PN junction between base and emitter and points in the direction in which conventional current travels, i.e. the direction of holes.

NPN:

For an NPN transistor, when the emitter-base is forward biased, holes from the base (p-type) start to flow to the emitter side (n-type) and electrons start to flow from the emitter to the base. The direction, however, represents the direction of the hole flow.

PNP:

For a PNP transistor, when the emitter-base junction is forward biased, holes from the emitter start to flow to the base. The direction of the arrow also indicates the same.

Notes:

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.
• 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.

No current will follow into the base when the emitter-base junction of a transistor is reverse biased. When no current is flowing into the base, no current will flow in the collector.
Now if the collector is also reverse biased, then the collector current will flow. This is called reverse active biasing.

Important Points:

Since the mobility of electrons is higher in NPN transistors, they are preferred more as it will result in a smooth flow of charge carriers.

Explanation:

Mobility (μ): It is defined as drift velocity per unit electric field applied i.e.

$\mu =\frac{v_d}{E}=\frac{q\tau }{m}$

m = effective mass of the charged particle

The effective mass of holes is more than that of electrons. This results in less mobility of holes.

NOTES:

NPN transistor: It is formed by sandwiching a thin layer of P-type semiconductor between two N-type semiconductors. Electrons are the carriers responsible for current flow.

PNP transistor: It is formed by sandwiching a thin layer of N-type semiconductor between two P-type semiconductors. Holes are the carriers responsible for current flow.

If an NPN transistor is properly biased in the active region, then most of the electrons from the emitter pass through the base to the collector.

• We know that the base-emitter junction of an NPN BJT in the active region will be forward biased, while the collector-base junction will be reversed biased
• The emitter region of an NPN transistor is heavily doped (n++) so that the diffusion current primarily consists of free electrons moving from the emitter into the base

NPN transistor with various currents is shown:

• Electrons flow into the emitter
• Holes flow out of the emitter
• Holes flow into the collector

A junction transistor is formed by sandwiching a thin layer of P-type semiconductor between two N-type semiconductors or by sandwiching a thin layer of n-type semiconductor between two P-type semiconductors.

Transistor consists of three main regions i.e. Emitter, Base, and Collector.

Emitter (E):

• It provides majority charge carriers by which current flows in the transistor.
• Therefore the emitter semiconductor is heavily doped.
• The emitter is always forward biased since its function is to provide charge carriers.

Base (B):

• The base region is thin and lightly doped. This reduces the base transit time which prevents the charge from the emitter to recombine at the base and pass directly to the collector (for maximum efficiency).
• It provides proper interaction between emitter and collector

Collector (C):

• The size of the collector region is larger than the two other regions and it is moderately doped.
• The main purpose of the collector is to collect majority charge carriers from the emitter. 

NPN transistors:

It is formed by sandwiching a thin layer of P-type semiconductor between two N-type semiconductors as shown:

PNP transistor:

It is formed by sandwiching a thin layer of N-type semiconductor between two P-type semiconductor as shown:

The common-emitter current gain is the ratio of the value of the transistor's collector current to the value of the transistor's base current in a transistor.

$\beta =\frac{I_c}{I_b}$

Note: 

The common base DC current gain is a ratio of the value of the transistor's collector current to the value of the transistor's emitter current.

$\alpha =\frac{I_c}{I_e}$

Since for a transistor I_e = I_b + I_c

α can now be written as:

$\alpha =\frac{I_c}{I_b+I_c}$

Dividing both the numerator and denominator by I_b, we get:

$\alpha =\frac{I_c/I_b}{1+I_c/I_b}$

Since $\beta =\frac{I_c}{I_b}$

$\alpha =\frac{\beta }{\beta +1}$

Important Differences between different transistor configuration is as shown:

The input resistance of the transistor is equal to the reciprocal of the slope of the input characteristic which is shown figure.

Input Characteristics:

The output voltage V_CE is kept constant and P.D. between the base and emitter V_BE is gradually increased. The base current I_B is measured. The curves obtained by plotting base current on the y-axis and V_BE on the x-axis at different values of V_CE are called input characteristics of the transistor. The reciprocal of the slope of the curve in the linear region gives input resistance Ri. For Ce mode is very small.

Output Characteristics:

The base current I_B is kept constant and the output voltage between the collector and emitter V_CE is gradually increased. The output current I_C is measured. The curves obtained by plotting collector current I_C on y-axis and V_CE on x-axis at different values of I_B are called output characteristics of the transistor.

A PNP transistor structure is as shown:

The arrow on the symbol for bipolar transistors indicates the PN junction between base and emitter and points in the direction in which conventional current travels, i.e. the direction of holes.

For a PNP transistor, when the emitter-base junction is forward biased, holes from the emitter start to flow to the base, i.e. in a PNP transistor, the current carriers are holes. The direction of the arrow also indicates the same.

NPN:

For an NPN transistor, when the emitter-base is forward biased, holes from the base (p-type) start to flow to the emitter side (n-type) and electrons start to flow from the emitter to the base. The direction, however, represents the direction of the hole flow.

Notes:

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.
• 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.

Various transistor configuration and their input and output impedances are given by: