EP 319-CHAPTER 4

PN JUNCTIONS

FORMATION OF PN JUNCTION

A p–n junction is a junction formed by joining p-type and n-type semiconductors together in very close contact. The term junction refers to the boundary interface where the two regions of the semiconductor meet. If they were constructed of two separate pieces this would introduce a grain boundary, so p–n junctions are more often created in a single crystal of semiconductor by doping, for example by ion implantation, d i f f u s i o n o f d o p a n t s , o r b y e p i t a x y .

**Remember that two excitation mechanism; diffusion and drift.

Diffusion current is due to the movement of the carriers from high concentration region towards to low

concentration region.

Drift current is due to the movement of the carriers under the influence of an applied electric field.

P-n junctions are formed by joining n-type and p-type semiconductor materials, as shown below. Since the n-type region has a high electron concentration and the p-type a high hole concentration, electrons diffuse from the n-type side to the p-type side. Similarly, holes flow by diffusion from the p-type side to the n-type side. If the electrons and holes were not charged, this diffusion process w o u l d c o n t i n u e u n t i l t h e concentration of electrons and holes on the two sides were the same.

• When the concentration of holes and

electrons are same, a depletion region is

formed. There aren’t any mobile charge

carriers in this region.

• An electric field (Ê) therefore builds up

in the so-called depletion region around

the junction to stop the flow.

• Depending on the materials used, a

‘built in’potential (Vbi) owing to Ê will be

formed.

The depletion region makes the p – n junction into

a diode, a device that conducts current easily in

one direction only.

• The electric field formed in the depletion region

acts as a barrier.

• External energy must be applied to get the

electrons to move across the barrier of the

electric field.

• The potential difference required to move the

electrons through the electric field is called the

barrier potential (built-in potential).

• The way is to move across the electric field

barrier give external voltage to the pn junction by

connecting a battery.

• The p-n junction may be connected to a battery

in two ways: (i) in forward bias (ii) in reverse bias

Energy levels in a p-n junction

Forward bias

Forward-bias occurs when the p-type semi-conductor

material is connected to the positive terminal of a battery

and the n-type semi-conductor material is connected to

the negative terminal. With a battery connected this way,

the holes in the p-type region and the electrons in the n-

type region are pushed towards the junction. This

reduces the width of the depletion zone. The positive

charge applied to the p-type material repels the holes,

while the negative charge applied to the n-type material

repels the electrons. As electrons and holes are pushed

towards the junction, the distance between them

decreases.

Forward Bias

This lowers the barrier in the potential. With increasing

forward-bias voltage, the depletion zone eventually

becomes thin enough that the zone’s electric field can’t

counteract the charge carrier motion across the p-n

junction, consequently reducing electrical resistance.

The electrons which cross the p-n junction into the p-

type material (or holes which cross into the n-type

material) will diffuse in the near neutral region.

Therefore, the amount of minority diffusion in the near-

neutral zones determines the amount of current that may

flow through the diode.

Forward Bias

• Once Ê is no longer large enough to stop the flow of

electrons and holes, a current is produced. The built in

potential reduces to Vbi –V and the current flow increases

exponentially with the applied voltage. This phenomenon

results in the Ideal Diode Law, expressed as

• where I is the current, I0 is the dark saturation current (the

diode leakage current density in the absence of light), V is

the applied voltage, q is the charge on an electron, k is

Boltzmann’s constant and T is absolute temperature.

Forward Bias • The practical result of the movements of electrons and holes is

summarised by the diode characteristic in Figure. Diode current I

increases with positive bias, growing rapidly above about 0.6 V; but with

negative bias the reverse current ‘ saturates ’ at a very small value Io.

Clearly this device only allows current flow easily in one direction.

Reverse Bias

Connecting the p-type region to the negative terminal of

the battery and the ntype region to the positive terminal

produces the reverse-bias effect. Because the p-type

material is now connected to the negative terminal of the

power supply, the ‘holes’ in the p-type material are pulled

away from the junction, causing the width of the depletion

zone to increase. Similarly, because the n-type region is

connected to the positive terminal, the electrons will also

be pulled away from the junction. Therefore the depletion

region widens, and does so increasingly with increasing

reverse-bias voltage.

Reverse Bias

This increases the voltage barrier, causing a high

resistance to the flow of charge carriers thus allowing

minimal electric current to cross the p-n junction. The

strength of the depletion zone electric field increases as

the reverse-bias voltage increases. Once the electric

field intensity increases beyond a critical level, the p-n

junction depletion zone breaks down and current begins

to flow, usually by either the Zener or the avalanche

breakdown processes. Both of these breakdown

processes are non-destructive and are reversible, so

long as the amount of current flowing does not reach

levels that cause the semi-conductor material to

overheat and cause thermal damage.

Photovoltaic Effect

When the solar cell (p-n junction) is illuminated, electron-

holes pairs are generated and acted upon by the internal

electric fields, resulting in a photo current (IL). The

generated photocurrent flows in a direction opposite to

the forward dark current. Even in the absence of external

applied voltage, this photocurrent continues to flow, and is

measured as the short circuit current (Isc). This current

depends linearly on the light intensity, because absorption

of more light results in additional electrons flowing in the

internal electric field force. The overall cell current I is

determined by subtracting the light induced current IL from

the diode dark current ID.