Unit 2 semiconductors

Introduction to semiconductors

Electronic Materials

• The goal of electronic materials is to generate and control the flow of an electrical current.

• Electronic materials include:1. Conductors: have low resistance which

allows electrical current flow2. Insulators: have high resistance which

suppresses electrical current flow3. Semiconductors: can allow or suppress

electrical current flow

Conductors

• Good conductors have low resistance so electrons flow through them with ease.

• Best element conductors include:– Copper, silver, gold, aluminum, & nickel

• Alloys are also good conductors:– Brass & steel

• Good conductors can also be liquid:– Salt water

Semiconductor Valence Orbit• The main characteristic

of a semiconductor element is that it has four electrons in its outer or valence orbit.

Crystal Lattice Structure• The unique capability of

semiconductor atoms is their ability to link together to form a physical structure called a crystal lattice.

• The atoms link together with one another sharing their outer electrons.

• These links are called covalent bonds.

2D Crystal Lattice Structure

Semiconductors can be Insulators• If the material is pure semiconductor material like

silicon, the crystal lattice structure forms an excellent insulator since all the atoms are bound to one another and are not free for current flow.

• Good insulating semiconductor material is referred to as intrinsic.

• Since the outer valence electrons of each atom are tightly bound together with one another, the electrons are difficult to dislodge for current flow.

• Silicon in this form is a great insulator.• Semiconductor material is often used as an insulator.

Semiconductors are mainly two types

1. Intrinsic (Pure) Semiconductors

2. Extrinsic (Impure) Semiconductors

Intrinsic Semiconductor

• A Semiconductor which does not have any kind of impurities, behaves as an Insulator at 0k and behaves as a Conductor at higher temperature is known as Intrinsic Semiconductor or Pure Semiconductors.

• Germanium and Silicon (4th group elements) are the best examples of intrinsic semiconductors and they possess diamond

Doping• To make the semiconductor conduct electricity,

other atoms called impurities must be added.• “Impurities” are different elements. • This process is called doping.

The Intrinsic Semiconductors doped with pentavalent impurities are called N-type Semiconductors. The energy level of fifth electron is called donor level. The donor level is close to the bottom of the conduction band most of the donor level electrons are excited in to the conduction band at room temperature and become the Majority charge carriers.

Hence in N-type Semiconductors electrons are Majority carriers and holes are Minority carriers.

N-type Semiconductor

Si

Si

SiPSi

Free electron

Impure atom (Donor)

E Ed

Ev

Valence band

Ec

Conduction band

Ec

Electronenergy

Distance

Donor levelsEg

Electronic Properties of Si Silicon is a semiconductor material.

– Pure Si has a relatively high electrical resistivity at room temperature.

There are 2 types of mobile charge-carriers in Si:– Conduction electrons are negatively charged;– Holes are positively charged.

The concentration (#/cm3) of conduction electrons & holes in a semiconductor can be modulated in several ways:

1. by adding special impurity atoms ( dopants )

2. by applying an electric field

3. by changing the temperature

4. by irradiation

Another Way to Dope• You can also dope a

semiconductor material with an atom such as boron that has only 3 valence electrons.

• The 3 electrons in the outer orbit do form covalent bonds with its neighboring semiconductor atoms as before. But one electron is missing from the bond.

• This place where a fourth electron should be is referred to as a hole.

• The hole assumes a positive charge so it can attract electrons from some other source.

• Holes become a type of current carrier like the electron to support current flow.

Si

Si

SiInSi

HoleCo-Valent bonds

Impure atom (acceptor)

E

Ea

Ev

Valence band

Ec

Conduction band

Ec

Electronenergy

temperature

Acceptor levels

Eg

• The dominant charge carriers in a doped semiconductor (e.g. electrons in n-type material) are called majority charge carriers. Other type are minority charge carriers

• The overall doped material is electrically neutral

Current Flow in N-type Semiconductors• The DC voltage source has a

positive terminal that attracts the free electrons in the semiconductor and pulls them away from their atoms leaving the atoms charged positively.

• Electrons from the negative terminal of the supply enter the semiconductor material and are attracted by the positive charge of the atoms missing one of their electrons.

• Current (electrons) flows from the positive terminal to the negative terminal.

Current Flow in P-type Semiconductors• Electrons from the negative

supply terminal are attracted to the positive holes and fill them.

• The positive terminal of the supply pulls the electrons from the holes leaving the holes to attract more electrons.

• Current (electrons) flows from the negative terminal to the positive terminal.

• Inside the semiconductor current flow is actually by the movement of the holes from positive to negative.

p- n junction formation

What happens if n- and p-type materials are in close contact?

Being free particles, electrons start diffusing from n-type material into p-material

Being free particles, holes, too, start diffusing from p-type material into n-material

Have they been NEUTRAL particles, eventually all the free electrons and holes had uniformly distributed over the entire compound crystal.

However, every electrons transfers a negative charge (-q) onto the p-side and also leaves an uncompensated (+q) charge of the donor on the n-side. Every hole creates one positive charge (q) on the n-side and (-q) on the p-side

Diode concepts

pn Junction Under Open-Circuit Condition• The diffusion of positive charge in one direction and negative charge in the other

produces a charge imbalance – this results in a potential barrier across the junction

Contd…

Fig (a) shows the pn junction with no applied voltage (open-circuited terminals).

Fig.(b) shows the potential distribution along an axis perpendicular to the junction.

Forward bias

if the p-type side is made positive with respect to the n-type side the height of the barrier is reduced more majority charge carriers have sufficient energy to surmount it the diffusion current therefore increases while the drift current remains the same there is thus a net current flow across the junction which increases with the applied voltage

10.1.4 Biasing the PN-Junction*Forward Bias:

Reverse bias• dc voltage negative terminal connected to the p region and positive

to the n region. Depletion region widens until its potential difference equals the bias voltage, majority-carrier current ceases.

if the p-type side is made negative with respect to the n-type side the height of the barrier is increased the number of majority charge carriers that have sufficient energy to surmount it rapidly decreases . The diffusion current therefore vanishes while the drift current remains the same thus the only current is a small leakage current caused by the (approximately constant) drift current the leakage current is usually negligible (a few nA)

*Reverse Bias: majority-carrier current ceases.* However, there is still a very

small current produced by minority carriers.

Biasing the PN-Junction* Reverse Breakdown: As reverse voltage reach certain value,

avalanche occurs and generates large current.

• Currents in a pn junction

Diode Characteristic I-V Curve

Shockley Equation* The Shockley equation is a theoretical result

under certain simplification:

0v whenapplicable not is equation This

Vn

vexpIi 0.1V,v when

C101.60q constant, sBoltzman' the is k

voltage thermal the is 300K at 0.026V q

TkV

t,coefficien emission the is 2 to 1 n current,

n saturatio(reverse) the is 300K at A10I where

1Vn

vexpIi

D

T

DsDD

19-

T

14-s

T

DsD

Symbol and Characteristic for the Ideal Diode

(a) diode circuit symbol; (b) i–v characteristic; (c) equivalent circuit in the reverse direction; (d) equivalent circuit in the forward direction.

Diode currentsSwitching times of diodeDiode equivalent modelsAvalanche and zener breakdown

Diode currents

Switching times of diode

• The switching time of a diode is defined as the time which a diode takes to change its state from forward biased state to reverse biased state or in other words the forward current through diode doesn’t reduce to reverse saturation current immediately as the reverse voltage is applied.

• In fact it takes time for the current to reduce from forward current to reverse saturation current. This time is also called reverse recovery time.

Charge distribution of diode in Forward Biased state

The red curve shows the level of concentration of minority carriers at different distances on the both sides of junction and the shaded blue part shows the increase in the concentration of minority carriers after forward biasing the diode. There is a difference in the peak level of minority carriers as we have the difference in the doping level of both sides.

Charge distribution of diode in Reverse Biased state

• When we reverse biase any diode, the minority carriers from both sides cross the junction and then recombine after reaching the other side. Hence the holes from n-side move towards p-side and after reaching p-type material become majority carriers

Diode switching times

• Now let’s analyze that what would happen when we change diode state from forward biase from reverse bias.

• Firstly we are in forward biase state when voltage applied is +V. diode is now forward biased

• Now we change the applied voltage to –V at time t=t1. i.e. diode is now reverse biased.

• As minority carrier concentration in both sides was large near junction in the forward bias, when we have instantly changed the state to reverse biased, those minority carriers start moving in the opposite direction. And due to large concentration of such minority carriers, the amount of current flowing remains the same, only direction changes as shown below:

Contd..• But the high reverse current

continues for small time because the concentration of the stored minority carriers start decreasing and the current also starts decreasing exponentially as shown

• The time gap t2 - t1 in which the reverse current is high (i.e. equal to I) is known as storage time and the time gap from t2 to t3 i.e. the time reverse current becomes equal to reverse saturation current is known as transient time.  The total time from t1 to t3 is known as reverse recovery time.

Effect of doping on reverse recovery time

IDEAL DIODE

• about the ideal diode, the diode is a device which acts as a short circuit when forward biased and acts as open circuit when reverse biased.

• Hence the behavior of ideal diode can be shown in the following graph:

Simplified Diode model /Equivalent model of diode when forward biased.

Constant voltage drop model

Diode Voltages

A conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium.

To forward bias a diode, the anode

must be more positive than the cathode or

LESS NEGATIVE.

To reverse bias a diode, the anode

must be less positive than the cathode or

MORE NEGATIVE.

Types of diodes

• Varactor diodes– a reversed-biased diode has two conducting regions

separated by an insulating depletion region– this structure resembles a capacitor– variations in the reverse-bias voltage change the

width of the depletion layer and hence the capacitance

– this produces a voltage-dependent capacitor– these are used in applications such as automatic

tuning circuits

Light-Emitting Light-Emitting Diodes:Diodes:

Light-emitting diodes are designed with a very large bandgap Light-emitting diodes are designed with a very large bandgap so movement of carriers across their depletion region emits so movement of carriers across their depletion region emits photons of light energy. Lower bandgap LEDs (Light-Emitting photons of light energy. Lower bandgap LEDs (Light-Emitting Diodes) emit infrared radiation, while LEDs with higher Diodes) emit infrared radiation, while LEDs with higher bandgap energy emit visible light. Many stop lights are now bandgap energy emit visible light. Many stop lights are now starting to use LEDs because they are extremely bright and starting to use LEDs because they are extremely bright and last longer than regular bulbs for a relatively low cost. last longer than regular bulbs for a relatively low cost.

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Schematic Symbol for a Light-Schematic Symbol for a Light-Emitting DiodeEmitting Diode

The arrows in the LED The arrows in the LED representation indicate representation indicate emitted light.emitted light.

Contd,..Contd,..Photodiodes:Photodiodes: While LEDs emit light, Photodiodes are sensitive to received While LEDs emit light, Photodiodes are sensitive to received

light. They are constructed so their pn junction can be light. They are constructed so their pn junction can be exposed to the outside through a clear window or lens.exposed to the outside through a clear window or lens.In Photoconductive mode the saturation current increases in In Photoconductive mode the saturation current increases in proportion to the intensity of the received light. This type of proportion to the intensity of the received light. This type of diode is used in CD players.diode is used in CD players.In Photovoltaic mode, when the pn junction is exposed to a In Photovoltaic mode, when the pn junction is exposed to a certain wavelength of light, the diode generates voltage and certain wavelength of light, the diode generates voltage and can be used as an energy source. This type of diode is used can be used as an energy source. This type of diode is used in the production of solar power.in the production of solar power.

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Schematic Symbols for Schematic Symbols for PhotodiodesPhotodiodes

Diode Applications Rectifier CircuitsSimple Half-Wave Rectifier

What would the waveformlook like if not an ideal diode?

Rectifier Circuits (contd)

Bridge Rectifier

Looks like a Wheatstone bridge. Does not require a enter tapped transformer.Requires 2 additional diodes and voltage drop is double.

Rectifier Circuits

Peak Rectifier

To smooth out the peaks and obtain a DC voltage, add a capacitor across the output.

• Transition and diffusion capacitances also refer to notes or text book

PN Junction Capacitance• A reverse-biased PN junction can be viewed as

a capacitor. The depletion width (Wdep) and hence the junction capacitance (Cj) varies with VR.

dep

sij W

C