Majority Carrier Diode

By Dr. Ghanshyam Singh

Junction Breakdown

• When a huge reverse voltage is applied to a- p n junction, the junction breaks down and c

onducts a very large current.

• Although, the breakdown process is not nat urally destructive, the maximum current mu

st be limited by an external circuit to avoid excessive junction heating.

• There are mechanisms dealing with the bre akdown: zener diode, tunneling diode and a valanche diode.

Tunneling Effect

• If a very high electric field is applied to a p-n junction in the reverse direction, a valence electron can make a transition from the valence band to the conduction band by penetrating through the energy bandgap called tunneling.

• The typical field for Si and GaAs is about 106 V/cm or higher.

Tunneling Effect

• To achieve such a high field, the doping con - - centration for both p and n regions must be

very high such as more than5 x 1017 cm-3.

• The breakdown voltage for Si and GaAs jun ctions about4Eg/e is the result of the tunneli

ng effect. With the breakdown voltage is mo re than6Eg/e , the breakdown mechanism is

the result of avalanche multiplication .• As the voltage is in between4Eg/e and6Eg/e

, the breakdown is due to a mix of both tunn eling effect and avalanche multiplication.

Backward Diode• a backward diode (also called back diode) is a

variation on a Zener diode or tunnel diode having a better conduction for small reverse biases (for example –0.1 to –0.6 V) than for forward bias voltages.

• The schematic symbol for the backward diode, annotated to show which side is P type and which is N; current flows most easily from N to P, backward relative to the arrow.

Current–voltage characteristics• The forward I–V characteristic is the same as that of

an ordinary P–N diode.

• The breakdown starts when reverse voltage is applied. In the case of Zener breakdown, it starts at a particular voltage. In this diode the voltage remains relatively constant (independent of current) when it is connected in reverse bias.

• The backward diode is a special form of tunnel diode in which the tunneling phenomenon is only incipient, and the negative resistance region virtually disappears. The forward current is very small and becomes equivalent to the reverse current of a conventional diode.

Energy Band diagram

Electron energy is on the vertical axis, position within the device is on the horizontal axis. The backward diode has the unusual property that the so-called reverse bias direction actually has more current flow than the so-called forward bias.

Applications of backward diodes• Detector

Since it has low capacitance and no charge storage effect, and a strongly nonlinear small-signal characteristic, the backward diode can be used as a detector up to 40 GHz.

• Rectifier

A backward diode can be used for rectifying weak signals with peak amplitudes of 0.1 to 0.7 V.

• Switch

Backward diode can be used in high speed switching applications.

Schottky diode• The Schottky diode also known as hot

carrier diode is a semiconductor diode with a low forward voltage drop and a very fast switching action.

• When current flows through a diode there is a small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.6–1.7 volts, while a Schottky diode voltage drop is between approximately 0.15–0.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency.

Schottky effect• In electron emission devices, especially electron guns,

the thermionic electron emitter will be biased negative relative to its surroundings. This creates an electric field of magnitude F at the emitter surface. Without the field, the surface barrier seen by an escaping Fermi-level electron has height W equal to the local work-function. The electric field lowers the surface barrier by an amount ΔW, and increases the emission current. This is known as the Schottky effect or field enhanced thermionic emission. It can be modeled by a simple modification of the Richardson equation, by replacing W by (W − ΔW). This gives the equation

ApplicationsVoltage clamping

• While standard silicon diodes have a forward voltage drop of about 0.7 volts and germanium diodes 0.3 volts, Schottky diodes’ voltage drop at forward biases of around 1 mA is in the range 0.15 V to 0.46 V which makes them useful in voltage clamping applications and prevention of transistor saturation. This is due to the higher current density in the Schottky diode.

Reverse current and discharge protection

• Because of a Schottky diode's low forward voltage drop, less energy is wasted as heat making them the most efficient choice for applications sensitive to efficiency.

Power supply

• Schottky diodes can be used in power supply circuits in products that have both an internal battery and a mains adapter input, or similar.

Heterojunction

A heterojunction is defi ned as a junction forme

d by two semiconducto rs with different energy

bandgaps Eg , different dielectric permittivities

s , different work functi on es , and different ele

ctron affinities eχ.

Heterojunction

• The difference energy betw een two conduction band ed ges and between two valenc

e band edges are represent ed by EC and EV , respectiv

ely, as

where Eg is the difference e nergy bandgap of two semic

onductors.

2 1CE e

1 1 2 2V g g g CE E e E e E E

Heterojunction

• Generally, heterojunction has to be formed between semiconductors with closely matched lattice constants.

• For example, the AlxGa1-xAs material is the most important material for heterojunction.

• When x = 0, the bandgap of GaAs is 1.42 eV with a lattice constant of 5.6533 Å at 300 K.

• When x = 1, the bandgap of AlAs is 2.17 eV with a lattice constant of 5.6605 Å .

Heterojunction

• We clearly see that the lattice constant is almost constant as x increased. The total built-in potential Vbi can be expressed by

where N1 and N

2 are the doping concentration

s in semiconductor1 and2 , respectively.

1 2bi b bV V V

2 21

1 1 2 2

1 12

1 1 2 2

bib

bib

N V VV

N N

N V VV

N N

Heterojunction

• The depletion widths x1 and x

2 can be found

by

1 2 21

1 1 1 2 2

1 2 12

2 1 1 2 2

2

2

bi

bi

N V Vx

eN N N

N V Vx

eN N N

Heterojunction

Ex. Consider an ideal abrupt heterojunction wi - th a built in potential of 1.6 V. The impurity

concentrations in semiconductor1 and2 ar e1 x 1016 donors/cm3 and3 x 1019 acceptors

/cm3 , and the dielectric constants are 12 an d 13 , respectively. Find the electrostatic pot

ential and depletion width in each material at thermal equilibrium.

Heterojunction

Soln

19

1 16 19

16

42 16 19

14 19

51 19 16 16 19

14 16

2

13 3 10 1.61.6 V

12 1 10 13 3 10

12 1 10 1.64.9 10 V

12 1 10 13 3 10

2 12 13 8.85 10 3 10 1.64.608 10 cm

1.6 10 1 10 12 10 13 3 10

2 12 13 8.85 10 1 10 1.

b

b

V

V

x

x

8

19 19 16 19

61.536 10 cm

1.6 10 3 10 12 10 13 3 10

Note: - Most of the built in potential is in the semic onductor with a lower doping concentration and

also its depletion width is much wider.

Metal-Semiconductor Junctions

• The MS junction is more likely known as the Sch- ottky barrier diode.

• Let’s consider metal ba nd and semiconductor b

and diagram before thecontact.

Metal-Semiconductor Junctions

• When the metal and se miconductor are joined,

electrons from the semi conductor cross over to

the metal until the Fer mi level is aligned (Ther

mal equilibrium conditi on).

• This leaves ionized don ors as fixed positive ch

arges that produce an i nternal electric field as

- -the case of one sided p n junction.

Metal-Semiconductor Junctions

• At equilibrium, equal number of electrons across the interface in opposite directions.

• Hence, no net transport of charge, electron current I - e equals to zero. The built in voltage Vbi = m - s .

• The barrier for electrons to flow from the metal to se miconductor is given by eb = e(m - χs) or it is called t

he barrier height of MS contact.

Metal-Semiconductor Junctions

• When a voltage is applied, the barrier heigh - t remains fixed but the built in voltage chan

ges as increasing when reverse biased and decreasing when forward biased.

Metal-Semiconductor Junctions• Reverse bias

• Few electrons move across the interface from metal t o semiconductor due to a barrier, but it is harder for

electrons in the semiconductor to move to the metal.

• Hence, net electron transport is caused by electrons moving from metal to semiconductor. Electron curre

nt flows from right to left which is a small value.

Metal-Semiconductor Junctions• Forward bias

• Few electrons move across the interface from metal to semiconductor, but many electrons move across t he interface from semiconductor to metal due to the

reduced barrier.

• Therefore, net transport of charge flows from semic onductor to metal and electron current flows from le

ft to right.

Metal-Semiconductor Junctions

• Under forward bias, the electrons emitted to the metal have greater energy than that of the metal electrons by about e(m - χs).

• These electrons are called hot-carrier since their equivalent temperature is higher than that of electrons in the metal.

• Therefore, sometimes, Schottky-diode is called “hot-carrier diode”.

Metal-Semiconductor Junctions

• This leads to the thermionic emission with t hermionic current density under forward bia

s as

/ /** 2 m s F Fe V kT eV kT

F sJ A T e J e

/** 2

*** *

0

where saturation current

. effective Richardson's constant

m se kT

sJ A T e

mA A

m

Metal-Semiconductor Junctions

• This behavior is referred to a rectification an d can be described by an ideal diode equati

on of

where V positive for forward bias and negati ve for reverse bias.

/ 1eV kTsJ J e

Metal-Semiconductor Junctions• - The space charge region width of Schottky diode is i

- - dentical to that of a one sided p n junction.

• Therefore, under reverse bias, they can contain the charges in their depletion region and this is called S chottky diode capacitance.

Metal-Semiconductor Junctions

Ex. A Schottky junction is formed between Au - and n type semiconductor of ND = 1016 cm-3 . Area of junction = 10-3 cm2 and me

* = 0.92 m0

. Work function of gold is 4.77 eV and eχs = 4.05 eV. Find current at VF = 0.3 volts.

Metal-Semiconductor Junctions

Soln

*** * 2

0

/** 2

/

4.77 4.05 / 0.02592 0.3 / 0.0259

2

3

120 0.92 110 A/ cm .K

1

110 300 e . 1

0.897 A/(cm )

10 0.897 0.897 mA

m s

e

e kTs

eV kTs

mA A

m

J A T e

J J e

e

I A J

Metal-Semiconductor Junctions

Ex. - Si Schottky diode of 100 μm diameter has(1/C2 ) v.s. VR slope of3 x 1019 F-2V-1 . Given r

= 11.9 for Si. Find NB for this semiconductor.

Metal-Semiconductor Junctions

Soln 2

2

4 -2 -1

2

22419 19 12

19 -3

21; [F/cm ]

2slope [cm F V ]

2

2

100 103 10 1.6 10 8.85 10 11.9

2

6.414 10 cm

bi Rj

j s B

s B

Bs

B

V V CC

C e N Area

e N

Nslope Area e

N

Ohmic contact

• This contact is defined as a junction that will not add a significant parasitic impedance to the structure on which it is used and will not sufficiently change the equilibrium-carrier densities within the semiconductor to affect the device characteristics.

• The I-V characteristic of ohmic contact is linear for an ideal case.

Ohmic contact

• A specific contact resistance RC is given by

• A good ohmic contact should have a small s pecific con tact resistance about 10-6 Ω.cm2 .

12

0

1.cm

VC

J

R V

Ohmic contact

• When the semiconduct or is heavily doped wit

h an impurity density o f 1019 cm-3 or higher, th e depletion layer of the

junction becomes very thin so that carriers ca

n tunnel instead of goi ng over the potential b

arrier.

THANK YOU