Chapter 5 (lec 14-20)

Question 1

What is a P-N Junction

Answer 1

A p-n junction refers to a metallurgial junction between a p type and n type semiconductor

Question 2

How does a P N junction work

Answer 2

The electric field and electron energy acorss the junction is controlled innovativley to allow or block the current flowing through the junction

Question 3

Why is a Pn junction important

Answer 3

• Fundamental building block of all semiconductor devices
• single most importantly invention
• invented in 1939 at the bell laboratory Nj
• Invented by Russell shoemaker Ohl

Question 4

What are the assumptions made before forming a junction (5)

Answer 4

1. Uniform P doping on one side of a sharp metallurgical junction and uniform n doping on the other side of the junction
2. One dimensional current (along x axis)
3. Thermal equilibrium ( no excess carrier)
4. no net current
5. The bulk material is quasi natural ( no electric field inside the material

Question 5

What are the assumptions made after forming a junction (7)

Answer 5

1. Fermi level of both n and p side align
2. Electrons diffuse n to p
   3 Uncompensated donor ions (Nd+) are left in the n side near the junction
3. Holes diffuse from p to n
4. Uncompensated acceptor ions (Na-) are left in the p sode near the junction
5. An electric field E is established from n to p due to the uncompensated acceptors and donors
6. The voltage associated with the electric field is called contact potential Vo

Question 6

What are the properties of equlibrium after forming a junction

Answer 6

1. Due to the electric field E electrons drift from P to n
2. Due to e field E holes drift from n to p
3. At some point an equilibrium is reached (drift = diffusion)
   4.No net current flows
4. This region is called the space charge region SCR or the depletion region as the region is now depleted of free carriers

Question 7

Ideal case assumptions for a pn junction

Answer 7

1. In the space charge region SCR there are no mobile carriers
2. In the quasi natural region (QNR) there is no electric field
3. The QNR sharply ends at -xp in the p side
4. The QNR sharply ends at +xp in the n side

Question 8

What are the 9 design parameters for a PN junction

Answer 8

1. Contact potential across the SCR Vo
2. Distribution of the electric field and energy across the SCR (E)
3. Distribution of electric potential across the SCR ( phy circle with line through it )
4. Width of the SCR (W)
5. Penetration of the SCR into the Pside and the Nside (-xp,xn)
6. Total charged in the Pside and Nside SCR(-Q , +Q)
7. Length of QNR (l)
8. Capacticance across the SCR and QNR Cj
9. Capacitance across the QNR, Cd

Question 9

Explain Band Bending

Answer 9

When an electric field exisits inside a material the band energies become a function of the position x the resulting variation of Ec and Ev and Ei with the position on the energy band diagram is called band bending

Question 10

List Assumptions when constructing Pn junction thermal equilibrium band diagrams

Answer 10

1. Vacuum energy level Eo must be continuous
2. Electron affinity is a property associated with crystal lattice and is constant for a given material
3. Fermi level must be constant
4. q phy-s and q-x are physical properties of the material

Question 11

Why do N side bands move down after forming a junction

Answer 11

1. Fermi level moved down to sastify the conditio of invariance of fermi level
2. As the work function is a constant for a particular material the vacuum level then must go down the N side
3. As the electron affinity is a constant for a particular material the vacuum level moded down, Ec will come down as well to sastify this condition
4. As Ec comes down Ec has a fixed relation with Ev by the band gap, Ev comes down
5. Ei is in the middle of the bandgap this Ei also moves down

Question 12

Explain the core concepts of fwrd reverse biases

Answer 12

• Manipulates the potentional barrier from n to p side to increase or decrease the current by applying a bias voltage
  -The fwd or rev bias voltage destroys the equilibrium condition of zero net current through the junction so that one type of current (drift or diffusion) dominates the other

Question 13

Explain the relationship for diffusion and drift current in both forward and reverse biases

Answer 13

• In fwd bias diffusion current is greater then drift current
• in reverse bias drift current is greater then diffusion current

Question 14

Explain the direction of minority carrier flow for fwd and rev bias

Answer 14

• in fwd minority carrier are injected into the QNR
• In rev minority carriers are extracted from the QNR

Question 15

Explain the bias voltage connection and direction of electric field for Fwd bias

Answer 15

• Postive terminal of the bias voltage source is connected to the p side and negative terminal of the bias voltage source is connected to the n side
• electric field due to the bias voltage is from p to n

Question 16

Explain the bias voltage connection and direction of electric field for reverse bias

Answer 16

• Postive terminal of the bias voltage source is connected to the inside and negative terminal of the bias voltage source is connected to the p side
• electric field due to the bias voltages is from n to p

Question 17

What is the transmission coeficatn T

Answer 17

The relative probability that an elecgtron will tunnel from region I to III through II barrier

Question 18

Give a breakdown of the tunneling process

Answer 18

When an electron encounters a potential energy barrier of height Vo greater than its energy but finite there is a finite probability that it will leak through the barrier
- If the barrier is infinite there is no tunneling
- The probability depends on sensitively on the energy width of the barrier
- for a wide potential barrier the probability of tunneling is proportional to e^-2alpha*a
- The wider or higher the potential barrier Vo the smaller the chance of tunneling

Question 19

What are the 5 zener breakdown requirements ( 2022 EXAM QUESTION LOOK AT GRAPH IN NOTES) lec 17

Answer 19

1. High Vr to realize high electric field to extract minority carriers
2. High Vr to align n side CB with p side VB (lowers the potential barrier)
3. High Vr narrows down SCR width w(a)
4. Heavy doping >5E17 /cm^3
5. Sharp junction to achieve small W

Question 20

Explain the zener breakdown mechanism (exam question 2022 lec 17 for diagram)

Answer 20

• With a suffciently high reverse bias Vr, n-side conduction bands align with p-side valence band
• if a, the width of the SCR (potential barrier) is narrow enough, electrons can tunnel through the SCR width from P-side valance band to inside conduction to generate a reverse current

Question 21

Explain the Avalanche breakdown (Exam question 2022 lec 17 for diagram)

Answer 21

high level - A high electric field across the SCR due to a reverse bias causes extensive impact ionization to create high electron and hole currents

1. A thermally generated electron (1) in
   the SCR gains kinetic energy due to
   strong reverse bias electric field
2. High kinetic energy electron breaks
   down a lattice bond when collide with
   an atom to create an electron hole
   pair (2 and 2’)
3. The newly created EHP acquire
   kinetic energy from the field and
   creates another electron EHP (3-3’)
   in the same manner
4. The process continues creating a
   stream of high energy electrons and
   holes similar to an avalanche

Question 22

What are the requirments for avalanche breakdown (lec 17, 2022 exam question)

Answer 22

• p+-n Junction (heavily doped p side,
  one sided junction)
• High doping
• High reverse bias

Question 23

what are the two types of capacitance in a p-n junction

Answer 23

1. diffusion capacitance (occurs when the junction is forward biased to to minority carrier motion across the QNR due to diffusion
2. Junction capacitance (due to fixed charges in SCR)

Question 24

Go over a long base diode and what assumption is made

Answer 24

• assume Na = Nd
• for a long diode the diffusion length Ln or Lp is small compared to the length of the p and n regions
• injected minority carriers decay exponentially to the equilibrium value before reaching the ohmic contacts

Question 25

Go over short based diodes

Answer 25

• A diode is called short if the diffusion length is ling compared to the lengths of the p or n side
• injected minority carriers decay linearly to zero at the ohmic contacts
• all the injected carriers diffuse across the n region before recombinaton
• In practice, most Si p-n junctions behave like short diodes, while laser diodes made in direct band gap (short lifetime) semiconductors often correspond to the long-diode case

Question 26

What is a nondegenerate semiconductor

Answer 26

if the fermi level Ef is at least 3kT above Ev or 3Kt below Ec

Question 27

Linearly graded junction

Answer 27

In a linearly graded junction the net dopant concentration varies gradually from the p type material to the n type
The junction is formed by two successive diffusion of opposite types of atoms

Question 28

What is a varactor diode

Answer 28

Voltage variable capicator- a change in bias voltage causes a change in diode capacitance - use for tuning

Question 29

What are the two types of semiconductor junctions

Answer 29

1. Schottky diode: a rectifying junction (polarity dependant current flow) like a p-n junction diode
2. Ohmic contact: a non - rectifying negligible loss contact used just for interconnection

Question 30

What is a schottky barrier

Answer 30

Refers to a metal semiconductor contact with a larger barrier height from the metal to the semiconductor or a low doping concentration that is less then the density of states in the conduction or valance band in the semiconductor
- current is mainly due to the majority carrier
- at moderate temp (300k) the dominant transport mechanism is thermoionic emission of majority carriers from the semiconductor over the potential barrier to the metal

Question 31

What are the advantages of using schottky diode

Answer 31

-Fast switching speed
- Lower contact potential results in low turn on voltage
- lower range of voltage drop
- more power efficant
- small depletion capacitance
- small junction capacitance