1. Lecture 2
OUTLINE
Semiconductor Basics
Reading: Chapter 2

2. Announcement Office Hours for tomorrow is cancelled
(ONLY for this week)
There will be office hours on Friday (2P-3P)
Thursday’s class will start at 4P

3. What is a Semiconductor?
Low resistivity => “conductor”
High resistivity => “insulator”
Intermediate resistivity => “semiconductor”
conductivity lies between that of conductors and insulators
generally crystalline in structure for IC devices
In recent years, however, non-crystalline semiconductors have become commercially very important
polycrystalline amorphous crystalline

4. Semiconductor Materials
Phosphorus
(P)
Gallium
(Ga)

5. Energy Band Description
For current flow, one needs to have electrons in the conduction band or holes in the valence band
Completely full or completely empty bands cannot carry current

6. Energy Band Description
Current due to electron flow and hole flow will add up

7. Silicon Atomic density: 5 x 1022 atoms/cm3
Si has four valence electrons. Therefore, it can form covalent bonds with four of its nearest neighbors.
When temperature goes up, electrons can become free to move about the Si lattice.

8. 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:
by adding special impurity atoms ( dopants )
by applying an electric field
by changing the temperature
by irradiation

9. Electron-Hole Pair Generation
When a conduction electron is thermally generated, a “hole” is also generated.
A hole is associated with a positive charge, and is free to move about the Si lattice as well.

10. Carrier Concentrations in Intrinsic Si
The “band-gap energy” Eg is the amount of energy needed to remove an electron from a covalent bond.
The concentration of conduction electrons in intrinsic silicon, ni, depends exponentially on Eg and the absolute temperature (T):

11. Doping (N type)
Si can be “doped” with other elements to change its electrical properties.
For example, if Si is doped with phosphorus (P), each P atom can donate a conduction electron, so that the Si lattice has more electrons than holes, i.e. it becomes “N type”:
Notation:
n = conduction electron
concentration

12. Doping (P type)
If Si is doped with Boron (B), each B atom can accept an electron (creating a hole), so that the Si lattice has more holes than conduction electrons, i.e. it becomes “P type”:
Notation:
p = hole concentration

13. Terminology donor: impurity atom that increases n
acceptor: impurity atom that increases p
N-type material: contains more electrons than holes
P-type material: contains more holes than electrons
majority carrier: the most abundant carrier
minority carrier: the least abundant carrier
intrinsic semiconductor: n = p = ni
extrinsic semiconductor: doped semiconductor

14. Intrinsic vs. Extrinsic Semiconductor

15. Electron and Hole Concentrations
Under thermal equilibrium conditions, the product of the conduction-electron density and the hole density is ALWAYS equal to the square of ni:
N-type material
P-type material

16. Dopant Compensation
An N-type semiconductor can be converted into P-type material by counter-doping it with acceptors such that NA > ND.
A compensated semiconductor material has both acceptors and donors.
N-type material
(ND > NA)
P-type material
(NA > ND)

17. Doping
What is the electron and hole density if you dope Si with Boron to 1018 /cm3 ?

18. Charges in a Semiconductor
Negative charges:
Conduction electrons (density = n)
Ionized acceptor atoms (density = NA)
Positive charges:
Holes (density = p)
Ionized donor atoms (density = ND)
The net charge density (C/cm3) in a semiconductor is

19. Carrier Drift
The process in which charged particles move because of an electric field is called drift.
Charged particles within a semiconductor move with an average velocity proportional to the electric field.
The proportionality constant is the carrier mobility.
Hole velocity
Electron velocity
Notation:
mp  hole mobility (cm2/V·s)
mn  electron mobility (cm2/V·s)

20. Velocity Saturation
In reality, carrier velocities saturate at an upper limit, called the saturation velocity (vsat).

21. Drift Current
Drift current is proportional to the carrier velocity and carrier concentration:
Total current Jp,drift= Q/t
Q= total charge contained in the volume shown to the right
t= time taken by Q to cross the volume
Q=qp(in cm3)X Volume=qpAL=qpAvht
 Hole current per unit area (i.e. current density) Jp,drift = q p vh

22. Conductivity and Resistivity
In a semiconductor, both electrons and holes conduct current:
The conductivity of a semiconductor is
Unit: mho/cm
The resistivity of a semiconductor is
Unit: ohm-cm

23. Resistivity Example
Estimate the resistivity of a Si sample doped with phosphorus to a concentration of 1015 cm-3 and boron to a concentration of 6x1017 cm-3.