Download CHAPTER 16

CHAPTER 16
INTRODUCTION TO
SEMICONDUCTORS
ATOMIC STRUCTURE
AND SEMICONDUCTORS

The basic structure of semiconductors

Silicon and Germanium Atoms
ATOMIC BONDING


The atoms within the crystal structure are held
together by covalent bonds
This sharing of valence electrons produces the
covalent bonds that hold the atoms together
CONDUCTION IN
SEMICONDUCTORS

An energy band
diagram for silicon
crystal occurs only
at a temperature of
absolute 0 K
Comparison of Semiconductors to
Conductors and Insulators

Pure semiconductive
materials are neither
insulators nor good
conductors because
current in a material
depends directly on
the number of free
electrons
Resistivity (ohm-cm)
Intrinsic (pure)
Conduction Electrons and Holes


An intrinsic (pure) silicon crystal at room
temperature has sufficient heat energy for
some valence electrons to jump the gap
from the valence band into the conduction
band, which become free electrons
When an electron jumps to the conduction
band, a vacancy is left in the valence band
within the crystal, called a hole.
Creation of electron-hole
Electron-hole pairs

Recombination occurs when a conduction-band
electron loses energy and fall back into a hole in
the valence band

Types of Current in Semiconductor
– Electron
Current
– Hole Current
Electron Current in intrinsic
silicon

When a voltage is applied across a piece of
silicon, the movement of free electrons is called
electron current. The current which flow opposite
with electron current is called hole current.
Hole current in intrinsic silicon
N-TYPE AND P-TYPE
SEMICONDUCTORS


The conductivities of silicon and
germanium can be increased and
controlled by the addition of impurities to
the intrinsic (pure) semiconductive
material called doping
The two categories of impurities are ntype and p-type
N-TYPE SEMICONDUCTOR

To increase the number of
conduction-band electron
in intrinsic silicon,
pentavalent impurity atom
with

five valence electrons (such
as arsenic (As), phosphorus
(P), and antimony (Sb) are
added.
n-type
Majority and Minority Carriers of
N-type Semiconductor


The electrons are called the majority
carries in n-type material ( the n stand for
the negative charge on an electron)
Holes which are not produced by the
addition of the pentavalent impurity atoms
are called minority carries
P-TYPE SEMICONDUCTOR


Trivalent impurity atom
(three valence electrons,
such as aluminum (Al),
Boron (B), and gallium
(Ga)) are added to
increase the number of
holes in intrinsic silicon
Atoms with three valence
electrons are known
acceptor atoms because
they leave a hole in the
semiconductor’s crystal
structure
p-type
Majority and Minority Carriers of
P-type Semiconductor


The holes are the majority carries in p-type
material
The Electron in p-type material are the
minority carries
Resistivity vs Concentration of
Number of free electrons (Si)
THE PN JUNCTION


The junction of silicon which it has
doped on one half with a trivalent
impurity and the other half with a
pentavalent impurity is called the pn
junction
The pn junction is the feature that
allows diodes , transistor, and other
devices to work
ขัน้ ตอนการเข้ าสู่สมดุลของรอยต่ อ PN
1) Free e- จานวนมากใน N เคลื่อนที่ไปมาอิสระ มีบางตัวเคลื่อนที่แพร่
ข้ ามไปยังฝั่ ง P
2) Free e-N รวมกับ holeP บริเวณขอบ
Atom ของ P ได้ รับ e- เกิน
Atom ของ N สูญเสีย e-
-> ประจุลบ
-> ประจุบวก
ขัน้ ตอนการเข้ าสู่สมดุลของรอยต่ อ PN
3) เกิดการสะสมประจุบริเวณรอยต่ อเกิดเป็ นกาแพงความต่ างศักย์ (Barrier
Potential)
: บริเวณรอยต่ อที่มีการสะสมประจุ = Depletion region
: ต้ านการไหล (แพร่ ) ของ e- จาก N -> P
4) Depletion region จะขยายการสะสมประจุ ทาให้ กาแพงความต่ างศักย์ (ความ
ต่ างศักย์ ท่ ีรอยต่ อ)เพิ่มสูงขึน้ จะต้ านการเคลื่อนที่ของ e- ทาให้
: จานวน e- จาก N ข้ ามไป P ลดน้ อยลงเรื่ อยๆ
ขัน้ ตอนการเข้ าสู่สมดุลของรอยต่ อ PN
VB
5) เข้ าสู่ภาวะสมดุล (Equilibrium state)
: หยุดการเคลื่อนที่ของ e- จาก N -> P
: เงื่อนไข
@ กาแพงความต่ างศักย์ (VB: Barrier Potential) สูง จน
ไม่ มี e- จาก N มีพลังงานสูงพอที่จะเอาชนะ VB ที่รอยต่ อ
จึงไม่ สามารถข้ ามรอยต่ อจาก N -> P ได้
VB (Si ) = 0.7 V; VB (Ge ) = 0.3 V
@ Depletion region ขยายกว้ าง
BIASING THE PN JUNCTION

Forward Bias


Forward bias is the condition that permits
current through a pn junction
The negative terminal of the VBIAS source is
connected to the n region, and the positive
terminal is connected to the p region
The Effect of the Barrier Potential
on Forward Bias
BIASING THE PN JUNCTION

Reverse Bias


Reverse bias is the condition that prevents
current through the pn junction
Reverse current is a very small current
produced by minority carries during reverse
bias
Energy Diagram for Reverse Bias


When a pn junction is reverse-biased, the n-region
conduction band remain at an energy level that prevents
the free electrons from crossing into the p-region
There are a few free minority electrons in the p-region
conduction band that flow down the ‘energy hill’ into the
n-region, and they combine with minority hole in the
valence band
DIODE CHARACTERISTICS

Diode Characteristic
Curve

Forward bias

As the forward
voltage
approaches the
value of the barrier
potential (0.7 V for
silicon and 0.3 V
for germanium),
the current begins
to increase
DIODE CHARACTERISTICS

Diode Characteristic
Curve

Reverse bias


As the voltage (VR)
increases to the
left, the current
remains near zero
until the
breakdown voltage
(VBR) is reached
When breakdown
occurs, there is a
large reverse
current that can
destroy the diode
Reverse Breakdown



If the external reverse-bias voltage is increased to
a large enough value, reverse breakdown occurs
When one minority conduction-band electron goes
toward the positive end of the pn junction, during
its travel, it collides with an atom and imparts
enough to knock a valence electron into the
conduction band
The rapid multiplication of conduction-band
electrons, known as an avalanche effect