This document discusses semiconductor electronics and band theory of solids. It explains that when two hydrogen atoms are brought together, their energy levels split into two, with one level slightly higher and one lower. When many atoms come together, the energy levels form bands. Insulators have a large band gap, semiconductors have a smaller band gap, and metals have overlapping bands. The document also covers p-n junction diodes, their characteristics and circuits, as well as bipolar junction transistors, how they are biased, and their input and output characteristics.

1. SEMICONDUCTOR ELECTRONICS

2. • situation when two hydrogen atoms are
brought together.
• interaction between the electrostatic fields of
the atoms split each energy level into two,
• gives one level slightly higher than before
and another one slightly lower
Band theory of solids

3. Band theory of solids
1 ATOM 2 ATOMS MANY ATOMS
BAND GAP

4. Insulators, Semiconductors and
Metals
VB
VB
CB
CB
CB
EG EG
VB

5. The Bond Theory
• 1. Ionic bonds: electrostatic attraction between the ions.
Atoms donate/accept electrons to become +vely or -vely
charged ions.
• 2. Covalent Bonds: Atoms joined by sharing valence
electrons.
• 3. Metallic bonds: exhibited by electrons with single
valence electrons, e.g. Cu, Na, Ag, Au. substances find
minimum energy configuration when they pool their
valence electrons. The electrons are no longer tied to
specific sites (atoms) but are unlocalised and are free to
travel throughout the metal forming an electron cloud.

6. Intrinsic and Extrinsic
semiconductors
intrinsic Extrinsic
(with P)
Extrinsic
with Al
(Gp 4 elements)

7. III
DIODE CIRCUITS AND
CHARACTERISTICS

8. • P-n junction:
• 1. depletion layer established on both sides
of the junction.
• 2. barrier (junction) potential is developed
across the junction.
• 3. Formation of junction and diffusion
capacitances.
DIODE CIRCUITS AND
CHARACTERISTICS

9. P-n junction (a) depletion region
+ + + + + + +
+ + + + + + +
- - - - - - - - - - -
- - - - - - - - - - - - -
p n
+ + + + + +
+ + + + + +
- - - - - -
- - - - - -
-
-
+
+
Depletion layer
with fixed ions
Free (mobile
charges)

10. (b) Junction or barrier potential
0
n
p
+ + + + + +
+ + + + + +
+ + + + + +
-
-
-
+
+
+
- - - - - -
- - - - - -
- - - - - -
Depletion
layer
VB

11. e ( Ie)
O ( Ih )
+ + + +
+ + + +
+ +
- - - - -
- - - - -
VA
0
I
VB - VA
q(VB – VA)
qVA
EC
EF
EV
Forward biased p-n junction.

12. Forward I-V characteristics.
Ge
Si
I
V
Vγ

13. Reverse Biased p-n Junction.
+ + + +
+ +
-
-
+
+
- - - -
- -
VA + VB
q(VB + VA)
qVA
O e

14. Reverse I-V characteristics
Forward
voltage
Reverse
voltage
I (μA)

15. Diode Law
For ideal diode:
•iD = 0, vD  0,
•vD = 0, iD > 0,
•With eqn Amps
kT
qV
I
I A













 1
exp
0

16. Diode Law
• 1.
• 2 VA = 0, I = 0
• 3
4
q
kT
VA 
1
2
3







kT
qV
I
I A
exp
0
q
kT
VA 


17. Rectifier circuits
i
RL
v
vi
vi

18. BIPOLAR JUNCTION
TRANSISTORS

19. Bipolar j. transistor
N P N
N P N
P N P
P N P
C
B
E
B
C
E

20. Biasing PN junctions.
• B-E junction forward biased by
VEE
• -ve terminal of VEE connected
to n-side
• flow of id across due to flow of
majority carriers (electrons)
from the N-type emitter
• become minority carriers in the
base
VE
E
B
C
E
Forward biased B-E (input)
junction

21. • C-B reverse biased by
VCC
• +ve of VCC connected to
N-type collector
• depletion region at the
junction widens
• current flowing from B-E
due to minority electrons
crossing the junction from
p-type base.
• constitute flow of reverse
current in the junction.
VCC
B
E
C
Reverse biased C-B
(output) junction

22. VEE VC
C
I
C
IB
Simultaneous biasing
• B is ground (0V)
• E is -ve wrt B & C is
+ve wrt B
• e- flow constitutes the
dominant type (in npn)
• IE = IC + IB
IE
e
npn

23. • IC sum of the injected &
thermally generated
minority carriers.
• if VEE is left open, & C-
B has normal reverse bias
• Then ICBO will flow
• Hence total I is
• IC = IC(Inj) + ICBO or (IC
= ICBO - IE)
• Where IC(Inj) is IC due to
carriers injected into the
base.
IE
IB
VEE VCC
IC
pnp

24. • portion of IC that survives after passing
through the B to become IC
E
Inj
C
I
I )
(



25. 9/10/2022
CB connection
IE
IB
IC
VCB =
Output
voltage
B
E C
VBE =
Input
Voltage
-
-
+
+
IE
IB
IC
VBC =
Output
voltage
B
E C
VEB =
Input
Voltage
+
+
-
-
NPN PNP
SCI 2010/2011 simiyuj@uonbi.ac.ke

26. CB Input Characteristics
• F-B diode (input is
across the forward-
biased B-E junction)
• greater the value of
VCB, the more readily
minority carriers in the
base are swept through
the B-E junction.
IE
(mA)
VCB = 0
VCB =
10V
VCB =
25V
VBE (V)
Increasing
output bias
(VBE = Φ(VCB, IE))

27. CB Output characteristics
• Active:
• IE = 0, IC = ICO
• IC rises with VCB & IC is
slightly less than IE since IC
= - IE and  ≈ 1
• Saturation
• VCB is +ve and the junction
is forward biased
• I.e C-B & E-B are forward
biased
satura
tion
Active
region
Cut
off
IE
ICO
(IC = Φ(VCB, IE))