1. Intro to Semiconductors and p-n junction devices
Homework: Chapter 11 Krane
Problems: 13, 14, 15, 16, 17, 23, 28, 33, 34, 35

2. pn Junction: Band Diagram
pn regions “touch” & free carriers move
Due to diffusion, electrons move from n to p-side and holes from p to n-side.
Causes depletion zone at junction where immobile charged ion cores remain.
Results in a built-in electric field (103 to 105 V/cm), which opposes further diffusion.
Note: EF levels are aligned across pn junction under equilibrium.
n-type
electrons EC EF EF EV
holes
p-type
pn regions in equilibrium – – EC – – + – + – EF + + – – – + + + – – + + – + + + EV
Depletion Zone

3. Forward Bias and Reverse Bias
Forward Bias : Connect positive of the positive end to positive of supply…negative of the junction to negative of supply
Reverse Bias: Connect positive of the junction to negative of supply…negative of junction to positive of supply.

4. PN Junction: Under Bias
Forward Bias: negative voltage on n-side promotes diffusion of electrons by decreasing built-in junction potential  higher current.
Reverse Bias: positive voltage on n-side inhibits diffusion of electrons by increasing built-in junction potential  lower current.
Equilibrium
Forward Bias
Reverse Bias
p-type
n-type
p-type
n-type
p-type
n-type –V +V e– e– e–
Majority Carriers
Minority Carriers

5. pn Junction: IV Characteristics
Current-Voltage Relationship
Forward Bias: current exponentially increases.
Reverse Bias: low leakage current equal to ~Io.
Ability of pn junction to pass current in only one direction is known as “rectifying” behavior.
Forward
Bias
Reverse Bias
Manifestly not a resistor: V=IR
Not Ohm’s law

6. N-channel MOSFET ID n n p
Gate
Drain
Source
oxide insulator
gate ID n n p
Without a gate-to-source voltage applied, no current can flow between the source and drain regions.
Above a certain gate-to-source voltage (threshold voltage VT), a conducting layer of mobile electrons is formed at the Si surface beneath the oxide. These electrons can carry current between the source and drain.

7. Metal-Oxide-Semiconductor FET
How does it work? There is no conduction between the source and drain normally (VGS = 0) because regardless of what voltage VDS you apply there is a reverse biased PN junction. Even apply a voltage VGS does not appear from the structure to have an obvious effect since it is not even attached - there is a thin SiO2 insulating layer in between!

8. Metal-Oxide-Semiconductor FET(2)
However when you apply a positive voltage the oxide behaves like a capacitor - since positive charge builds up on one side, there must be an equal and opposite charge on the other side.
This charge must come from the substrate. Since it is P-type there are not many electrons but those that are present are all sucked up to the gate oxide. This creates a region that is very thin, but very rich in electrons, converting P-type to N-type locally. This “channel” is enhanced by applying higher positive biases.

9. IG vs. VGS Characteristic
Consider the current IG (flowing into G) versus VGS : IG G D +  S
VDS
oxide
semiconductor + 
VGS IG
The gate is insulated from the semiconductor, so there is no significant steady gate current.
always zero!
VGS

10. ID vs. VDS Characteristics
Next consider ID (flowing into D) versus VDS, as VGS is varied: G ID +  S D
VDS
oxide
semiconductor + 
VGS ID
Above threshold (VGS > VT):
“inversion layer” of electrons appears, so conduction between S and D is possible
VGS > VT
zero if VGS < VT
VDS
Below “threshold” (VGS < VT): no charge  no conduction

11. The MOSFET as a Controlled Resistor
The MOSFET behaves as a resistor when VDS is low:
Drain current ID increases linearly with VDS
Resistance RDS between SOURCE & DRAIN depends on VGS
RDS is lowered as VGS increases above VT
NMOSFET Example: ID
VGS = 2 V
VGS = 1 V > VT
VDS
IDS = 0 if VGS < VT

12. ID vs. VDS Characteristics
The MOSFET ID-VDS curve consists of two regions:
1) Resistive or “Triode” Region: 0 < VDS < VGS  VT
2) Saturation Region:
VDS > VGS  VT
“CUTOFF” region: VG < VT

13. Resistive: ID ~ (VGS – VT)VDS
MOSFET regions of operation
Cutoff: VGS <= VT
ID = 0
Resistive: ID ~ (VGS – VT)VDS
Saturation: ID ~ (VGS – VT)2

14. MOSFET Uses
A MOSFET can be used as
A linear amplifier: Input voltage applied between
gate and source; output voltage
appears between source and drain or
An electronic switch: Switches between no
current conduction between source and drain,
and heavy conduction between source and
drain as voltage applied between gate and
source changes from low to high

15. Importance
The transistor is probably the most important invention of the 20th century….
Transistors are central to the Integrated Circuit, and therefore, all electronic devices of the information age, such as: pc’s, cellular phones, ipods, pda’s, intelligent cars and buildings…….