Let's take a closer look at what happens if VDS keeps increasing beyond
VDSAT.
The "extra" voltage (VDS-VDSAT) is dropped across "delta L", while the
voltage drop across the resistor remains VDSAT.thereofre, ID saturates.
Accumulation: Holes accumulate at
ECE 474: Principles of
Electronic Devices
Day 32
MOSFET Operation and
Principles
(a)
S
B
D
metal (Al)
G
N
+
N
+
P
SiO2
+
P-type substrate (bulk)
(b)
G
(c)
D
S
G
GATE
Drain
Source
Bulk
B
D
S
B
VGS
VGS
G
G
+
x
D
S
y
N
B
z
ID
VDS
N+
x
P - substrate (bulk)
ch
What have we discussed so far:
1. pn junctions: band diagrams, zero bias, equilibrium, forward bias, reverse bias (band diagrams for each case) equations for Vbi, diode equation. Applet
(http:/jas.eng.buffalo.edu/education/pn/biasedPN/index.html)
2. Metal
Example 2: n-channel JFET. Silicon. p+ gate; and Nd=7x1017 cm-3 for the channel. a=1 m. Z= 10-2
cm. L=5x10-4 cm. Plot ID vs VD for various values of VG.
JFET Review.
MESFET
ONLINE APPLETS: http:/jas.eng.buffalo.edu/
JFET: Junction Field Effect Transistor
Let's now synchronize the equations with those in the book.
Example: n-channel JFET. Silicon. Na=1018 cm-3 for the p+ gate; and Nd=1016 cm-3 for the channel.
a=1 m. Find the pinch-off voltage and the voltage in the d
Heterojunctions: Junctions that consist of two latticematched semiconductors with different bandgaps.
Steps for drawing the band diagram for an
heterojunction (page 236 of textbook)
1. Align the Fermi level with the two semiconductor
bands separated. Leav
Energy band diagram for metal to p-type semiconductor (metal workfunction < semiconductor
workfunction) -Schottky junction
Energy band diagram for metal to p-type semiconductor (metal workfunction > semiconductor
workfunction) - Ohmic Contact
To Summarize
Small Signal model of the pn junction:
Shottky Diode (Metal-Semiconductor junction):
Steps for drawing the energy band diagram for a metalsemiconductor junction:
1) Draw the Fermi energy level first (constant across
the junction).
2) Draw the conduction a
Forward Bias:
For the forward bias analysis, we will need a different expression for Vbi (Vo in textbook).
Let's start with the expression for the complete expression for the hole current in a pn junction:
Remember that, using Gauss, the electric field ca
Zero Bias.check
Now, Reverse Bias.
Junction Capacitance
Image taken from internet: http:/upload.wikimedia.org/wikipedia/commons/f/fa/Pn-junctionequilibrium-graphs.png
Review of material covered after the test (before the break).
So far, we have studied the case of pn- junction under zero-bias. Most of the time, in practice, we use pn- junctions under
an applied electric field. An example of this is when a pn- junction
Space Charge Width
Example:
An abrupt silicon junction @ zero bias and T=300 K has dopant concentrations of:
Na=10^17 cm^-3 and Nd=5x10^15 cm^-3
a) Calculate the fermi energy level on each side of the junction
b) Determine Vbi
c)Determine xn, xp, and the
If the Fermi Energy level changes as a function of distance
through a semiconductor in thermal equilibrium, an electric field
exists in the semiconductor.
If we assume full ionization (i.e.
We get:
There will be a potential difference through the semicond
Carrier Transport Phenomena: Drift
Carrier Transport Phenomena: Diffusion
Mobility: How well the particle moves under and electric field
Dn: How well an electron moves in a semiconductor as a redult of a density gradient
Non-uniformly doped semiconductors
Pauli exclusion principle: only 1 electron per quantum state.
W e know by now that the allowed energy bands are divided into discrete energy levels. In order to calculate electron and hole concentrations is necessary to determine the
density of these allo