ch1

Case 1
Case 2
1.3.1 Models
3. Small signal model__AC model
When a diode is operating in the small range,it can be a small-signal incremental resistance.
v That is: rd = D i D
Load line
Find two special points (5V,0mA) and (0V, 2.5mA) Intersection__quiescent point or Q-point
0.5 vD/V 0 0.62 1 2 3 4 5
1.3.2 DC Analysis
PS
(2) using the model analysis.
1.1.2. Extrinsic Semiconductor
1. phosphorus+silicon=N-type semiconductor Holes present because of thermal energy What are the majority carriers in n-type materials? What are the minority carriers in n-type materials?
Ch1. Semiconductor Materials and Diodes
1.1 Semiconductor Materials and Properties 1.2 The PN Junction 1.3 Diode Circuits_DC Analysis and Models 1.4 Diode Circuits_AC Equivalent Circuit 1.5 Other Diode Types
1.2.1 The Equilibrium PN Junction
1. Depletion region(space-charge region)_PN junction 2. Electric field 3. Drift current IDR = diffusion current IDF _Balance
1.2.3 Reverse-Biased PN Junction
4. When the voltage is increased to some point, breakdown occurs and current increases rapidly. The voltage at this point is called the breakdown voltage. 5. Avalanche breakdown Zener diode operate in the breakdown region
1.1.1 Intrinsic Semiconductor
2. When T increases, free electrons and "holes"are created In pure semiconductor, the concentration of electrons and holes are equal, and very small, so it has very small conductivity.
Redundant electron Donor Impurity Positive charge
N-type semiconductor
1.1.2. Extrinsic Semiconductor
2. boron+silicon=P-type semiconductor What are the majority carriers in P-type materials? Acceptor Impurity What are the minority carriers in P-type materials?
1.2.3 Reverse-Biased PN Junction
6. Junction capacitance Cj
VR C j = C j 0 1 + V bi
1 2
VR__applied voltage Vbi__build-in voltage
VR
1.2.4 PN Junction Diode
Ideal Model
VD = 0 V I D = VPS / R = 2.5 mA
PS
Piecewise model__1
Vγ = 0 . 7 V (Silicon Diode) I D = (VPS Vγ ) / R = 2.15 mA
iD
(a) Diode circuit
di D gd = dv D
∵ i D = I S (e v D / VT 1)
small-signal conductance
I DQ I S v D / VT e Q = Q= VT VT 1 VT ∴ rd = = I gd DQ
VT 26(mV) = Room T(T=300K) rd = ( ) I DQ I DQ (mA)
iD vD
Applied electric field
1.2.2 Forward-Biased PN Junction
3. The current-voltage (v-i) characteristic The current is an exponential function of voltage.
1. Diode__ has one PN junction in it
+ vD
iD
circuit symbol
Photo of real diode
1.2.4 PN Junction Diode
2. Temperature Effect
T increases→required forward-bias voltage decrease
Negative charge Motion of holes Hole
P-type semiconductor
1.2 The PN Junction
1.2.1 The Equilibrium PN Junction Holes diffuse from P- to N-region. Electrons diffuse from N- to P-region
iD
iD
Conducting state
+ vD iD + vD (vD <0, iD =0)
iD =0 + vD -
Reverse bias O vD
(vD <0, iD =0)
(a)
(b)
(c)
1.3.1 Models
2. Piecewise model
Two linear approximations
PS
Sol: For diode
iD = I S (e v D / VT 1)
For R and VPS branch
VPS = VD + I D R
(a) Diode circuit
3.0 2.5 2.2 2.0 1.5 1.0 i D/mA Q-point
(b) Conventional circuit
iD = I S (e vD / VT 1)
IS__reverse-bias saturation current VT __thermal voltage at room temperature VT =0.026V 4. Vγ __turn-on, cut-in voltage
1.2.3 Reverse-Biased PN Junction
Storage time ts Turn-off time tf
1.3 Diode Circuits_DC Analysis and Models 1.3.1 Models
Describing i-v characteristic, when analyzing circuit
1. Ideal model
1.2.3 Reverse-Biased PNபைடு நூலகம்Junction
3. The reverse current flow will be due to the drift current IDR, known as the reverse saturation (or leakage) current IS.
Positive voltage is applied to N, reverse-biased PN. The two fields are in the same direction. The results 1. Depletion region is increased, high resistance.
P
N
IDF
IDR
1.2.2 Forward-Biased PN Junction
Positive voltage is applied to P, forward-biased PN. The two fields are opposite. The results are: 1. Depletion region is reduced, low resistance. 2. Majority carriers flow across PN junction more easily. IDF>>IDR , iD=IDF-IDR D-region electric field
iD = I S ( e v D / VT 1)
IS and VT are functions of T.
T1 T0 T1 > T0
1.2.4 PN Junction Diode
3. Switching Transient
When forward-bias, excess carrier is stored in both regions. When switching from forward to reverse, it need time to remove.
1.3.2 DC Analysis
E.g.1.1 Determine the diode voltage and current for the circuit shown in figure, Assume PS VPS=5V , R=2k (1) using the graphical analysis.
P
N
hole
Electron
1.2.1 The Equilibrium PN Junction
1. Depletion region(space-charge region)_PN junction 2. Electric field
P
PN Junction
N
Depletion region Electric field
are:
2. Majority carriers cannot cross the junction. Minority carriers sweep across PN easily. IDF<<IDR , iD=IDR D-region electric field
iD
vD
Applied electric field
Conductor : conduction electrons__electrical conductivity
1.1.1 Intrinsic Semiconductor
1. Silicon, germanium__single-crystal structure
At temperature T=0oK, silicon is an insulator.
Ch1. Semiconductor Materials and Diodes
1.1 Semiconductor Materials and Properties 1.1.1. Intrinsic Semiconductor
Materials
Insulator : electrons in bonding mechanism __cannot move Semiconductor : silicon, germanium, gallium arsenide