SCHOTTKY BARRIER DIODE
•The Schottky diode (named after German physicist Walter H.
Schottky; also known as hot carrier diode) is a semiconductor diode
with a low forward voltage drop and a very fast switching action.
•When current flows through a diode there is a small voltage drop
across the diode terminals. A normal silicon diode has a voltage
drop between 0.6–1.7 volts, while a Schottky diode voltage drop is
between approximately 0.15–0.45 volts. This lower voltage drop
can provide higher switching speed and better system efficiency.
CONSTRUCTION
• A metal–semiconductor junction is formed between a metal and a
semiconductor, creating a Schottky barrier (instead of a
semiconductor–semiconductor junction as in conventional diodes).
• Typical metals used are molybdenum, platinum, chromium or
tungsten; and the semiconductor would typically be N-type
silicon.
• The metal side acts as the anode and N-type semiconductor acts as
the cathode of the diode. This Schottky barrier results in both very
fast switching and low forward voltage drop.
Reverse recovery time
• The most important difference between the p-n and
Schottky diode is reverse recovery time, when the diode
switches from conducting to non-conducting state. Where
in a p-n diode the reverse recovery time can be in the order
of hundreds of nanoseconds and less than 100 ns for fast
diodes, Schottky diodes do not have a recovery time, as
there is nothing to recover from (i.e. no charge carrier
depletion region at the junction).
• The switching time is ~100 ps for the small signal diodes,
and up to tens of nanoseconds for special high-capacity
power diodes. With p-n junction switching, there is also a
reverse recovery current, which in high-power
semiconductors brings increased EMI noise. With
Schottky diodes switching essentially instantly with only
slight capacitive loading, this is much less of a concern.
CONSTRUCTION AND APPLICATIONS
• It is often said that the Schottky diode is a "majority
carrier" semiconductor device. This means that if the
semiconductor body is doped n-type, only the n-type
carriers (mobile electrons) play a significant role in normal
operation of the device. The majority carriers are quickly
injected into the conduction band of the metal contact on
the other side of the diode to become free moving
electrons. Therefore no slow, random recombination of nand p- type carriers is involved, so that this diode can cease
conduction faster than an ordinary p-n rectifier diode. This
property in turn allows a smaller device area, which also
makes for a faster transition.
• This is another reason why Schottky diodes are useful in
switch-mode power converters; the high speed of the diode
means that the circuit can operate at frequencies in the
range 200 kHz to 2 MHz, allowing the use of small
inductors and capacitors with greater efficiency than would
be possible with other diode types. Small-area Schottky
diodes are the heart of RF detectors and mixers, which
often operate up to 50 GHz.
Limitations
• The most evident limitations of Schottky diodes are the
relatively low reverse voltage ratings for silicon-metal
Schottky diodes, typically 50 V and below, and a relatively
high reverse leakage current. Some higher-voltage designs
are available; 200V is considered a high reverse voltage.
• Reverse leakage current, because it increases with
temperature, leads to a thermal instability issue. This often
limits the useful reverse voltage to well below the actual
rating.
• While higher reverse voltages are achievable, they would
be accompanied by higher forward voltage drops,
comparable to other types; such a Schottky diode would
have no advantage
Applications
• Voltage clamping
• Reverse current and discharge protection
• Power supply
7
*Other Two-Terminal Devices
*Schottky Barrier Diodes
Two-Terminal Devices Having A
Single p-n Junction
Schottky
Tunnel
Varactor
Photodiode
Solar Cell
Other Two-Terminal Devices
Of A Different Construction
Photoconductive Cell
LCD (Liquid-Crystal Display)
Thermistor
Schottky-Barrier Diode
*Surface-Barrier/Hot-Carrier Diode
Schottky-Barrier Diode
Areas of Application
Very high frequency range
Lower noise figure
Low-voltage or high-current power supplies
AC-to-DC converters
Radar systems
Schottky TTL logic
Fig. 20.1 Passivated hot-carrier diode
Gold leaf metal contact
Anode (+)
Metal
Silicon dioxide screen
Metal semiconductor junction
Metal contact
Cathode (-)
Fig. 20.2 Comparison of characteristics
of hot-carrier and p-n junction diodes
ID
Hot
carrier
diode
p-n
junction
diode
VD
p-n
junction
diode
Hot
carrier
diode
Fig. 20.3 Schottky (hot-carrier) diode:
(a) equivalent circuit; (b) symbol
(a)
Fig. 20.4 Approximate equivalent
circuit for the Schottky diode
Fig. 20.5 Motorola Schottky barrier devices.
(Courtesy Motorola Semiconductor Products, Incorporated
IO Average rectified forward current (amperes)
VRRM
(Volts)
Case
Anode
Cathode
59-04
Plastic
MBR4040
800
430-2
(DO-21)
Metal
MBR4030
IN5833
MBR2530
IN5830
MBR1530
IN5827
IN5824
IN5821
MBR4035
MBR2535
MBR1535
MBR335M
MBR335P
MBR135P
MBR4020PF MBR4030PF MBR4035PF
IN5834
800
MBR4020
MBR2534
800
IN5832
IN5831
800
MBR2520
MBR1540
500
IN5829
IN5828
500
MBR1520
IN5825
500
IN5826
MBR340M
500
IN5823
MBR340P
200
MBR330M
IN5822
MBR320M
MBR140P
250
MBR330P
MBR130P
IN5819
Max VF @ IFM = IO
MBR320P
MBR120P
IN5818
50
IN5820
IN5817
TJ Max
257
(DO-5)
Metal
100
40
TC @ Rated IO (ºC)
257
(DO-4)
Metal
60
Metal
5.0
35
IFSM (Amps)
267
Plastic
MBR030
30
MBR020
20
51-02
(DO-7)
Glass
800
85
80
85
80
75
70
50
125ºC
Fig 20.6 Characteristics curves for Hewlett-Packard 50822300 series of general-purpose Schottky barrier diodes.
100
Forward current (mA)
10
IF
1
.1
0.1 0
Temperature Coeffiecient
10A
-2.3mV/ºC
100A
-1.8mV/ºC
1.0mA
-1.3mV/ºC
10mA
-0.7mV/ºC
100mA
-0.2mV/ºC
T = 100ºC
T = 25ºC
100
200
300
400
Forward voltage (mV)
500
600
700
T = -50ºC
I-V Curve Showing Typical Temperature Variation for 5082-2300 Series Schottky Diodes
(a)
100
500
Reverse current (nA)
2900
2303
100
50
2301
2302
2305
100
5
10
Reverse voltage (V)
5082-2300 Series Typical Reverse Current vs. Reverse Voltage at TA = 25ºC
(b)
15
1.2
1.0
Capacitance (pF)
0.8
0.6
2900
2303
0.3
2301
2302
2305
0.2
0
4
8
12
16
20
VR-Reverse voltage (V)
5082-2300 Series Typical Capacitance vs. Reverse Voltage at TA = 25ºC
(c)
Schottky diode I-V characteristics
Schottky diode is a metal-semiconductor (MS) diode
Historically, Schottky diodes are the oldest diodes
MS diode electrostatics and the general shape of the MS diode IV characteristics are similar to p+n diodes, but the details of
current flow are different.
Dominant currents in a p+n diode
– arise from recombination in the depletion layer under small
forward bias.
– arise from hole injection from p+ side under larger forward
bias.
Dominant currents in a MS Schottky diodes
– Electron injection from the semiconductor to the metal.
Current components in a p+n and MS Schottky diodes
p+
M
n
n-Si
dominant
negligible
B
IR-G
Ir-g
negligible
dominant
I-V characteristics
 qV A



I  I s  e kT  1 




where
*
Is  AA T
2

e
B
kT
where B is Schottky barrier height, VA is applied voltage, A is
area, and A* is Richardson’s constant.
The reverse leakage current for a Schottky diode is generally
much larger than that for a p+n diode.
Since MS Schottky diode is a majority carrier devices, the
frequency response of the device is much higher than that of
equivalent p+ n diode.