The name of this diode is given after the German physicist Walter.H.Schottky. Other than the name Schottky diode, it is also referred to as Schottky barrier diode or as hot carrier diode. This is a diode with semiconductor-metal junction.
This device can simply rectify frequencies greater than 300 MHz. Its forward voltage drop is also very low (0.15 to 0.45 V). This results in higher switching speed and improved system efficiency. The junction in the diode is formed by the metal (such as gold, tungsten, chromium, platinum, molybdenum or certain silicides) and N-type doped silicon semiconductor. Here, anode is the metal side and cathode is the semiconductor side.
The symbol of Schottky diode is in figure below.
Construction of Schottky Diode
At one ending, there is a junction formed between the metal and lightly doped n-type semiconductor. This is unilateral junction. At the other ending, the metal and heavily doped semiconductor contact is present. It is called Ohmic bilateral contact (see Figure 2 and Figure 3). In this contact, no potential is present and is non-rectifying.
If the doping of semiconductor is increased, the depletion layer width decreases. When the width is decreased to a certain level, the charge carriers will tunnel easily through the depletion region. When the doping is very high, the junction can never act as a rectifier and it will become an ohmic contact. This diode can be a diode and an ohmic contact simultaneously.
When a Schottky diode is in unbiased condition, the electrons lying on the semiconductor side have very low energy level when compared to the electrons present in metal. Thus, the electrons cannot flow through the junction barrier which is called Schottky barrier. If the diode is forward biased, electrons present in the N-side gets sufficient energy to cross the junction barrier and enters into the metal. These electrons enter into the metal with tremendous energy. Consequently these electrons are known as hot carrier. Thus the diode is so called as hot-carrier diode.
The equivalent circuit of the device (Schottky diode) with typical values of the components is shown below.
The above circuit can be approximated as shown below. This approximated circuit is used in many applications.
The Schottky diode has some unique features when compared to normal P-N junction diode.
- It is a unipolar device. This is due to the absence of significant current flow from metal to N-type semiconductor (minority carriers in the reverse direction is absent). But P-N junction diode is a bipolar device.
- No stored charge due to the absence of holes in the metal. As a result, schottky diode can quickly switch than other diodes and noise is also relatively low.
- Lower barrier potential (0.2 – 0.25 V) compared to P-N diode (0.7 V)
Comparison of V-I characteristics of Schottky Diode, PN Junction Diode and Point Contact Diode
Current Components in Schottky Doide
The current condition in this diode is through electrons (majority carriers) in N-type semiconductor.
IDiffusion → Diffusion current (result of concentration gradient and diffusion current density of electrons)
Dn → Diffusion constant of electrons.
q → Electronic charge = 1.6 ×1019 C.
ITunneling → Tunneling current
IThermonic emission → As a result of electron ejection due to thermal energy (thermionic emission), this current will be produced across the electrodes.
Advantages of Schottky Diode
Advantages of Schottky diode are showing below-
- It has fast recovery time due to very low quantity of stored charge. So this diode is used for high speed switching application.
- It has low turn on voltage.
- It has low junction capacitance.
- Voltage drop is low.
Disadvantages of Schottky Diode
Disadvantages of Schottky diode are showing below-
- Reverse leakage current.
- Low reverse voltage rating.
Application of Schottky Diode
- Used in Switched-mode power supplies.
- Used in reverse current protection.
- Used in discharge protection.
- Used in voltage clamping application.
- Used in RF mixer and Detector diode.
- Used in solar cell application