What is a Schottky Diode (Hot Carrier Diode)?
A Schottky diode (also known as the hot-carrier diode or Schottky barrier diode) is a semiconductor diode formed by the junction of a semiconductor with a metal. Schottky diodes have a low forward voltage drop (0.15 to 0.45 V) and a very fast switching action.
Just like a regular diode, a Schottky diode will conduct a current in the forward direction when sufficient forward voltage is applied.
But a silicon PN junction diode has a typical forward voltage of 600–700 mV (0.6–0.75 V), while Schottky’s forward voltage is 150–450 mV (0.15–0.45 V).
This lower forward voltage requirement enables Schottky diodes to have higher switching speeds and increased efficiency.
Here, the anode is the metal side and the cathode is the semiconductor side.
The symbol of the Schottky diode is shown in the figure below.
Construction of Schottky Diode
At one end, there is a junction formed between the metal and lightly doped n-type semiconductor. This is a unilateral junction.
At the other ending, the metal and heavily doped semiconductor contact are 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 the 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 a very low energy level when compared to the electrons present in the metal.
Thus, the electrons cannot flow through the junction barrier which is called the Schottky barrier. If the diode is forward biased, electrons present in the N-side get sufficient energy to cross the junction barrier and enters the metal.
These electrons enter into the metal with tremendous energy. Consequently, these electrons are known as hot carriers. Thus the diode is called a 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 the 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, the 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)
VI characteristics of Schottky Diode, PN Junction Diode, and Point Contact Diode
The graph below compares the V-I characteristics of a Shottky Diode, PN Junction Diode, and a Point Contact Diode.
Current Components in Schottky Diode
The current condition in this diode is through electrons (majority carriers) in N-type semiconductor.
IDiffusion → Diffusion current (the result of the 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
The advantages of a Schottky diode include:
- It has a fast recovery time due to the very low quantity of stored charge. So this diode is used for high-speed switching application.
- It has a low turn-on voltage.
- It has low junction capacitance.
- The voltage drop is low.
Disadvantages of Schottky Diode
The disadvantages of a Schottky diode include:
- Reverse leakage current.
- Low reverse voltage rating.
Application of Schottky Diode
Some of the applications of a Schottky diode are:
- 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