Schottky Diode

The Schottky diode is formed by a direct contact between a metal and a lightly doped semiconductor, creating a rectifying junction without any p-type region. Because it relies entirely on majority carrier transport, it switches far faster than conventional p-n junction diodes and has a lower forward voltage drop, making it indispensable in high-speed and power electronics.

Core Concept Explanation

When a metal is brought into contact with a lightly doped n-type semiconductor, electrons flow from the semiconductor into the metal until the Fermi levels align at equilibrium. This creates a space charge region (depletion region) in the semiconductor adjacent to the metal. The resulting energy barrier is called the Schottky barrier and its height is denoted phi_B. Unlike a p-n junction, there is no p-type region and no minority carrier injection in normal forward operation.

Under forward bias the barrier seen by electrons in the semiconductor is reduced, allowing majority carrier electrons to flow over the barrier into the metal by thermionic emission. This is the dominant current mechanism. Because only majority carriers participate and there is no minority carrier storage, the Schottky diode has essentially zero reverse recovery time. This makes it ideal for high-frequency rectifiers and fast switching circuits.

The forward voltage drop of a Schottky diode is typically 0.25 V to 0.45 V, significantly lower than the 0.6 to 0.7 V of a silicon p-n junction. This lower drop reduces power dissipation in rectifier circuits and improves efficiency in switched-mode power supplies.

Mathematical Expression

The current in a Schottky diode follows the thermionic emission model. The saturation current density J0 depends exponentially on the barrier height:

J0 = A** T^2 exp(-phi_B / kT), where A** is the Richardson constant (approximately 110 A/cm^2/K^2 for n-type Si), T is absolute temperature, phi_B is the Schottky barrier height in volts, and k is Boltzmann constant. The total forward current follows I = I0 * (exp(qV/nkT) - 1) with ideality factor n close to 1 for a good Schottky diode, unlike p-n junctions where n can approach 2 at low current levels.

The barrier height phi_B depends on the metal chosen. Platinum on n-Si gives phi_B approximately 0.85 eV, giving a very low leakage current but higher forward drop. Aluminum on n-Si gives phi_B around 0.72 eV. For lower forward drop applications, metals like molybdenum or titanium are chosen.

Practical Understanding

Schottky diodes are used as clamp diodes in TTL and Schottky TTL logic families to prevent transistors from entering deep saturation, which would cause slow recovery. They are also used as the rectifying diodes in switch-mode power supplies operating at hundreds of kilohertz where even a few nanoseconds of reverse recovery in a p-n diode would cause significant switching loss.

A practical limitation is that the Schottky diode has a higher reverse leakage current compared to a p-n junction of the same forward voltage rating. This is because the barrier height is lower. At elevated temperatures this leakage increases substantially, which can cause thermal runaway in power circuits if not properly managed.

Given:
n-type Si Schottky diode: phi_B = 0.8 eV, T = 300 K, A** = 110 A/cm^2/K^2, Area = 0.01 cm^2

Why this formula applies:
Schottky current is dominated by thermionic emission over the barrier

Formula:
I0 = A * A** * T^2 * exp(-phi_B * q / kT)

Substitution:
I0 = 0.01 * 110 * (300)^2 * exp(-0.8 / 0.02585)
   = 0.01 * 110 * 90000 * exp(-30.95)

Calculation:
exp(-30.95) = 2.78e-14
I0 = 0.01 * 110 * 90000 * 2.78e-14
I0 = 99000 * 2.78e-14

Final Answer: I0 = 2.75e-9 A = 2.75 nA (saturation current)

Exam Tip: Schottky diode has NO minority carrier storage, so reverse recovery time is nearly zero. If a GATE question asks which diode is suitable for high-frequency rectification or fast switching, the answer is Schottky. Also, ideality factor n is close to 1 for Schottky, close to 2 for recombination-dominated p-n junctions.

• Schottky barrier height phi_B is determined by the work function difference between metal and semiconductor.
• Under forward bias, barrier on semiconductor side reduces by qV, increasing thermionic emission current exponentially.
• No minority carriers are injected, so there is no stored charge and no reverse recovery time.
• Higher phi_B gives lower leakage but higher forward drop; lower phi_B gives lower Vf but more reverse leakage.
• Schottky TTL uses clamped transistors to prevent saturation by shunting base-collector junction with a Schottky diode.

Quick Revision

• Schottky diode = metal + lightly doped n-type semiconductor; no p-region.
• Current mechanism: thermionic emission of majority carriers (electrons) over Schottky barrier.
• Forward voltage drop 0.25-0.45 V, much less than 0.6-0.7 V for p-n junction.
• No minority carrier storage, zero reverse recovery time, ideal for high-speed applications.
• I0 = A * A** * T^2 * exp(-phi_B/kT); barrier height phi_B controls leakage.
• Applications: SMPS rectifiers, Schottky TTL logic clamp, microwave mixer diodes.
• Exam trap: Schottky is a majority carrier device. Minority carrier injection and reverse recovery are nearly absent.