6.4 Schottky diodes

Schottky diode fundamentals

A Schottky diode forms at the junction between a metal and a semiconductor, creating a rectifying contact with distinct advantages over traditional pn junction diodes. Its low forward voltage drop and fast switching speed make it a workhorse in power electronics, RF circuits, and high-speed digital systems.

This section covers how the junction forms, what determines the barrier height, and why real devices don't always behave the way simple theory predicts.

Metal-semiconductor junction

When a metal and semiconductor are brought into intimate contact, electrons flow from the semiconductor into the metal until the Fermi levels of both materials align. This charge transfer creates a potential barrier at the interface called the Schottky barrier, and it depletes carriers from the semiconductor side of the junction.

• The barrier height depends on two material properties: the metal's work function and the semiconductor's electron affinity
• Common metals used in Schottky contacts include aluminum (Al), platinum (Pt), titanium (Ti), and tungsten (W)
• The choice of metal directly affects the barrier height and, consequently, the diode's electrical characteristics

Schottky barrier formation

The Schottky barrier arises from the mismatch in work functions between the metal and the semiconductor. For an electron in the semiconductor conduction band to enter the metal, it must have enough energy to overcome this barrier.

The barrier height for an n-type semiconductor is given by:

$\phi_B = \phi_m - \chi_s$

where $\phi_m$ is the metal work function and $\chi_s$ is the semiconductor electron affinity.

• A higher barrier height means a larger forward voltage drop but lower reverse leakage current
• A lower barrier height gives a smaller voltage drop but more leakage in reverse bias
• This tradeoff is central to Schottky diode design

Fermi level pinning

In practice, changing the metal often doesn't change the barrier height as much as the equation above predicts. This is Fermi level pinning: the barrier height stays nearly constant regardless of the metal work function.

• Surface states and defects at the metal-semiconductor interface trap charge and effectively "pin" the Fermi level to a fixed position within the bandgap
• This limits your ability to tune barrier height simply by choosing different metals
• Fermi level pinning is especially pronounced in covalent semiconductors like Si and GaAs
• Interfacial layer engineering (inserting a thin dielectric between metal and semiconductor) is one technique used to reduce pinning effects

Depletion region width

Near the metal-semiconductor interface, free carriers in the semiconductor are swept away, forming a depletion region. This region behaves much like the depletion region in a pn junction.

• The depletion width depends on the applied voltage, the semiconductor doping concentration, and the dielectric constant
• Higher doping narrows the depletion region; reverse bias widens it
• The depletion width directly affects the diode's capacitance and breakdown voltage
• Voltage-dependent modulation of the depletion width is the operating principle behind varactor diodes used in tuning circuits

Current-voltage characteristics

The I-V characteristics describe how current flows through a Schottky diode as a function of applied voltage. Several transport mechanisms contribute, and real devices deviate from ideal behavior due to parasitic effects.

Thermionic emission theory

Thermionic emission is the dominant current transport mechanism in most Schottky diodes. Electrons in the semiconductor that have enough thermal energy to surmount the Schottky barrier flow into the metal.

The current density is described by the Richardson-Dushman equation:

$J = A^* T^2 \exp\left(-\frac{q\phi_B}{kT}\right) \left[\exp\left(\frac{qV}{kT}\right) - 1\right]$

where $A^*$ is the effective Richardson constant, $T$ is temperature, $q$ is the electron charge, $\phi_B$ is the barrier height, $k$ is Boltzmann's constant, and $V$ is the applied voltage.

• This theory applies best to diodes with relatively high barriers and moderate doping
• Wide-bandgap Schottky diodes (e.g., on SiC or GaN) typically exhibit thermionic emission-dominated transport

Diffusion theory

In diffusion-limited transport, the current is controlled by how quickly carriers can diffuse through the semiconductor near the junction rather than by how many can jump over the barrier.

• Current density is proportional to the carrier concentration gradient near the interface
• This mechanism dominates in diodes with low barrier heights and high doping concentrations
• Heavily doped silicon Schottky diodes often fall into this regime

Combined thermionic emission-diffusion theory

Most real Schottky diodes operate in a regime where both thermionic emission and diffusion play a role. The combined theory accounts for both mechanisms and provides a more accurate description of I-V behavior across a wider range of barrier heights and doping levels.

• The resulting current expression is more complex but reduces to either pure thermionic emission or pure diffusion in the appropriate limits
• This framework is the most general and physically accurate for intermediate cases

Ideality factor

Real Schottky diodes don't perfectly follow thermionic emission theory. The ideality factor $n$ quantifies this deviation. The forward current is often written as:

$J \propto \exp\left(\frac{qV}{nkT}\right)$

• An ideal diode has $n = 1$
• Real devices typically have $n$ between 1 and 2
• Sources of non-ideality include barrier height inhomogeneity, thin interfacial layers, image force lowering, and recombination within the depletion region
• Extracting $n$ from experimental I-V data is a standard way to assess diode quality

Series resistance effects

Every real diode has parasitic series resistance from the semiconductor bulk, the ohmic back contact, and interconnect wiring. This resistance doesn't matter much at low currents, but at high forward currents it causes a noticeable voltage drop that makes the I-V curve deviate from exponential behavior.

• At high currents, the I-V curve becomes nearly linear instead of exponential
• Series resistance can be minimized by using low-resistivity substrates, optimizing contact geometry, and reducing interconnect length
• Extracting series resistance from I-V data is important for accurate device modeling

Reverse bias behavior

Under reverse bias, the Schottky barrier blocks most current flow, but small leakage currents still exist. At sufficiently high reverse voltages, breakdown occurs.

Reverse saturation current

When the diode is reverse-biased, a small current still flows due to thermionic emission of electrons from the metal into the semiconductor (over the barrier in the reverse direction).

• This reverse saturation current increases strongly with temperature because more electrons gain enough thermal energy to cross the barrier
• It also depends exponentially on the barrier height: lower barriers produce higher leakage
• Reverse leakage affects power dissipation and noise performance, especially in sensitive circuits

Breakdown mechanisms

Under high reverse bias, two breakdown mechanisms can occur:

• Avalanche breakdown: The electric field in the depletion region accelerates carriers to high enough energies to ionize lattice atoms on impact, creating an avalanche of new carriers. This is the dominant mechanism in lightly doped, wide-depletion-region devices.
• Tunneling (Zener) breakdown: At very high fields, electrons can quantum-mechanically tunnel through the barrier without needing enough energy to go over it. This dominates in heavily doped devices where the depletion region is narrow.

Both mechanisms cause a rapid increase in reverse current and can damage the device if current isn't limited externally.

Edge effects and guard rings

The electric field at the edges of a Schottky contact is higher than at the center due to field crowding at the junction periphery. This non-uniform field distribution can cause premature breakdown and elevated leakage current at the edges.

• Guard rings solve this problem. These are heavily doped p-type regions implanted around the perimeter of the Schottky contact.
• The guard ring redistributes the electric field, preventing it from peaking at the edges
• Proper guard ring design can significantly increase the breakdown voltage of a Schottky diode

Capacitance-voltage characteristics

C-V measurements reveal important device parameters and are the basis for several applications, including varactors and high-frequency tuning circuits.

Junction capacitance

The depletion region separates positive and negative charges, acting like a parallel-plate capacitor. The junction capacitance is:

$C_j = A \sqrt{\frac{q \epsilon_s N_D}{2(V_{bi} - V)}}$

where $A$ is the junction area, $\epsilon_s$ is the semiconductor permittivity, $N_D$ is the donor concentration, $V_{bi}$ is the built-in potential, and $V$ is the applied voltage.

• Junction capacitance is inversely related to depletion width: reverse bias widens the depletion region and reduces capacitance
• Forward bias narrows the depletion region and increases capacitance
• This voltage-dependent capacitance is what makes Schottky diodes useful as varactors

Diffusion capacitance

When the diode is forward-biased, minority carriers are injected and stored near the junction. The charge storage associated with these carriers gives rise to diffusion capacitance.

• Diffusion capacitance is proportional to the forward current and the minority carrier lifetime
• It becomes significant at low frequencies and high forward bias
• In Schottky diodes, diffusion capacitance is much smaller than in pn junctions because Schottky diodes are majority-carrier devices with minimal minority carrier injection

High-frequency C-V analysis

High-frequency C-V measurement is a standard technique for extracting Schottky diode parameters.

1. Apply a DC bias voltage to the diode and superimpose a small AC signal (typically in the MHz range)
2. Measure the capacitance as a function of DC bias voltage
3. Plot $1/C^2$ vs. $V$

This plot yields a straight line for a uniformly doped semiconductor. From the slope, you can extract the doping concentration $N_D$, and from the voltage-axis intercept, you can determine the built-in potential $V_{bi}$ (and hence the barrier height).

At high frequencies, diffusion capacitance is negligible, so the measurement reflects only the junction capacitance.

Schottky diode applications

The low forward voltage drop, fast switching, and low capacitance of Schottky diodes make them well-suited for several important applications.

Rectifiers and power electronics

Schottky diodes are the standard choice for output rectification in switched-mode power supplies (SMPS) and voltage regulators, especially at low output voltages.

• The forward drop of 0.2-0.5 V (vs. 0.6-0.7 V for Si pn diodes) significantly reduces conduction losses
• Fast switching eliminates the reverse recovery losses that plague pn junction diodes in high-frequency converters
• Particularly valuable in low-voltage, high-current applications like DC-DC converters for portable electronics

High-frequency devices

Schottky diodes are used extensively in RF and microwave circuits as mixers, detectors, and switches.

• Their low junction capacitance and absence of minority carrier storage enable operation well into the GHz range
• In an RF mixer, the Schottky diode's nonlinear I-V characteristic is used to multiply (mix) two signals, producing sum and difference frequencies for downconversion
• Schottky diodes also serve as clamping diodes in high-speed digital logic to prevent transistor saturation (as in Schottky TTL)

Microwave and millimeter-wave detectors

At microwave and millimeter-wave frequencies, Schottky diodes function as square-law detectors, rectifying incoming high-frequency signals into a measurable DC voltage.

• Used in radar receivers, radiometers, and wireless communication front ends
• Millimeter-wave imaging systems for security screening and non-destructive testing rely on Schottky diode detectors
• Their high sensitivity and low noise make them competitive even at frequencies above 100 GHz

Solar cells and photovoltaics

Schottky barrier solar cells use the built-in electric field at the metal-semiconductor junction to separate photogenerated electron-hole pairs.

• Fabrication is simpler and cheaper than pn junction solar cells since only a metal deposition step is needed
• Efficiency is generally lower than pn junction cells due to higher recombination at the metal-semiconductor interface
• Best suited for low-cost, low-power applications such as solar-powered sensors and disposable electronics
• Example: Au/Si Schottky barrier cells have been demonstrated for wireless sensor network power supplies

Comparison with pn junction diodes

Choosing between a Schottky diode and a pn junction diode depends on the application requirements. Here are the key differences:

Forward voltage drop

• Schottky diodes: typically 0.2-0.5 V
• Si pn junction diodes: typically 0.6-0.7 V

The lower forward drop in Schottky diodes comes from the metal-semiconductor barrier being smaller than the built-in potential of a pn junction. This translates directly into lower conduction losses, which matters most in low-voltage, high-current circuits like battery-powered devices.

Reverse recovery time

Schottky diodes are majority-carrier devices, meaning they don't store minority carriers the way pn junctions do. When the voltage reverses, there's no stored charge to remove.

• Schottky diode reverse recovery: picoseconds to nanoseconds
• pn junction diode reverse recovery: nanoseconds to microseconds

This difference is why Schottky diodes dominate in high-frequency switching applications.

Switching speed

The switching speed of a Schottky diode is limited primarily by its RC time constant (junction capacitance × series resistance) and external circuit parasitics, not by minority carrier dynamics.

• This makes Schottky diodes much faster than pn junctions for the same geometry
• Critical for RF mixers, high-speed clamping, and fast rectification

Reverse leakage current

This is the main disadvantage of Schottky diodes. The lower barrier height that gives a small forward drop also allows more current to leak in reverse bias.

• Schottky diodes: nanoamperes to microamperes of leakage
• pn junction diodes: picoamperes to nanoamperes of leakage

For low-noise or high-sensitivity applications (photodetectors, precision analog circuits), pn junction diodes are often preferred because of their lower leakage.

Schottky barrier lowering techniques

Reducing the effective barrier height can improve forward conduction and reduce the forward voltage drop. Several approaches exist to engineer the metal-semiconductor interface.

Interfacial layer engineering

A thin layer (typically a few nanometers) is inserted between the metal and the semiconductor. This layer modifies the charge distribution at the interface and can create dipoles that shift the barrier height.

• The interfacial layer can be an insulator (e.g., a thin $SiO_2$ layer), a different semiconductor, or even a molecular monolayer
• Depending on the material and thickness, the barrier can be either lowered or raised
• This technique also helps mitigate Fermi level pinning by decoupling the metal from the semiconductor surface states

Barrier height inhomogeneity

Real metal-semiconductor interfaces aren't perfectly uniform. Local variations in surface composition, crystallographic orientation, and defect density create a distribution of barrier heights across the contact area.

• Current preferentially flows through low-barrier "patches," so the effective barrier height is lower than the average
• Surface treatments like chemical etching, annealing, or intentional texturing can be used to control the degree of inhomogeneity
• Nanoscale surface roughness can create low-barrier regions that enhance forward current

Graded composition semiconductors

Using a semiconductor with a composition that varies with depth (a graded layer) creates a built-in electric field that assists carrier transport across the interface.

• For example, a graded $Al_xGa_{1-x}As$ layer on GaAs has a varying bandgap and electron affinity, which effectively lowers the barrier seen by electrons
• Graded layers are grown using techniques like molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD)
• This approach is especially useful in III-V semiconductor Schottky diodes for high-frequency and optoelectronic applications

Reliability and failure mechanisms

Long-term reliability depends on how well the metal-semiconductor interface holds up under electrical stress, elevated temperatures, and radiation exposure.

Electromigration

High current densities can physically transport metal atoms along the contact, a process called electromigration. Over time, this creates voids (where metal has been removed) and hillocks (where metal has accumulated).

• Voids increase the local series resistance and can eventually cause open-circuit failure
• Electromigration is accelerated by high temperature and high current density
• Using refractory metals like tungsten (W) or titanium nitride (TiN) as barrier layers helps resist electromigration
• Proper contact geometry that avoids current crowding also extends device lifetime

Thermal stability

At elevated temperatures, atoms from the metal and semiconductor can interdiffuse across the interface, changing the barrier height and degrading device performance.

• Interdiffusion can increase leakage current and shift the I-V characteristics
• Silicide-forming metals (e.g., Ti, Co, Ni on Si) are often used because the resulting metal silicide forms a stable, well-defined interface
• Diffusion barrier layers (e.g., TiN, TaN) between the contact metal and the semiconductor help maintain interface integrity at high temperatures
• Thermal stability is especially critical for devices operating in harsh environments, such as automotive or aerospace electronics