Schottky Diode

1. Theory

Definition: A Schottky diode is a rectifying metal-semiconductor junction, usually a metal on lightly doped N-type silicon.
Schottky barrier: The metal work function and semiconductor electron affinity set a barrier height $ϕ_{B}$ that electrons must cross to enter the metal.
Depletion region: The depletion region lies entirely in the semiconductor, like a one-sided junction.
Majority-carrier device: Current is carried by majority electrons only, with no significant minority-carrier injection or storage.
Low turn-on voltage: The barrier is lower than a Si PN junction’s built-in potential, giving a forward drop of about 0.15–0.45 V.
Image-force lowering: The field at the interface lowers the effective barrier, and the lowering grows with reverse bias.

2. Working Principle

Forward bias: The semiconductor-side barrier drops and electrons cross into the metal by thermionic emission, so current rises exponentially.
Reverse bias: The barrier from the metal side stays fixed, so only a small thermionic leakage flows.
Rising leakage: Image-force barrier lowering makes reverse leakage keep growing with reverse voltage instead of saturating, and it is much larger than a PN diode’s.
Fast switching: With no stored minority charge, reverse recovery is only the RC discharge of the junction capacitance.
Voltage limit: High blocking voltage needs a thick, lightly doped drift layer, which raises series resistance, so Si Schottky diodes are mostly limited to about 200 V.

3. Applications

Low-voltage power rectification: Output rectifiers in switch-mode supplies, where the low forward drop saves conduction loss.
Freewheeling and catch diodes: Fast, low-loss diodes in DC-DC converters and motor drivers.
RF mixers and detectors: Standard nonlinear element from microwave up to THz frequencies.
Solar panel bypass diodes: Low-drop bypass across shaded cell strings.
Transistor clamping: Prevents saturation of BJTs (Schottky TTL) to speed up switching.

Name	Formula	Related Formulae	Variables/Constants
Saturation current	$I_{S} = A A^{*} T^{2} exp (- \frac{q ϕ _{B}}{k _{B} T})$	$J = A^{*} T^{2} exp (- \frac{q ϕ _{B}}{k _{B} T}) [exp (\frac{q V}{k _{B} T}) - 1] (see Metal-Semiconductor Contacts page)$	$I_{S} : r e v er se s a t u r a t i o n c u r r e n t$ $A : d i o d e (co n t a c t) a r e a$ $A^{*} : e f f ec t i v e R i c ha r d so n co n s t an t$ $k_{B} : B o l t z mann co n s t an t$
Practical I-V equation (with series resistance)	$I = I_{S} [exp (\frac{V - I R _{s}}{n V _{T}}) - 1]$	$V = I R_{s} + n V_{T} ln (\frac{I}{I _{S}} + 1)$	$n : i d e a l i t y f a c t or (\approx 1, d e v ia t i o n s in d i c a t e ima g e - f or ce l o w er in g, t u nn e l in g, or in t er f a ce s t a t es)$ $R_{s} : ser i es (b u l k + co n t a c t) r es i s t an ce$ $V_{T} : t h er ma l v o l t a g e, k_{B} T / q$ $k_{B} : B o l t z mann co n s t an t$
Cut-in or turn-on voltage (Rearranging practical I-V equation)	$V = \frac{n k _{B} T}{q} ln (\frac{I}{I _{S}} + 1)$		$k_{B} : B o l t z mann co n s t an t$
Small-signal (dynamic) resistance	$r_{d} = \frac{n V _{T}}{I}$	$r_{d} (V = 0) = \frac{n V _{T}}{I _{S}}$	$r_{d} : in cr e m e n t a l d i o d e r es i s t an ce a t t h e bia s p o in t$
Total diode capacitance (majority-carrier device)	$C_{t o t a l} \approx C_{j}$	$C_{j} = \frac{ε _{s} A}{W} (see Metal-Semiconductor Contacts page)$	$C_{t o t a l} : s ma l l - s i g na l c a p a c i t an ce, w i t h n o s i g ni f i c an t d i f f u s i o n (s t or a g e) c a p a c i t an ce t er m$
Reverse leakage current (barrier-lowering enhanced)	$I_{R} (V) = A A^{*} T^{2} exp (- \frac{q [ ϕ _{B} - Δ ϕ _{B} ( V )]}{k _{B} T})$	$Δ ϕ_{B} = \frac{q E _{ma x}}{4 π ε _{s}} (see Metal-Semiconductor Contacts page)$	$I_{R} : r e v er se l e ak a g e c u r r e n t, w hi c h g r o w s w i t h ∣ V ∣ r a t h er t han s a t u r a t in g$ $k_{B} : B o l t z mann co n s t an t$
Carrier transit time (depletion region)	$t_{t r} \approx \frac{W}{v _{s a t}}$	$W = \frac{2 ε _{s} ( V _{bi} - V )}{q N _{D}} (see Metal-Semiconductor Contacts page)$	$t_{t r} : t im e f or c a r r i er s t o cr oss t h e d e pl e t i o n r e g i o n$ $v_{s a t} : c a r r i er s a t u r a t i o n v e l oc i t y$
Cutoff frequency	$f_{T} = \frac{1}{2 π R _{s} C _{j}}$		$f_{T} : f i g u r e - o f - m er i t c u t o f f f r e q u e n cy$
Reverse breakdown voltage (one-sided junction)	$B V \approx \frac{ε _{s} E _{cr i t}^{2}}{2 q N _{D}}$		$B V : r e v er se b r e ak d o w n v o l t a g e$ $E_{cr i t} : cr i t i c a l (b r e ak d o w n) e l ec t r i c f i e l d, ma t er ia l - d e p e n d e n t$
Reverse recovery time (negligible, RC-limited)	$t_{r r} \approx few \times R_{s} C_{j}$	$t_{r r}^{P N / P I N} \approx τ ln (1 + \frac{I _{F}}{I _{R}}) (see PN Junction Diode / PIN Diode pages)$	$t_{r r} : s w i t c hin g - o f f t im e, se t p u r e l y b y c a p a c i t i v e d i sc ha r g e t h r o ug h R_{s} s in ce t h er e i s n o s t or e d min or i t y c ha r g e t er m τ t o d i sc ha r g e$

Important Questions

Ques: Why does a Schottky diode switch so much faster than a PN diode (negligible reverse recovery)?

Conduction in a Schottky diode occurs via majority-carrier thermionic emission over the barrier rather than minority-carrier injection and diffusion, so there is no significant minority charge stored in the semiconductor bulk under forward bias the way there is in a PN or PIN diode. With no meaningful diffusion (storage) capacitance to discharge on turn-off, switching speed is instead limited only by the much smaller depletion (junction) capacitance and the carrier transit time across the already-thin depletion region — this is why Schottky diodes are the standard choice for high-frequency rectification and fast switching applications where a PN diode’s reverse recovery time would be a limiting factor. Reference: S.M. Sze and Kwok K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007.

Ques: Why does Schottky diode reverse leakage current keep increasing with reverse bias instead of saturating, unlike an ideal PN diode?

Thermionic emission current depends exponentially on the barrier height, and that barrier height is itself reduced by the image-force (Schottky) barrier-lowering effect, which grows with the peak electric field at the interface — and that field itself increases with reverse bias. This creates a self-reinforcing effect: increasing reverse voltage raises $E_{ma x}$ , which lowers $ϕ_{B}$ , which exponentially increases the leakage current, so the “reverse saturation current” of a Schottky diode is not truly constant the way the diffusion-current-limited saturation current of an ideal PN diode is. This gradually rising reverse leakage is one of the key practical tradeoffs against the Schottky diode’s fast switching advantage. Reference: S.M. Sze and Kwok K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007.

Quartz 5

Explorer