Trapped Plasma Avalanche Triggered Transit (TRAPATT) Diode

1. Theory

Definition: A TRAPATT (TRApped Plasma Avalanche Triggered Transit) diode is an avalanche diode, usually a Si $P^{+}$ -N- $N^{+}$ structure, driven at very high current density so that a dense electron-hole plasma forms and is then slowly extracted.
High current density: TRAPATT mode starts when the bias current density exceeds $q N_{B} v_{s a t}$ , so the avalanche front moves faster than carriers can drift.
Avalanche shock front: The ionizing front sweeps through the depletion region, filling it with plasma much denser than the doping.
Trapped plasma: The plasma collapses the field behind the front to a low value, so carriers are trapped and drift only slowly.
Same structure as IMPATT: The device is IMPATT-like; the mode is set by the drive current and circuit, not by a different physical structure.

2. Working Principle

Charging: The diode charges like a capacitor until the field exceeds breakdown everywhere ahead of the moving front.
Plasma formation: The avalanche front races through the device, the diode voltage collapses, and a plasma fills the active region.
Plasma extraction: At low voltage and high current, the plasma is slowly swept out by the bias current; this phase sets the cycle time.
Recovery: Once the plasma is gone, the voltage jumps back up and the cycle repeats.
Square-wave operation: The near-rectangular voltage and current waveforms give high efficiency (about 20–60%, versus 10–30% for IMPATT).
Low frequency, high noise: Slow plasma extraction limits operation to roughly 0.5–10 GHz, and the device is very noisy and needs a complex harmonic-tuned circuit.

3. Applications

Current status: Largely superseded by GaN and GaAs transistor power amplifiers.
Historical role: High-efficiency pulsed sources for phased-array radar, radar altimeters and other L- to S-band transmitters.

Name	Formula	Related Formulae	Variables/Constants
Avalanche zone (ionization front) velocity	$v_{z} = \frac{J _{0}}{q N _{B}}$	$v_{z} ≫ v_{s a t} (TRAPATT condition; contrast IMPATT where v_{z} ≲ v_{s a t})$	$v_{z} : v e l oc i t y a t w hi c h t h e cr i t i c a l - f i e l d (i o ni z in g) p o in t a d v an ces t h r o ug h t h e d i o d e$ $J_{0} : D C bia s c u r r e n t d e n s i t y$ $N_{B} : d o p in g o f t h e a c t i v e (l i g h tl y d o p e d) r e g i o n$
TRAPATT-mode onset condition	$J_{0} > q N_{B} v_{s a t}$	$(contrast: IMPATT mode when J_{0} ≲ q N_{B} v_{s a t}, see IMPATT Diode page)$	$v_{s a t} : c a r r i er s a t u r a t i o n v e l oc i t y$
Trapped plasma charge (per unit area)	$Q_{pl} = q p_{pl} W, p_{pl} ≫ N_{B}$		$Q_{pl} : e l ec t r o n - h o l e pl a s ma c ha r g e l e f t b e hin d t h e a d v an c in g i o ni z a t i o n f r o n t$ $p_{pl} : t r a pp e d pl a s ma d e n s i t y, m u c h l a r g er t han t h e d o p in g N_{B}$ $W : a c t i v e l a y er w i d t h$
Plasma extraction (discharge) time	$t_{pl} \approx \frac{Q _{pl}}{J _{0}} = \frac{q p _{pl} W}{J _{0}}$	$t_{pl} ≫ avalanche zone transit time, and dominates the RF period$	$t_{pl} : t im e f or t h e bia s c u r r e n t t o s w ee p o u t (n e u t r a l i z e) t h e t r a pp e d pl a s ma c ha r g e$
TRAPATT oscillation frequency (approximate)	$f_{T R A P A T T} \approx \frac{1}{t _{pl}} = \frac{J _{0}}{q p _{pl} W}$	$f_{T R A P A T T} ≪ f_{0} = \frac{v _{s a t}}{2 W _{d}} (IMPATT transit-time frequency, see IMPATT Diode page)$	$f_{T R A P A T T} : o p er a t in g f r e q u e n cy, t y p i c a l l y l o w er t han t h e nai v e t r an s i t - t im e f r e q u e n cy f or t h e s am e d im e n s i o n s$
RF conversion efficiency (approximate)	$η \approx \frac{2}{π} \cdot \frac{V _{R F}}{V _{D C}}$	$η_{T R A P A T T} (\sim 20 - 60%) ≳ η_{I M P A T T} (\sim 10 - 30%) (see IMPATT Diode page)$	$η : D C - t o - R F co n v er s i o n e f f i c i e n cy$

Important Questions

Ques: What physically distinguishes TRAPATT mode from IMPATT mode, given both devices rely on avalanche multiplication in a similar structure?

The distinguishing factor is current density. At the lower current densities typical of IMPATT operation, the point of critical field advances through the diode at or below the carrier saturation velocity, so the usual picture of carriers drifting across a well-defined drift region (as described on the IMPATT Diode page) remains accurate. At the much higher current densities used in TRAPATT operation, $J_{0} > q N_{B} v_{s a t}$ means the ionizing front itself advances faster than carriers can actually be transported by drift — the point of impact ionization effectively outruns normal carrier transport, leaving a dense electron-hole plasma trapped behind it in a region where the field has since collapsed to a low value. Rather than cleanly drifting out as in transit-time operation, this trapped plasma must instead be slowly discharged through the external bias current, which is the “trapped plasma” that gives TRAPATT its name. Reference: H.J. Prager, K.K.N. Chang, and S. Weisbrod, “High-power, high-efficiency silicon avalanche diodes at ultra high frequencies,” Proc. IEEE 55, 586 (1967); S.M. Sze and Kwok K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007.

Ques: Why does TRAPATT achieve higher conversion efficiency than IMPATT?

IMPATT’s negative resistance relies on a phase-delayed, roughly sinusoidal relationship between current and voltage, which inherently limits how much of the DC voltage swing can be converted to useful RF power. TRAPATT’s plasma-discharge mechanism instead produces a current waveform closer to a rectangular pulse — the diode voltage collapses sharply once the plasma forms and stays low throughout the slow plasma-extraction phase, then recovers sharply once the plasma clears — allowing a much larger fraction of the available voltage swing to be captured as RF output rather than being lost to phase-mismatched reactive power. This is the physical basis for TRAPATT’s typically higher efficiency (often 20–60%, compared to IMPATT’s 10–30%). Reference: S.M. Sze and Kwok K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007.

Ques: Why does the plasma extraction time, rather than a simple transit time, set TRAPATT’s operating frequency?

The avalanche zone itself sweeps through the diode very quickly (at velocity $v_{z}$ , which by the TRAPATT-mode condition is already faster than saturation velocity), but the resulting trapped plasma charge $Q_{pl}$ can only be removed as fast as the external bias current can neutralize it — and since $Q_{pl}$ is a substantial charge set by the plasma density (much greater than the doping) and the active-layer volume, this plasma extraction phase takes considerably longer than the initial avalanche zone transit. Because this slow discharge phase dominates the total RF cycle time, TRAPATT diodes of similar physical dimensions to an IMPATT diode typically operate at noticeably lower frequencies than the simple transit-time formula would predict, which is part of why TRAPATT and IMPATT diodes, despite closely related structures, tend to be used in different frequency/power regimes in practice. Reference: S.M. Sze and Kwok K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007.

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