Microwave Engineering

Avalanche Transit
Time Devices

Master the physics of negative resistance, impact ionization, and high-frequency oscillations in IMPATT, TRAPATT, and BARITT diodes.

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Fundamental Physics

Avalanche Transit Time devices rely on two key phenomena: Impact Ionization (Avalanche) and Carrier Transit Time.

Key Formula

f = vd / 2W

Where vd is drift velocity and W is depletion width. The factor of 2 arises from the 180° phase delay.

Impact Ionization

Under a high reverse bias electric field (> 3x105 V/cm), carriers gain enough kinetic energy to knock valence electrons into the conduction band, creating electron-hole pairs. This "avalanche" multiplication results in a high current.

Transit Time Delay

The generated carriers drift through the depletion region at saturation velocity (vs). This drift time introduces a delay between the AC voltage and the AC current, creating a 180° phase shift.

Negative Resistance

When the avalanche delay (approx 90°) is combined with the transit time delay (approx 90°), the total phase shift between voltage and current approaches 180°. This means the current flows "against" the voltage, mathematically equivalent to a negative resistance (R = -V/I).

I V

Current (Green) lags Voltage (Red) by 180°

IMPATT / READ

The IMPATT Diode

Structure & Mechanism

The IMpact Avalanche Transit Time diode is a p-n junction reverse biased to breakdown. The structure typically consists of a narrow Avalanche Region and a wide Drift Region.

  • 1

    Injection: Avalanche multiplication injects a pulse of electrons into the drift region.

  • 2

    Drift: Electrons drift at saturation velocity (vs ≈ 107 cm/s) across the depletion width.

  • 3

    Collection: They arrive at the terminal when the AC voltage is at its minimum (negative peak), delivering power.

Performance

Freq: 1GHz - 300GHz
Power: Watts to Kilowatts (Pulsed)

Cross-Sectional View

P+ (Anode)
Avalanche Zone
Drift Region (Intrinsic/N-)
N+ (Cathode)
E-Field

N+-P-I-P+ (Read) Structure

Voltage & Current Waveforms

TRAPATT Mode: Trapped Plasma Avalanche Triggered Transit

TRAPATT

TRAPATT Mode

The TRapped Plasma Avalanche Triggered Transit mode is a high-efficiency mode of IMPATT operation.

1

Plasma Formation

A high current pulse triggers a dense plasma of electrons and holes, shorting the diode (Voltage drops to near zero).

2

Trapping & Extraction

The plasma is trapped. As it is slowly extracted, the field rebuilds. The voltage is high when current is low (efficient).

Advantage: Much higher efficiency (20-60%) compared to IMPATT (10-15%).
Disadvantage: Noisier and limited to lower frequencies.

BARITT

The BARITT Diode

The BArrier Injection Transit Time diode operates on the principle of thermionic injection rather than avalanche breakdown.

Mechanism

It consists of a Metal-Semiconductor-Metal (MSM) or P-N-P structure. Forward biasing the first junction injects minority carriers (holes) into the drift region. This injection is delayed by the RC time constant of the barrier.

Characteristics

  • Lower noise than IMPATT (no avalanche noise).
  • Lower power output.
  • Used in local oscillators for radar and communication systems.

P-N-P Structure

P+
N (Drift)
P+
Anode (+) Cathode (-)

ATT Device Simulator

Visualize carrier drift and phase relationships.

0V 50V 100V
Short Medium Long

Live Metrics

Frequency
10.0 GHz
Phase Delay
180°
*Adjust width to change transit time. Adjust voltage to change avalanche intensity (IMPATT) or injection current (BARITT).
Electron
Hole

Comparative Analysis

Parameter IMPATT TRAPATT BARITT
Full Name Impact Avalanche Transit Time Trapped Plasma Avalanche Transit Time Barrier Injection Transit Time
Principle Avalanche Multiplication Trapped Plasma Formation Minority Carrier Injection
Efficiency Low (10-15%) High (20-60%) Medium (5-10%)
Noise Figure High (Noisy) Very High Low (Quiet)
Frequency High (100-300 GHz) Low (1-10 GHz) Medium (Up to 25 GHz)