Report

TED’s are semiconductor devices with no junctions and gates. They are fabricated from compound semiconductors like GaAs, InP, CdTe etc. TED’s operate with hot electrons whose energy is much greater than the thermal energy. Invented by J.B Gunn Gunn Effect: Above some critical voltage (Corresponding to Electric field of 2k-4k V/cm) the current passing through n-type GaAs becomes a periodic fluctuating function of time. Frequency of oscillation is determined mainly by the specimen, not by the external circuit. Period of oscillation is inversely proportional to the specimen length and is equal to the transit time of electrons between the electrodes The current waveform was produced by applying a voltage pulse of 16V and 10ns duration to an n-type GaAs of 2.5 x 10-3 cm length. The oscillation frequency was 4.5Ghz Explanation for Gunn Effect: Ridley – Watkins – Hilsum (RWH) Theory Two concepts related with RWH Theory. ◦ Differential negative resistance ◦ Two valley model Fundamental concept of RWH Theory. Developed in bulk solid state III-V compound when a voltage is applied Differential negative resistance make the sample electrically unstable. Data for two valleys in GaAs Conductivity of n-type GaAs: e = Electron charge μ = Electron mobility = Electron density in the lower valley = Electron density in the upper valley is the electron density According to RWH theory, in order to exhibit negative resistance the energy band structure of semiconductor should satisfy ◦ The energy difference between two valleys must be several times larger than the thermal energy (KT ~ 0.0259eV) ◦ The energy difference between the valleys must be smaller than the bandgap energy (Eg) ◦ Electron in lower valley must have a higher mobility and smaller effective mass than that of in upper valley Possessed by GaAs, InP, CdTe etc In GaAs, at electric fields exceeding the critical value of Ec ≈ 3.2 kV/cm the differential mobility is –ve. When the field exceeds Ec and further increases, the electron drift velocity decreases. Gunn Oscillation Mode: ◦ (f x L) = 107 cm/s and (n x L) > 1012 /cm2 ◦ Cyclic formation of High field domain Stable Amplification Mode LSA Oscillation Mode Bias-circuit ◦ (f x L) = 107 cm/s and 1011/cm2 < (n x L) >1012/cm2 ◦ (f x L) >107 cm/s and 2 x 104 < (n/f) > 2 X105/cm2 ◦ (f x L) is small. L is very small. When E=Eth current falls as Gunn oscillation begins, leads to oscillation in bias circuit (1KHz to 100MHz) Condition for successful domain drift: Transit time (L/vs) > Electric relaxation time Frequency of oscillation = vdom/Leff. Gunn diode with a resistive circuit -> Voltage change across diode is constant-> Period of oscillation is the time required for the domain to drift from the cathode to anode. Not suitable for microwave applications because of low efficiency. Gunn diode with a resonant circuit has high efficiency. There are three domain modes for Gunn oscillation modes. 1. Transit time domain mode, (Gunn mode) 2. Delayed domain mode ◦ Here domain is collected while ◦ New domain cannot form until E rises above threshold again. ◦ , ◦ Also called inhibited mode. ◦ Efficiency: 20% 3. Quenched domain mode: ◦ If bias field drops below Es, domain collapses before it reaches anode. ◦ When the bias field swings above Eth, a new domain starts and process repeats. ◦ Frequency of oscillation is determined by resonant circuit. ◦ Efficiency : 13% Limited Space charge Accumulation Mode (LSA) Most Important mode for Gunn oscillator. Domain is not allowed to form. Efficiency : 20% Power: 1W (Between 4HHz and 16GHz) Gain Bandwidth product : >10dB Average gain : 1 – 12 dB Noise figure : 15 dB In radar transmitters Air traffic control (ATC) and Industrial Telemetry Broadband linear amplifier Fast combinational and sequential logic circuit Low and medium power oscillators in microwave receivers As pump sources