TRAPATT and

BARITT diodes

TRAPATT diodes

• TRAPATT- trapped plasma avalanche triggered transit mode

• It is a high efficiency microwave generator capable of generating oscillations of several hundreds of MHz to several GHz.

• It consists of a basic p-n junction diode operated in the reverse biased condition to current densities higher than that for normal avalanche operation.

• The doping is such that the DC electric field in the depletion region is well above the saturated drift-velocity level.

Principle of operation• Consider a p+-n-n+ diode. In TRAPATT mode, a high field

avalanche zone propagates through the diode and fills the depletion layer with a dense plasma of electrons and holes that become trapped in the low-field region.

• As shown in the figure below, let a square wave current be applied to the p+-n-n+ diode.

• At point A, the electric field is large and uniform throughout the diode but its magnitude is less than that required for avalanche breakdown.

• The current density is expressed as

• Where s=semiconductor dielectric permittivity of diode

Voltage and current waveforms for TRAPATT diode

• At the point A, the diode current is switched on. Thisleads to generation of carriers due to avalanche effect.The diode therefore charges up to a point B like acapacitor.

• By the time the voltage reaches point B, sufficientnumber of carriers have been generated and the particlecurrent exceeds the external current . This causes theelectric field to decrease throughout the depletion regionand the voltage decreases.

• Even though the electric field has decreased, it is stillsufficient for avalanche to continue. So a dense plasma ofelectrons and holes is created.

• The electric field and hence the voltage keeps decreasingfrom point B to C.

• Some of the electrons and holes drift out of the depletion region. As the field keeps decreasing, the remaining plasma gets trapped within the depletion layer. The voltage reaches a point D at this time.

• It takes a long time to remove the trapped plasma because the total plasma charge is large compared to the applied current. At point E, the plasma is removed.

• Removal of the plasma leaves behind residual charges. As these residual charges are removed, the voltage increases from point E to F.

• At point F, all the carriers generated internally have been removed. The diode then regains its initial state and voltage rises from point F to G.

• At G, the external current goes to 0. The voltage remains constant for one half cycle until current is switched on and the cycle repeats.

• The electric field can be expressed as

• Where NA = doping concentration of the n region

• x = distance

• The value of t at which the electric field reaches Em at a given distance x into the depletion region is obtained by setting E(x,t)=Em

• Differentiating w.r.t t, we get the avalanche zone velocity

• The avalanche zone sweeps quickly across the diode leaving behind highly conducting plasma of holes and electrons whose space-charge depresses the voltage to low values.

• Since the drift velocity depends on the field, the electrons and holes will drift at velocities determined by the low-field mobilities and the transit time of the carriers can be much longer than

• Where vs= saturated carrier drift velocity

• Thus the TRAPATT mode can operate at comparatively low frequencies.

Power output and efficiency

• RF power is delivered to the load when the diode is placed in a circuit where the diode effective negative resistance is matched to the load at the output frequency.

• To date, the highest pulse power of 1.2 kW has been obtained at 1.1GHz and the highest efficiency of 75% has been achieved at 0.6GHz.

• The TRAPATT mode generally exhibits higher noise figures than the IMPATT mode and the upper operating frequency appears to be practically limited to below the millimeter wave region.

BARITT diodes• BARITT- barrier injected transit time diodes

• The carriers are generated in BARITT mode by minority carrier injection from forward-biased junctions instead of being extracted from the plasma of the avalanche region as in TRAPATT mode.

• These diodes have different structures like p-n-p, p-n-v-p, p-n-metal and metal-p-metal.

• For p-n-v-p BARITT diode, the forward-biased p-n junction emits holes into the v(intrinsic) region and are collected at the p contact.

• The diode exhibits a negative resistance for transit angles between and 2 . The optimum transit angle is approximately 1.6 .

Principles of operation

• A crystal n-type Si wafer with 11Ω-cm resistivity and 4x1014 per cubic cm doping is made of a 10 m thin slice.

• This n-type Si wafer is sandwiched between 2 PtSi Schottky barrier contacts.

• The energy band diagram at thermal equilibrium is shown below where n1 and n2 are the barrier heights for the metal-semiconductor contacts.

• For PtSi-Si-PtSi structure, n1= n2=0.85eV

• The hole barrier height p2 for the forward-biased contact is about 0.15 eV.

• The mechanisms responsible for microwave oscillations are derived from:

M-n-M diode

1. The rapid increase of the carrier injection process caused by the decreasing potential barrier of the forward-biased metal-semiconductor contact.

2. An apparent 3 /2 transit angle of the injected carrier that traverses the semiconductor depletion region.

3. The rapid increase in terminal current with applied voltage (above 30V) is caused by thermionic hole injection into the semiconductor as the depletion layer of the reverse-biased contact reaches through the entire device thickness. The critical voltage is

Microwave performance

• The continuous wave(CW)microwave performance of the M-n-M type BARITT diode was obtained over the entire C-band of 4 to 8GHz.

• The maximum power observed was 50mW at 4.9GHz. The maximum efficiency was about 1.8%.

• The noise figure of BARITT diodes is lower than that of Si IMPATT diodes.