Explain Read diode. Give the electric field distribution, doping profile, voltage and current characteristics of read diode.

Read diode is nothing but the INPATT diode. The IMPATT diode is a convenient structure for a theoretical analysis and physical understanding but due to complicated nature IMPATT structure the prediction could not be immediately verified experimentally.

The first successfully experimental observation of microwave oscillations from Impatt was made in 1965 by Johnston and Lee from the ordinary p-n junction. Later in 19 65 Misawa showed with the help of small signal analysis that a negative resistance could be possessed by a junction diode of any arbitrary doping profile.

Thus the basic properties of charge carriers in semiconductor underlying the impatt phenomenon are

1. The structure of drift velocity of electrons and holes under a high electric field which leads to a transit time delay for the mobile charge carriers which is proportional to the width of the space charge layer.
2. Impact ionization leading to avalanche multiplication of charge carriers which leads to an avalanche delay of 90° or more between the applied RF voltage and the generated RF current.

Thus the device exhibits a high frequency negative resistance when the combined phase delay between current and the AC voltage due to avalanche process and the finite transit time of the drift carriers lie between 90° and 270°.

Fig. 21(a) structure of SDR Read diode or IMPATT diode, (b) Electric field distribution and (c) Doping profile.

The two common types of Impatt structure are SDR (Signal Drift Region) and DDR (Double Drift Region). The doping and field profile of both SDR and DDR structure are as shown in Fig. 21 and Fig. 22. Again SDR Impatt may be either a p⁺nn⁺ type or its complimenting n⁺pp⁺ type. In the SDR diodes the width of the avalanche zone around the field maximum located either p⁺n or n⁺p (depends upon the type of structure) is finite and extends up to a small fraction of the uniformly throughout the avalanche zone and is mostly encountered in the region near the metallurgical junction (either p⁺n or n⁺p ) where the electric field is the highest. The avalanche zone and drift zone together occupy the space charge layer which covers the lowly doped n or p regions.

The DDR structure was proposed by Scharfetter in the year 1970 have the type of structure n⁺np p⁺. It was conceive that charge carriers of both types could be utilized to produced microwave power when they travel in opposite directions in two separate drift regions on either side of the avalanche zone. The structure, doping and electric field distribution are shown in Fig. 22.

Fig. 22(a) Structure of Read diode or DDR IMPATT diode, (b) Electric field distribution and (c) Doping profile.

In DDR diode the carrier multiplication takes place in the central high field avalanche zone and generated electrons and holes move through the respective drift zone on n and p – side, which are finally collected through n⁺ and p⁺ contact region. The added drift region in the case of DDR Impatt diode has the edge over its SDR counterpart, such as higher breakdown TF power output. The central avalanche region causes a reduction in minority carriers storage and thereby increases the efficiency of double drift device.

Later it was found that the efficiency of the diode can be enhanced by constructing its avalanche layer through modification of doping profile in the lower conductivity region of the diode structure. An impurity bump (a region of higher doping density) can be introduced as various position of the depletion layer of the diode. A bump beside the junction will lead to high - low doping profile and when it is situated slightly away from the junction the profile is known as low – high - low profile. These modification profiles will push the value of field maximum. Thus the electric field value is enhanced within the avalanche zone which in turn augments the carrier multiplication process due to high value of carrier ionization rates near the metallurgical junction. This phenomenon would help to localize the avalanche zone and thereby would cause an enhancement of the device efficiency.

Fig. 23(a) Structure of Read diode or DDR IMPATT diode, (b) Doping profile and (c) Electric field distribution.

Due to the modification of doping profile a sharp increase of electric field in the avalanche region takes place in the low – high – low structure, this in turn sharply localizes the avalanche zone and causes a large region of considerable electric field to become a multiplication free drift zone. The large drift voltage drop in these devices is thus mainly responsible for the higher power output and the higher efficiency of the devices. Low –high - low and high – Low diodes which can further enhance the power output and efficiency of the devices. The doping profile and field profile of high – low and low – high – low DDR structure are as shorn in Fig 23.

Applications:

1. In the final power stage of solid state microwave transmitter (RADER) for communication purpose.
2. In the transmitter of television system.
3. Used in TDM and FDM system.
4. As a microwave source in the laboratory.
5. As a missile seeker head.