Avalanche Breakdown Characteristics of Wide Band GaP vis-à-vis Low Band Gap Junctions and High RF Power/Low Noise Generation in ZnS DD IMPATTs

Ionization threshold in a semiconductor is determined through the value of band gap energy (Eg, < 1 eV to > 5 eV), which monitors the avalanche breakdown phenomenon of corresponding reverse biased p-n junctions. Further the p-n junction under avalanche breakdown can generate high frequency (hf) power (between 10 and 400GHz and beyond) in Impatt mode, as negative resistance oscillator, performance of which again depends on band gap energy of the semiconductor. Computer aided studies on breakdown and hf performance characteristics of flat DD (ppnn) Impatts based on conventional materials with moderate band gap energy ( Si, GaAs and InP having Eg=1.12, 1.36 and 1.26 eV respectively) as well as wide band gap semiconductors (ZnS, SiC and GaN, with respectively values of Eg =3.68, 2.9 and 3.4 eV) are presented. The p-n junctions of materials having wide band gap energy needs sufficiently high reverse electric field/reverse bias voltage to initiate avalanche breakdown, as compared to the same for materials with low gap. For 3 micron pp-nn type flat doped junction, the breakdown field and breakdown voltage for ZnS diode become 170 V/micron and 661 V respectively as against 40 V/micron and 72 V for corresponding Si junction. Similar trend has been observed for SiC, GaN and InP, GaAs diode junctions. The rf analysis of the corresponding Impatt diodes shows that the Device efficiency (21.1%) and negative resistance (0.228 × 10 Ω.m) for 60 GHz (1 micron) ZnS DDRs are respectively around 1.9, 2.4 times of the values of the same parameters of corresponding Silicon DDR. The inherent avalanche noise associated with ZnS diode has been computed to be only 7% of the noise generated in Si Impatt diode. The performance of ZnS diode is also compared 230 Shankar P. Pati, Pravash R. Tripathy and Santosh K. Dash with diodes based on GaAs, InP, GaN and SiC diodes. The computed value of efficiency & rf power output remains low, noise level becomes high for GaAs, InP, SiC and GaN devices compared to ZnS DDD. Computed values of rf power from 60 GHz Si, GaAs, InP, SiC, GaN and ZnS DDRs have been found respectively to be 18.9, 66.65, 119.95, 1532.7, 979.99 and 1987.4 W. RF power generation also have been investigated by considering respective diode area to be 50a, 10a, 5a,2a, and a where a is 10m for different frequency ranges (11 GHz-140GHz) for ZnS-based IMPATTs which indicates similar trend of high rf power realization from ZnS DDDs in this entire frequency range. Indexing Terms: Band Gap Energy, ZnS, GaN, SiC, InP, GaAs, Si, Flat DDD, Impatt, mm-power, Avalanche Noise. Introduction Ionization threshold in a semiconductor is determined through the value of band gap energy (normally the value of band gap energy, Eg, remains in the range of 0.66 eV to 5.1 eV for practical semiconductors), which monitors the avalanche breakdown phenomenon of reverse biased p-n junctions based on the same semiconductor. Further reverse biased p-n junction under avalanche breakdown can generate high frequency (hf) power (fundamental frequency ranges between 10 and 400 GHz) in impatt mode, as negative resistance oscillator, performance of which is supposed to depend on band gap energy of the semiconductor. Under harmonic mode the frequency can be still pushed to 1000 GHz, which is usually not achievable by any other solid state semiconductor device. Further advantage of impatt class of device is that the junction device can be fabricated from any semiconductor as the device physics is not typical to particular semiconductor. The negative resistance is produced due to combined phenomena of transit time delay and avalanche phase delay, making the total phase delay between rf current and voltage between π/2 and 3π/2. The efficiency and rf power output fall when the frequency is pushed to mm-wave/submm-wave bands. Research work is being carried out by different groups of scientists/engineers to enhance the efficiency and mm-wave power from this class of device even at beyond mm-wave frequencies. New materials are being studied and explored for impatt diodes with a view to achieve the above mentioned objectives. The indication from report of better microwave performance from GaAs and InP impatt over silicon diode due their higher band gap provides motive to study the device characteristics of wide gap semiconductor based impatt diodes vis-à-vis impatt diodes fabricated from narrow gap semiconductor. The authors have selected ZnS, SiC & GaN (respective values of Eg =3.68, 2.9 and 3.4eV) as prospective wide band semiconductors and considered Si, InP & GaAs (having Eg=1.12, 1.36 and 1.26 eV respectively) as moderate band gap semiconductors and compared breakdown as well as hf characteristics of p-n junction impatts with above mentioned base semiconductors. The properties of ZnS impatt diodes are observed to excel over the rest. This has given reasons to the authors to provide due emphasis to ZnS as one Avalanche Breakdown Charecteristics of Wide 231 prospective material for high efficiency and high rf power generation associated with low noise. Since the device technology is now very advanced and a lot of experimental reports are available on Silicon Impatts, first the properties of ZnS Impatts are compared with those of Silicon diodes. A review of recent reports [1-4] on high field transport properties and ionization rate data in ZnS indicate some interesting features like very high value of electron ionization rate at electric field above 1.5×10 V/m and its sharp fall below this field. These properties are supposed to identify ZnS as a prospective material for impatt device. Flat doped ZnS impatt diodes (pp-nn) have been analyzed for a wide range of doping/frequencies through use of a three phase generalized computer algorithm, which can be used to compute microwave/mm-wave properties and also to estimate the generation of avalanche noise for any form of impatt represented in figure 1. The results have been compared with Si Impatts designed for corresponding frequencies. The results indicate high values of breakdown electric field (Em) and breakdown voltage (VB) for ZnS diode. The avalanche zone becomes localized leading to possible realization of optimum high efficiency (η > 28% at 18 GHz) and enhanced value of diode negative resistance (Zr). High VB would make the diode capable of handling high power and high η and Zr help in producing high rf power. In addition the computed value of mean square avalanche noise voltage (NV) and Noise measure (NM) are observed to be lower by an order of magnitude compared to corresponding conventional devices based on common semiconductors. However, the advantages of ZnS impatts are observed to be moderated at frequencies beyond 140 GHz. Thus ZnS may prove to be a promising high Eg material for use in Impatt diode fabrication in order to realize high rf power with high η and low noise form X-band to D-band (10140 GHz). Analysis also has been carried out for Impatt diodes based on GaAs & InP low band gap materials and SiC & GaN wide band gap materials IMPATTs. A comparative account of the rf performance of the devices are presented in this paper which indicates possibility of high mm-wave power generation associated with low noise from ZnS devices compared to Si, GaAs, InP, SiC and GaN-based Impatts. 232 Shankar P. Pati, Pravash R. Tripathy and Santosh K. Dash Figure 1: Schematic Double Drift Structure of Impatt Diode. Theory and Simulation Method Extracting the values of electron ionization rate (α) at different electric field values [2], a usual exponential (α=a.exp (-b/Eγ) as well as high order polynomial form (α=a0+a1E+a2E +a3E +. +anE ) of equations for α~E, are framed following usual curve fitting technique. The values of ‘a’, ‘b’ and γ in the exponential form and those of a1 to a8 are determined/used for the high field and low field zones. The drift velocities are taken from [4]. The electron and hole ionization rate are taken to be equal as ZnS is a very wide band gap semiconductor. Ionization rate, drift velocities and other material parameters for other semiconductors are taken from recent available experimental reports. A double iterative computer method to solve Poison, Carrier Continuity and space charge equations simultaneously subject to fulfillment of boundary conditions in electric field and carrier currents has been framed ensuring speedy convergence. The basic equations for DC analysis are, Poisson’s equation, x E ∂ ∂ = ∈ q (ND-NA+p-n) (1) and the current continuity equation is given by,