Impatt diode layer structure and field distribution

When applied voltage exceeds the breakdown voltage, impact ionization starts and avalanche breakdown occurs. Electron-hole pairs are generated in the l_a region near the metallurgical border, where the field strength is high enough for impact ionization, and then are divided by the field. These new charge carriers cause current which stops after they leave the p-n junction.

Drift velocity distribution (v(E))

Drift velocity tends to velocity of saturation (v_s=10⁵ m/s), for Si-dot line

Gunn diode

carriers movement

Gunn diode layer structure and field distribution

The structure of Gunn diode is n-type GaAs with high-concentration doped contact regions, used to make ohmic contacts. Gunn diode has no junctions in its structure. The main advantage of Gunn diode is its structure simplicity that leaded to wide use of this device.

Drift velocity distribution

The complex form of distribution is caused by the region of negative differential mobility. In this region bunching of carriers takes place.

2) Generation frequency of IMPATT diode and GD with moving domain

IMPATT diode

,

V_s=10⁵ (m/s) – velocity of saturation; f₀ – generation frequency; L_device – active length

Generation frequency:

Gunn diode

,

V_s=10⁵ (m/s) – velocity of saturation; f₀ – generation frequency; L_device – active length

Generation frequency:

Answer: generation frequency: IMPATT diode f₀=6.25 GHz, Gunn diode f₀=12.5 GHz.Links: lecture materials on MWEl "Semiconductor devices",

Lebedev AI "Physics of Semiconductor Devices", 2008

Task №3 Transistors

1. Compare max possible base thickness of a bipolar transistor and gate length of a FET used on frequency . Estimate transit angle in both cases. Link with the solution of №5 from the first task.

2. Explain the trend of using in modern transistors of such materials as GaN, SiC and diamond.

3. Compare advantages and disadvantages of using in microwave range of HEMT-devices and ballistic transistors. Calculate the thickness of the heavily-doped region of HEMT with , if the junction potential

4. Draw (qualitatively) input and output VCC of three MESFET having similar size but made of Si, GaN, GaAs.

5. Draw series of input and output VCC and noise factor on one plot. Explain good noise characteristics of MESFET. Use solution of №7 from the first task for analysis.

Given:

Find: 1. θ (BJT and FET)

2. Explain the trends of using of different materials

3. Compare HEMT and ballistic transistors. A_k (HEMT)

4. VCC of MESFET

5. VCC and noise factor of MESFET

Solution:

1.1 BJT. Supposing f₀ is cutoff frequency.

Cutoff frequency:

— transit time that can be found a using base thickness:

— coefficient that depends on doping (for uniform doping );

—carrier diffusion coefficient, that can be found with the use of Einstein equation and the value of carrier mobility. For electrons in Si:

Then the base thickness: ;

Transit angle:

1.2 FET. Supposing the carrier's velocity is velocity of saturation. Transit time:

Cutoff frequency:

Then:

Transit angle:

Comparing with exercise №5 from task №1 we can see, that the result is right, because we have got transit angles orders near one radian, while transit length is approximately one μm. Estimated value of transit angle in exercise №5 from task №1 was π radian while the length had the order of 10 μm.

Answer: 1. BJT: W=0.312 μm; θ=0.22 rad; 2. L_g=1.45 μm; θ=1 rad

2. Explanation of the trend of using in modern transistors of such materials as GaN, SiC and diamond

GaN gallium nitride. This is a semiconductor of type . Gallium nitride is a direct-gap semiconductor with a large band gap (3,4 eV at 300 K). Transistors made of this material have high power and frequency for operation in the microwave range, the maximum operating temperatures compared with microwave transistors based on traditional Si, GaAs, Ge.

Silicon carbide SiC. This is a semiconductor of type . Silicon carbide also belong to the wide bandgap semiconductors. Devices based on this materials have a greater power in comparison with Si and GaAs devices on the same frequencies.

Diamond (Carbon). Usually diamonds are excellent electrical insulators, but some of them can be used as semiconductors: some blue diamonds are natural semiconductors. The conductivity and blue color originate from boron impurity. Boron substitutes for carbon atoms in the diamond lattice, donating a hole into the valence band.

Let's give a comparative description of the materials on the basis of some properties in a table:

Parameter	Si	GaAs	SiC	GaN	C
Band gap, eV	1,1	1,4	3,2	3,5	5,45
Critical field strength, *106 V/cm	0,3	0,4	3	3,3	10
Thermal conductivity, W/m*K	1,5	0,5	4,9	>1,5	22
Electron mobility, cm2/V*s	1350	8500	700	2000	2000
Electron drift velocity of saturation, *107 cm/s	1,0	2,0	2,0	2,7	2,7

Comparison of different materials' characteristics

The maximum band gap (3 times greater than that of silicon technology in silicon carbide and gallium nitride, and 5 times greater than that of diamond) determines the possibility of operation of transistors based on gallium nitride, silicon carbide and diamond at high temperatures and radiation levels. Thus also for this reason, a small leakage current is formed. That is, devices based on these materials can operate at extreme conditions. For these devices also have high value of the critical electric field strength. That is, breakdown occurs at much higher voltage.

Devices based on gallium nitride showed record values ​​of specific power output density. On the basis of silicon carbide transistors IGBT power electronics are built - these transistors are cheap replacement for powerful MOSFETs. Nanodiamond transistors, like diamonds themselves, are incredibly resistant to temperature and radiation — and apparently they also require less power to switch than their silicon-based counterparts. One of the problem about them is their scale. An important factor is also a high thermal conductivity, which greatly increases the maximum operating temperature.

The disadvantage of these three materials is that the mobility of charge carriers is much lower than mobility in gallium arsenide. This leads to the fact that the performance of devices on silicon carbide, gallium nitride and diamond structures is significantly lower.

3.1. The advantages and disadvantages of the use of microwave-HEMT devices and ballistic transport transistors

HEMT-transistors - FETs with high electron mobility in the channel. This type of transistor is different from a MESFET by the presence of heterojunction between the barrier and the undoped channel layers. On the border of the metallurgical section of the heterojunction there forms a charge storage well, where a 2D-gas is forming. The material for the channel layer initially has high electron mobility. But the channel layer is not doped, so the scattering of the electron gas is minimal, and thus the mobility remains very high. Due to the high mobility the transistor has significantly higher performance.

The increase in the electron mobility increases the saturation velocity and a reduction of the resistance of ​​the drain and source (such resistance has a negative impact on the steepness of the VCC). On the other hand, for the creation of high-power devices it is necessary to make the drain current density was as high as possible. This requires to increase the surface density of electrons in the channel (QW). This will lead to a reduction in the electron mobility. The difficulty is to find a compromise between power and performance.

Modern HEMT-transistors are the most high-frequency and high-speed devices of all types of field-effect transistors. These transistors also have a low noise level.

Ballistic transistors. This type of transistor is based on the phenomenon of ballistic transport that occurs in structures with ultrashort (<500 Å) channel. Charge carriers moving in a strong electric field, flies channel for a time less than the momentum relaxation time, so they can never experience a single act of scattering. Charge carriers in a transistor are moving as quickly as possible (speed at any point of the channel is determined only by the value of the electrostatic potential at this point) - it allows to provide the minimum possible transit time for a given geometry of the device and the maximum slope of the VCC.

The structure with the channel length of 250 Å was created in 1999. The formed drain current was 85% of the maximum possible value corresponding purely ballistic regime, because it is quite difficult to provide a pure ballistic regime. When reducing the length of the channel, scattering of charge carriers on charged impurities increases strongly. These impurities are located in the heavily doped gate, in the halo region and heavily doped source and drain regions. Also, increasing of the intensity of the transverse electric field increases the scattering on phonons and surface roughness.

The main advantages of these transistors are small shot noise at low temperatures, small size, low power consumption and high speed switching. These transistors are a perspective course for the terahertz range.

3 .2. To calculate the thickness of a HEMT heavily-doped region let's use the following expression:

,

φ_bin - junction potential, [V]

N_d - carrier concentration, [cm^-3]

ε - permittivity, [F∙m^-1]

A_k - region thickness, [m]

q - elementary charge,

Material used is Al_0.3Ga_0.7As-GaAs, the heavily-doped layer is the Al_0.3Ga_0.7As part;

ε _Al0.3Ga0.7As=12,9-2,84

Answer: =

4. Input and output VCC of three MESFET having similar size but made of Si, GaN, GaAs

U_ds

I_ds

Output VCC

Expression for linear part of VCC:

Since the dimensions of the transistors are the same, the drain current will be determined by the parameters of the semiconductor. In particular - electron mobility. In GaAs electron mobility is much higher than that of silicon and gallium nitride. Therefore, the slope of the linear portion of the characteristic (increase of drain current) is the biggest. The breakdown voltage is determined by the band gap. In this case, gallium nitride has the largest biggest band gap. The greater the gap, the higher the breakdown voltage.

The structure is symmetric so the negative branch will be the same.