1. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Introduction 2013.01.26 SD Lab. SOGANG Univ. Gil Yong Song

2. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Contents 1. Introduction ideal and typical power device characteristics Unipolar power devices Bipolar power devices MOS-Bipolar power devices Ideal drift region for unipolar power device 2. Material Properties and Technology Fundamental properties Other properties relevant to power devices Fabrication Technology

3. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Introduction

4. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Introduction 1950’s : The industry relied upon silicon bipolar devices, such as bipolar power transistors and thyristors. 1970’s : The advent of MOS technology for digital electronics enabled the creation of a new class of devices in the 1970s for power switching applications. 1980’s : The merger of MOS and bipolar physics enabled creation of yet another class of devices (IGBT). - Power devices are required for systems that operate over a broad spectrum of power levels and frequencies.

5. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Introduction MOSFET IGBT Thyristor - Thyristor : low frequency. IGBT : medium frequency. MOSFET : high frequency.

6. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Ideal and Typical Power Device Characteristics Ideal case - zero on-state voltage drop. - zero leakage current. - zero switching time. Practical case - finite voltage drop when carrying current on the on-state leading to ‘conduction’ power loss. - finite leakage current when blocking voltage in the off-state creating power loss. - Doping concentration and thickness of drift region control the breakdown voltage.

7. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Ideal and Typical Power Device Characteristics Ideal transistor conducts - on-state with zero voltage drop - off-state with zero leakage current - active region controlled by gate bias Practical transistor conducts - finite resistance - finite leakage current while operating in the off-state - finite breakdown voltage uniform non-uniform

8. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Unipolar Power Devices Bipolar power devices - Power loss problem that degrade the power management efficiency. -> use unipolar power diode! Schottky rectifier (example of unipolar power diode) - utilize a metal-semiconductor barrier to produce current rectification. - drift region : to support the reverse blocking voltage.

9. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Unipolar Power Devices - The most commonly used unipolar power transistor is MOSFET. D-MOSFET - planar-gate structure - p-base, N+ source region - the voltage blocking capability is determined by the doping and thickness of drift region. U-MOSFET - It has a gate structure embedded within a trench etched into the Si surface. - The N-type channel is formed on the sidewall of the trench. - The Si U-MOSFET structure was developed to reduce the on-state resistance.

10. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Bipolar Power Devices

11. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. MOS-Bipolar Power Devices -IGBT(insulated gate bipolar transistor) : the most widely used Si high voltage device for power switching -The structure is similar to that for the power MOSFET by combining the physics of bipolar and MOSFET. -four-layer parasitic thyristor

12. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Ideal Drift Region for Unipolar Power Devices -Drift region is designed to support the blocking voltage. -Assumption : abrupt junction -Specific resistance of the ideal drift region is given by -The depletion width under breakdown conditions is given by, -The doping concentration in the drift region is given by, -Combining by these relationships,

13. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Ideal Drift Region for Unipolar Power Devices -The improvement in drift region resistance for GaAs in comparison with silicon is largely due to its much greater mobility for electrons. -The improvement in drift region resistance for SiC in comparison with silicon is largely due to its much larger critical field for breakdown.

14. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Materials Properties and Technology

15. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Fundamental Properties -Only the properties for 4H-SiC have been included here

16. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Energy Band Gap Energy Bang Gap Si : 1.12eV, 4H-SiC : 3.26eV (3 times larger than Si) - Large band gap → small generation in the depletion regions → small leakage current -Intrinsic carrier concentration -For silicon, -For 4H-SiC -The intrinsic carrier concentration can be calculated as a function of temperature. -The bulk generation current is negligible for the determination of the leakage current in SiC. Room temperature

17. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Energy Band Gap -Built in potential -For Si, ionized impurity concentrations are equal to the doping concentration because of the small dopant ionization energy levels. But, this doesn’t apply to SiC because of the much larger dopant ionization levels. -(fig 2.4) The built in potential for SiC is much larger than that for Si due to the far smaller values for the intrinsic carrier concentration. -(fig 2.5) The zero bias depletion width for SiC is much larger than for Si for the same doping concentration.

18. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Impact Ionization Coefficients

19. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Electron mobility

20. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Donor and Acceptor Ionization Energy levels -In Si, the donor and acceptors ionization energy is small (50mV). -SiC have a large ionization energies for donors and acceptors. -The most widely used Donor of 4H-SiC : nitrogen (100meV) -Acceptor of 4H-SiC : aluminum (200meV) Recombination Lifetimes -The recombination process is band-to-band but is usually assisted by the deep levels. -Deep levels can produce leakage current by the generation of carriers in the depletion region. -SiC (unipolar device) has a low lifetime and diffusion length → fast switching speed -but bipolar devices have a higher bulk lifetime → slow switching speed

21. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Metal-Semiconductor Contacts -Ohmic contact : metal-semiconductor contacts with low barrier height and high doping level in semiconductor (tunneling process) -The contact resistance determined by the tunneling process is dependent upon the barrier height and doping level -The barrier heights of metal-SiC contact are large due to large band gap. -Schottky barrier rectifier : Titanium, Nikel

22. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Fabrication Technology

23. SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Fabrication Technology