Metal Semiconductor Field Effect Transistor MESFET an Overview
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Metal Semiconductor Field Effect Transistor (MESFET) : an Overview Subject: Electronics; Paper: Semiconductor Device (ELC 202) Keywords: MESFET, Analytical Modeling Dr. Sutanu Dutta Department of Electronics Vidyasagar University
What is MESFET ? x y Fig. 1. 1: A schematic diagram of a MESFET Fig. 1. 2 Drain Characteristics of a MESFET and Transfer characteristics of a MESFET S. M. Sze, Physics of semiconductor Device
Modeling of drain current Assumptions n Poisson's Equation Gradual channel approximation n Abrupt depletion layer n Constant mobility Current Density Drain Current Integrating both side of Id from 0 to L (1. 1)
Field dependent mobility (Si) n The above modeling approach is appropriate for long channel devices (L >> a). n At low electric field electron mobility is constant and electron velocity is field dependent but at large electric field electron velocity becomes constant and mobility becomes field dependent. (1. 2) (1. 3) Fig. 1. 3: Velocity –Field characteristics Si S. M. Sze, Physics of semiconductor Device
Field dependent mobility for Compound semiconductors For Ga. As MESFET (1. 4) For Si. C and Ga. N MESFET (1. 5) Fig. 1. 4: Velocity field characteristics of Ga. As, In. N, Ga. N and Ai. N For Ga. As MESFET, β= 4. For Ga. N and Si. C MESFET, β=1. 7 and 0. 84, respectively M FAbusaid et al 1986 and Turin et al 2006
Two region Model for E < Ec for E ≥ Ec Current in region I (1. 6) Fig. 1. 5: MESFET operated under two region model Current in region II Id = q vs Z Nd a(1 –uc) (1. 7) L 1 = z L {(uc 2 -u 12)-2/3(uc 3 -u 13)}/ (1 -uc) (1. 8) VI = Vp(uc 2 -u 12) (1. 9) VII = (2 a /π) Fs Sinh{π(L-L 1)/2 a} (1. 10) S. M. Sze, Physics of semiconductor Device
Wide band gap Semiconductors § § § The WBG semiconductors like Ga. N and Si. C are generally popular as a channel material of a MESFET for high temperature and high power applications. This is due to their low thermal-generation rates and large break down voltage. Ga. N have high electron mobility in comparison with other wide band gap materials and It supports the development hetero-structure technology with its related alloys. But thermal conductivity is low for Ga. N and thermal energy cannot dissipated quickly from it, which limits performances. In some cases, the materials having large thermal conductivity like Si. C is used as a substrate material. The performance of the device operated in high temperature and high power environment is significantly improved when Si. C is chosen as channel material. This is due to some unique properties of Si. C like large band gap (3. 26 e. V), high break down voltage (3 MV cm-1), high saturation electron velocity (2. 107 cm/s), high critical field (2 MV/cm), high thermal conductivity (4. 9 Wcm-1 K-1) and low relative permittivity (9. 66) of the material. The Si. C MESFETs thus can withstand higher temperature (873 K) in comparison with the other conventional channel materials and has becoming a promising candidate for high temperature and high power electronics. Casady et al 1996, Zolper et al 1998 and Deng et al 2009
Analytical models of Si. C MESFET n For Si. C the electric field dependence of the electron mobility can be described following the Caughey-Thomas model (1. 11) n Where µ 0 is the low field mobility, vs is the saturation velocity and E is the electric field strength. β is the curvature parameter having its value 0. 84 (approximated as 1). In region I, the current equation is driven by field dependent mobility considering above equation given by (1. 12) It is assumed that the electric field is such that the electron velocity saturates at x = L 1 and for x>L 1 the current equation is governed by velocity saturation model. (1. 13) where uc is the normalized depletion layer width at x = L 1 and Vp is the pinch off voltage of the device.
Surface state effect § In a MESFET structure, there are some electron traps originating from various defects during material processing at the un-gated regions between gate to source or drain. These traps can capture electrons and the subsequent reduction of drain current due to such electron capture near the semiconductor surface is known as surface state effects. § The surface state effects can be mitigated introducing a passivation layer of Si 3 N 4 at the surface of the un-gated regions of the channel. § Sometimes a small p type spacer layer has been introduced in the un-gated regions so that electrons moving towards the surface are restricted due to the depletion region of P-N junctions formed in the spacer layer at the surface. Ref: http: //www. nt. chalmers. se/mve/wbg. htm Mukherjee et al 2004 and Kun et al 2012
Saturated Velocity Model For large applied drain field velocity saturation of electron may occurs near to source end and drain current can be given as For the channel having non uniform doping S. M. Sze, Physics of semiconductor Device, Wiley India Edition, page
Comparative Table Ref: http: //www. nt. chalmers. se/mve/wbg. htm
Self Aligned and Non self aligned MESFET Self Aligned MESFET Non-Self Aligned MESFET
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