Answers of Questions
Chapter 1
1-1. Please classify the power semiconductor discrete devices.
A: power rectifiers and power switches.
功率整流器和功率开关
1-2. Please list the name of popular power switch.
A: power MOSFET, power BJT, IGBT, etc.
功率MOS管,功率BJT管,IGBT等
1-3. Define the ideal power semiconductor device.
A: It must be able to control the flow of power to loads with ZERO power dissipation.
零功耗(包括导通损耗,关断损耗,开关损耗,均为零)
1-4.
Drawing out the switching waveform of ideal power switch.
A:
1-5. List the device name you known that has the normally-off performance.
A: Normally-off: BJT, Enhancement MOSFET, VDMOS, Trench MOS, IGBT,
Normally-on: JFET, Depletion MOSFET.
1-6. Which is preferable for normal system, normally-on device or normally-off? Give the reason.
A: For normal system, normally-off device is preferable as it has the power dissipation lower than normally-on device’s.
常关器件更好。因为开通功耗(有电流)大于关断功耗(近似无电流),所以常关器件比常开器件具有更低功耗。
1-7. List the international semiconductor company name you know that products the power semiconductor devices.
A: TI, Onsemi, Freescale, Fairchild, ST, Infineon, NXP, Vishay, ADI, Maxim, ROHM
1-8. List the special points of power semiconductor devices comparing with the digital semiconductor devices.
A: higher voltage and larger current, bigger/special process technology, longer life cycle.
功率器件通常应用在高压、大电流环境,大线宽、特殊工艺,生命周期长。
数字器件通常应用在低压、小电流环境,小线宽、
工艺,生命周期短。
Chapter 2
2-1. what is meant by impact ionization coefficient of hole(electron)?
耗尽区内,一个空穴(电子)在电场作用下,通过1cm的长度上产生的电子-空穴对的数目,定义为空穴(电子)的碰撞电离系数
(
)。
2-2. what is a plane junction?
平行平面结是一维二极管,理想PN结,具有一维性质的电场。
2-3. define the punch-through diode structure.
PiN二极管,反向耐压最大时i区全耗尽,称为穿通型二极管结构。
2-4. why does the impact ionization coefficient increase with the increasing of the electric field and it decrease with the increasing of temperature?
碰撞电离系数是电场的强
(
)∝E7,电场增加碰撞电离系数增大;随着温度升高,晶格振动加剧,载流子更容易与晶格碰撞失去能量,产生的电子-空穴数目减少,所以温度升高碰撞电离系数减小。
2-5.
一个电子(空穴)在耗尽区内与晶格原子发生碰撞,产生一个电子-空穴对,器件即发生雪崩击穿,
2-6. Which structure has the largest breakdown voltage under the same junction depth: plane junction, cylindrical junction and spherical junction? Why?
平行平面结具有最大BV。
圆柱结(二维场)和球面结(三维场)因为曲率的存在,电场集中,更易击穿。
E(圆柱)=√Ex2+Ey2
E(球面)=√Ex2+Ey2+Ez2
2-7. Explain the effect of the oxide charger on the breakdown voltage in the field plate design.
氧化层电荷是正电荷,浓度在1010~1012/cm2 。由于额外正电荷的引入,对N+P结,耗尽层边缘会展宽,器件的BV增加;相对P+N结则产生负作用,耗尽层缩小,器件BV降低。
Chapter 3
3-1. Define the high-level injection
所谓大注入,即注入半导体中的非平衡少子浓度接近或者超过平衡时多子的浓度。
3-2. Define the conductivity modulation
PiN二极管,正向导通时,对i区,P+注入大量空穴,N+注入大量电子,电子-空穴对浓度可以从1014上升到1018,电导率急速上升,这叫电导调制效应。其可使PiN二极管承担小压降,大电流。
3-3/4. Drawing out the structure of JBS/MPS and describing its operation mechanism.
图见page 97/98,(注意JBS漂移区比MPS要薄)
JBS,采用肖特基势垒低的金属与半导体结合;正向时,PN结不导通,只有电子参与导电,开关速度快;反向时,PN结耗尽区随电压升高展宽,最终连成一体,隔离肖特基势垒,其耐压由PN结来承受,泄漏电流小,由于漂移区比MPS薄,BV比MPS低。
MPS,正向电压<0.5V时,PN结不导通,工作类似于肖特基二极管。正向电压>0.6V时,PN结导通,空穴注入漂移区,产生电导调制效应,压降小电流大,由于电子空穴都参与导电,开关速度低于JBS;反向耐压机制与JBS一样,但由于漂移区更厚,BV较JBS高。
3-5. What is the overshoot of the power PiN rectifiers
对PiN管,开启时如果电流变化过快(di/dt较大),其正向压降开始时会超过在稳态时传输相同电流的压降,此现象叫功率PiN管的电压过冲。因为此时i区没有完全被电导调制,电阻还很大,则产生较大压降。
3-6. Explain why the Si Schottky diode cannot operate at high-voltage condition?
硅基肖特基二极管是多子器件,无空穴参与导电,反向时N-承担耐压;无电导调制效应,正向导通时N-区电阻很大;根据Ron.sp∝BV2~2.5,电压升高导通电阻(导通损耗)急剧上升。所以SBD一般应用<600V。
3-7. which structure has the higher leakage current ,Schottky diode or PiN diode?Why?
肖特基二极管泄漏电流更大。
PiN泄漏电流由2部分组成,空间电荷区的产生电流,中性区P+和N+少子的扩散电流。
SBD泄漏电流除了同PiN的相似的2部分,还有金半接触带来的热电子发射电流,并且后者比前2者大得多。
Chapter 4
4-1. What is the meant by reach through breakdown (or punch through breakdown)?
随着BC结反偏电压增加,BC结耗尽层展宽并接触到BE结耗尽层的时候,BE结势垒变低,发射极电子可以越过更低的一个势垒,被反偏BC结电场抽走,此现象叫穿通击穿。
4-2. What is the meant by BVCBO and BVCEO?
BVCBO,开发射极击穿电压;发射极开路,集电极与基极之间的击穿电压。
BVCEO,开基极击穿电压;基极开路,集电极与发射极之间的击穿电压。
4-3. The collector efficiency under what condition is much larger than unit?
当CB结的偏置电压可以和雪崩击穿电压相比拟的时候,集电极效率远大于1.
4-4. Why does the power BJT can operate at high-level current when it locates at its saturation region?
BJT在饱和区的时候VBE>0,VBC>0,VCE很小;BC结正偏,相当于PN-N+二极管工作在正偏情况下,发生大注入现象(电导调制效应),电阻降低,所以在饱和区可以传导大电流。
4-5. what should be carefully considered when you design the base of the power BJT?
1,掺杂不能太淡或者基区不能太薄,否则容易发生穿通击穿
2,掺杂很高或者基区很厚,增益太低,需要更大控制电流IB
所以需要一个折中。
4-6. why ICEO>ICBO?
ICBO是二极管泄漏电流,ICEO会被BJT放大,有公式ICEO=(1+?)ICBO, 所以ICEO>ICBO。
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