P-n diode performance limitations

Transcription

1 P-n diode performance limitations P-n diode I-V I V to V V to 0.7 V; I forw up to 100 A, V rev up to 1000V The turn-on voltage is relatively high (>0.7 V) 1

2 Switching processes in p-n diodes are relatively slow When a square wave voltage is applied to a p-n diode, it is forward biased duirng one halfcycle and reverse biased during the next half-cycle V s I V d R Under forward bias, the current is I V V R s Under reverse bias, the current is almost equal to zero d V s I forward reverse t Using regular p-n diodes, this pulsed current waveform can only be obtained with low frequency pulses t 2

3 Switching processes in p-n diodes (cont.) However, if the pulse frequency is high the reverse current shows significant increase V s t V d I ideal V s I R I t practical t Real p-n diode transient at high frequency High frequency 3

4 Charge storage and Diode transients L n L p Recall the injected carrier distribution at forward bias -x p x n At reverse bias the steady- state minority carrier concentration is very low. But not immediately after switching from the forward bias! -x p x n 4

5 Schottky Diodes Schottky diode has low forward voltage drop and very fast switching speed. Schottky diode consists of a metal - semiconductor junction. There is no p-n junction in Schottky diode. In Schottky diode, there is no minority carrier injection In 1938, Walter Schottky formulated a theory predicting the Schottky effect. metal semiconductor 5

6 Band diagrams of p-n and Schottky diodes In Schottky diode, the depletion region occurs only in the semiconductor region as metal has extremely high electron (hole) concentration. E F E F p n n metal 6

7 Schottky Barrier Formation Work function (Φ): Energy difference between Fermi level and vacuum level. It is a minimum energy needed to remove an electron from a solid. Φ Vaccum level (outside the solid) Φ Electron Affinity (X s ): Energy difference between the conduction band edge and the vacuum level. Vaccum level (outside the solid) X 7

8 continued Schottky Barrier Formation Metal n-type semiconductor before contact Vacuum level (outside the solid) In metals, the conductance band edge and the valence band E v are the same (both at E F level) Φ m X s Φ s E Fs E Fm metal semiconductor 8

9 continued Schottky Barrier Formation After Contact (with n- type material): Vacuum level (outside the solid) Φ m X s Φ s E F Schottky barrier for electrons metal semiconductor 9

10 continued Schottky Barrier Formation Before contact (with p-type material): Vacuum level (outside the solid) X s Φ s Φ m E Fs E Fm metal semiconductor 10

11 continued Schottky Barrier Formation After contact (with p-type material): Vacuum level (outside the solid) X s Φ s Φ m E Fs Schottky barrier for holes metal semiconductor 11

12 Schottky diode characteristics Using energy voltage relationships: Φ m = q φ m and X s = q χ s, we can find: The Schottky barrier height at equilibrium, φ b = φ m χ s metal semiconductor The built-in voltage, V bi qφ m The depletion region charge density, qχs qφ s Vbi = φ m φ s qφ bo qv bi ρ = qn d Note: there is no depletion region in metal E F The depletion region width, x n = 2εε0Vbi qn d x n 12

13 Schottky diode under bias V R V F metal N type metal N type metal N type q(v bi +V R ) qv bi q(v bi -V F ) E F E F E F x n x n x n Reverse bias Equilibrium Forward bias 13

14 Schottky diode current Schottky diode has the same type of current - voltage dependence as a p-n diode: I SCH qv = IS exp 1 kt However, important difference is that in Schottky diodes, the current is NOT associated with electron and hole ACCUMULATION (injection, diffusion and recombination) as in p-n diodes. q(v bi -V) E F The current flow mechanism in Schottky diodes is a thermionic emission. The thermionic emission is the process of electron transfer OVER the Schottky barrier 14

15 continued Schottky diode current The saturation current parameter I s in Schottky diodes depends on the Schottky barrier height: A is the diode area. * qφ 2 b Is = AT exp A kt B A * is the Richardson s constant: A * = * 2 4π qmnk 3 h where m n is the electron effective mass, h is the Planck constant and k is the Boltzmann constant. 15

16 Microwave Schottky diodes HSCH-9161 Millimeter Wave GaAs Schottky Diode (Agilent) 16

17 Ohmic contacts + - p-type n-type Any semiconductor device has to be connected to external wires in order to form an electronic circuit in combination with other circuit elements. In the case of a p-n diode, for example, contacts have to be provided to both p-type and n-type regions of the device in order to connect the diode to an external circuit. 17

18 Ohmic contacts Ohmic contacts must be as low-resistive as possible, so that the current flowing through a semiconductor device leads to the smallest parasitic voltage drop. In good Ohmic contacts, the voltage drop that occurs across the contact must be low and proportional to the current (so that the contacts do not introduce any nonlinearities). Since such contact I-Vs follow the Ohm's law, they are usually called ohmic contacts. xp Ohmic contact qv 1 kt I S p-n junction Ohmic contacts to semiconductors are often made using Schottky contacts 18

19 Rectifying Schottky contacts n-type semiconductor n-type Φ m > Φ s metal semiconductor Rectifying Schottky contact creates an electron depletion region at the metal-semiconductor interface 19

20 Rectifying Schottky contacts p-type semiconductor p-type Φ m < Φ s metal semiconductor Rectifying Schottky contact creates a hole depletion region at the metal-semiconductor interface 20

21 Non - rectifying Schottky contacts Schottky contacts (Rectifying contacts) Ohmic Contacts (Non-rectifying contacts) Criteria: n-type Φ m > Φ s p-type Φ m < Φ s Criteria: n-type Φ m < Φ s p-type Φ m > Φ s 21

22 Non - rectifying Schottky contacts Ohmic Contact to n-type semiconductor Φ m < Φ s Majority carriers are electrons; there is no potential barrier for electrons in both forward or reverse directions: Non-rectifying Schottky contact creates an electron accumulation region at the metal-semiconductor interface. The electron concentration in the contact region is higher than that in the bulk. The resistance of the contact region is low. 22

23 Non - rectifying Schottky contacts Ohmic Contact to p-type semiconductor Φ m > Φ s Majority carriers are holes; there is no potential barrier for holes in both forward or reverse directions: Non-rectifying Schottky contact creates a hole accumulation region at the metal-semiconductor interface. The hole concentration in the contact region is higher than that in the 23 bulk. The resistance of the contact region is low.

24 Ohmic Contact under bias Ohmic contact to n-type semiconductor E F Positive bias at metal V Negative bias at metal metal N type V metal N type E F E F No barrier, so almost no contact voltage drop The voltage is evenly distributed in the bulk Electron reservoir at the interface 24

25 continued Ohmic Contact under bias Ohmic contact to p-type semiconductor E F Positive bias at metal Negative bias at metal V V metal N type metal N type E F E F Hole reservoir at the interface 25

26 Issue: Tunneling Schottky contacts Not for all semiconductors, it is possible to find the metal with Φ m > Φ s If the condition Φ m > Φ s is not met, the Schottky contact creates a depletion region at the Metal Semiconductor interface. Solution: heavily doped semiconductor Depletion region width = W E F W E F 1 W ~ N D Low-doped material large W Highly-doped material small W Metal - n-type contact example Schottky contact to a heavily doped semiconductor creates a tunneling contact with very low effective resistance. 26

27 Tunneling Schottky contacts for high voltage devices: only sub-contact regions are heavily doped Top metal contact p + -type material (heavily doped) d p n-type material; N D and d n are chosen to provide the required operating voltage d n n + sub-contact layer Bottom metal contact 27

28 Sub-contact doping by annealing During high-temperature annealing, metal atoms diffuse into semiconductor and create donor impurities. The contact material needs to be properly chosen to create donor (acceptor in p-materials) type of impurities. Top metal contact p + -type material (heavily doped) d p n-type material; N D and d n are chosen to provide the required operating voltage d n n + annealed region Bottom metal contact 28

29 The contact resistance A quantitative measure of the contact quality is the specific contact resistance, ρ c, which is the contact resistance per unit contact area. sandwich type devices also called vertical geometry devices The contact resistance of each contact in a sandwich-type structure ( VERTICAL structure): R CV =ρ CV /A, where A is the contact area. ρ CV is specific contact resistance for vertical structures: [ρ CV ] = Ω cm 2 Typical current densities in sandwich type devices can be as high as 10 4 A/cm 2. Hence, the specific contact resistance of 10-5 Ω cm 2 is needed to maintain a voltage drop on the order of 0.1 V. 29

30 Contact resistance of planar structures active layer substrate (device holder ) W Planar, or lateral geometry device structure Current In planar structures, contact resistance is inversely proportional to the contact width W but no longer proportional to the total contact area. The current density is larger near the contact edge. The contact resistance of planar structures is typically given by the contact resistance per unit width, R c1. The lateral contact resistance R C and unit-width contact resistance R C1 are related as: R C = R W c1 30

31 Sheet (per square) resistance of thin films L W t The resistance R of a thin semiconductor film between the two contacts, L R = ρ tw For thin films, commonly used thin film characteristic is so called resistance per square or sheet resistance : ρ R sq = t L R Rsq W = When L = W, R = R sq 31

32 Transmission Line Model (TLM) method to determining contact resistance L=1μm 2μm 3μm W t Resistance R n,n+1 between two adjacent contacts in the TLM pattern, R 2R n,n+ 1 = c + R sq L n,n+ 1 W Where L n,n+1 is the distance between the contacts number n and n+1, R sq is the resistance of the semiconductor film per square, 32

33 Transmission Line Model (TLM) plot From the Y axis intercept we can find the value of R C. From the slope of R (L) plot we can find the film resistance per square: Resistance (Ω) ΔR ΔL Δ R= Rsq W 2R c ΔL Distance between contact pads L (μm) 33